PDF generated using the open source mwlib toolkit. See http://code.pediapress.com/ for more information.PDF generated at: Fri, 12 Nov 2010 17:43:32 UTC

Galaxies

ContentsArticlesOverview 1

Galaxy 1Galaxy formation and evolution 19Galaxy merger 25Galaxy morphological classification 26Hubble sequence 31

Short-scale structure 36

Dark matter halo 36Galactic bulge 39Galactic corona 41Galactic disc 42Galactic halo 43Ionization cone 43Low-ionization nuclear emission-line region 44Relativistic jet 46Supermassive black hole 49

Large-scale structure 53

Galaxy groups and clusters 53Galaxy supercluster 56Galaxy filament 60

Types of galaxies 65

Active galaxy 65Barred lenticular galaxy 71Barred irregular galaxy 72Barred spiral galaxy 72Blazar 77Blue compact dwarf galaxy 80Dark galaxy 81Disc galaxy 83Dwarf elliptical galaxy 84Dwarf galaxy 85

Dwarf spheroidal galaxy 88Dwarf spiral galaxy 89Elliptical galaxy 90Faint blue galaxy 92Field galaxy 93Flocculent spiral galaxy 94Grand design spiral galaxy 95Host galaxy 96Interacting galaxy 96Intermediate spiral galaxy 98Irregular galaxy 100Lenticular galaxy 101Low surface brightness galaxy 102Luminous infrared galaxy 103Lyman-alpha emitter 104Lyman-break galaxy 105Magellanic spiral 106Pea galaxy 107Peculiar galaxy 115Polar-ring galaxy 115Protogalaxy 117Quasar 118Radio galaxy 125Ring galaxy 130Seyfert galaxy 131Spiral galaxy 132Starburst galaxy 137Type-cD galaxy 140Unbarred lenticular galaxy 143Unbarred spiral galaxy 144

Appendix 146

Brightest cluster galaxy 146Galaxy color-magnitude diagram 147List of galaxies 148Fossil group 165

References

Article Sources and Contributors 166Image Sources, Licenses and Contributors 170

Article LicensesLicense 173

1

Overview

Galaxy

NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, isabout 17,000 parsecs in diameter and approximately 20 million parsecs

distant.

A galaxy is a massive, gravitationally boundsystem that consists of stars and stellar remnants,an interstellar medium of gas dust, and animportant but poorly understood componenttentatively dubbed dark matter.[1] [2] The name isfrom the Greek root galaxias [γαλαξίας],literally meaning "milky", a reference to theMilky Way galaxy. Typical galaxies range fromdwarfs with as few as ten million (107) stars,[3]

up to giants with a hundred trillion (1014)stars,[4] all orbiting the galaxy's center of mass.Galaxies may contain many star systems, starclusters, and various interstellar clouds. The Sunis one of the stars in the Milky Way galaxy; theSolar System includes the Earth and all the otherobjects that orbit the Sun.

Historically, galaxies have been categorizedaccording to their apparent shape (usuallyreferred to as their visual morphology). A common form is the elliptical galaxy,[5] which has an ellipse-shaped lightprofile. Spiral galaxies are disk-shaped assemblages with dusty, curving arms. Galaxies with irregular or unusualshapes are known as irregular galaxies, and typically result from disruption by the gravitational pull of neighboringgalaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induceepisodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies thatlack a coherent structure could also be referred to as irregular galaxies.[6]

There are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe.[7] [8] Most galaxies are1,000 to 100,000[9]  parsecs in diameter and are usually separated by distances on the order of millions of parsecs (ormegaparsecs).[10] Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average densityless than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations calledclusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arrangedinto sheets and filaments, which surround immense voids in the universe.[11]

Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies.Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. Theyare proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Waygalaxy appears to harbor at least one such object within its nucleus.[12]

Galaxy 2

EtymologyThe word galaxy derives from the Greek term for our own galaxy, galaxias (γαλαξίας), or kyklos galaktikos,meaning "milky circle" for its appearance in the sky. In Greek mythology, Zeus places his son born by a mortalwoman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and willthus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: shepushes the baby away and a jet of her milk sprays the night sky, producing the faint band of light known as theMilky Way.[13]

In the astronomical literature, the capitalized word 'Galaxy' is used to refer to our galaxy, the Milky Way, todistinguish it from the billions of other galaxies. The term Milky Way first appeared in the English language in apoem by Chaucer.

"See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt."—Geoffrey Chaucer. The House of Fame, c. 1380.[14]

When William Herschel constructed his catalog of deep sky objects, he used the name spiral nebula for certainobjects such as M31. These would later be recognized as immense conglomerations of stars, when the true distanceto these objects began to be appreciated, and they would be termed island universes. However, the word Universewas understood to mean the entirety of existence, so this expression fell into disuse and the objects instead becameknown as galaxies.[15]

Observation historyThe realization that we live in a galaxy, and that there were, in fact, many other galaxies, parallels discoveries thatwere made about the Milky Way and other nebulae in the night sky.

The Milky Way

Galactic Center of Milky Way and a meteor

The Greek philosopher Democritus (450–370 B.C.)proposed that the bright band on the night sky knownas the Milky Way might consist of distant stars.[16]

Aristotle (384–322 B.C.), however, believed the MilkyWay to be caused by "the ignition of the fieryexhalation of some stars which were large, numerousand close together" and that the "ignition takes place inthe upper part of the atmosphere, in the region of theworld which is continuous with the heavenlymotions."[17] The Neoplatonist philosopherOlympiodorus the Younger (c. 495-570 A.D.) criticizedthis view, arguing that if the Milky Way weresublunary it should appear different at different timesand places on the Earth, and that it should have parallax, which it does not. In his view, the Milky Way was celestial.This idea would be influential later in the Islamic world.[18]

The Arabian astronomer, Alhazen (965–1037), made the first attempt at observing and measuring the Milky Way's parallax,[19] and he thus "determined that because the Milky Way had no parallax, it was very remote from the Earth and did not belong to the atmosphere."[20] The Persian astronomer Abū Rayhān al-Bīrūnī (973–1048) proposed the Milky Way galaxy to be "a collection of countless fragments of the nature of nebulous stars."[21] The Andalusian

Galaxy 3

astronomer Ibn Bajjah ("Avempace", d. 1138) proposed that the Milky Way was made up of many stars that almosttouch one another and appear to be a continuous image due to the effect of refraction from sublunary material,[17] [22]

citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects arenear.[17] The Syrian-born Ibn Qayyim Al-Jawziyya (1292–1350) proposed the Milky Way galaxy to be "a myriad oftiny stars packed together in the sphere of the fixed stars".[23]

Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to studythe Milky Way and discovered that it is composed of a huge number of faint stars.[24] In 1750 Thomas Wright, in hisAn original theory or new hypothesis of the Universe, speculated (correctly) that the galaxy might be a rotating bodyof a huge number of stars held together by gravitational forces, akin to the solar system but on a much larger scale.The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk.[25] In a treatise in1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way.

The shape of the Milky Way as deduced from star counts by WilliamHerschel in 1785; the solar system was assumed to be near the

center.

The first attempt to describe the shape of the MilkyWay and the position of the Sun in it was carried out byWilliam Herschel in 1785 by carefully counting thenumber of stars in different regions of the sky. Heproduced a diagram of the shape of the galaxy with thesolar system close to the center.[26] [27] Using a refinedapproach, Kapteyn in 1920 arrived at the picture of asmall (diameter about 15 kiloparsecs) ellipsoid galaxywith the Sun close to the center. A different method by

Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk withdiameter approximately 70 kiloparsecs and the Sun far from the center.[25] Both analyses failed to take into accountthe absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler quantifiedthis effect in 1930 by studying open clusters, the present picture of our galaxy, the Milky Way, emerged.[28]

Distinction from other nebulae

Sketch of the Whirlpool Galaxy by Lord Rosse in 1845

In the 10th century, the Persian astronomer, Abdal-Rahman al-Sufi (known in the West as Azophi),made the earliest recorded observation of theAndromeda Galaxy, describing it as a "small cloud".[29]

Al-Sufi also identified the Large Magellanic Cloud,which is visible from Yemen, though not from Isfahan;it was not seen by Europeans until Magellan's voyagein the 16th century.[30] [31] These were the first galaxiesother than the Milky Way to be observed from Earth.Al-Sufi published his findings in his Book of FixedStars in 964.

In 1750 Thomas Wright, in his An original theory ornew hypothesis of the Universe, speculated (correctly)that Milky Way was a flattened disk of stars, and thatsome of the nebulae visible in the night sky might beseparate Milky Ways.[25] [32] In 1755 Immanuel Kant introduced the term "island universe" for these distant nebulae.

Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest nebulae (celestial objects with a nebulous appearance), later followed by a larger catalog of 5,000 nebulae assembled by William Herschel.[25] In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical

Galaxy 4

and spiral nebulae. He also managed to make out individual point sources in some of these nebulae, lending credenceto Kant's earlier conjecture.[33]

In 1912, Vesto Slipher made spectrographic studies of the brightest spiral nebulae to determine if they were madefrom chemicals that would be expected in a planetary system. However, Slipher discovered that the spiral nebulaehad high red shifts, indicating that they were moving away at rate higher than the Milky Way's escape velocity. Thusthey were not gravitationally bound to the Milky Way, and were unlikely to be a part of the galaxy.[34] [35]

In 1917, Heber Curtis had observed a nova S Andromedae within the "Great Andromeda Nebula" (Messier objectM31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average,10 magnitudes fainter than those that occurred within our galaxy. As a result he was able to come up with a distanceestimate of 150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds thatspiral nebulae are actually independent galaxies.[36]

Photograph of the "Great Andromeda Nebula" from 1899, lateridentified as the Andromeda Galaxy

In 1920 the so-called Great Debate took place betweenHarlow Shapley and Heber Curtis, concerning thenature of the Milky Way, spiral nebulae, and thedimensions of the Universe. To support his claim thatthe Great Andromeda Nebula was an external galaxy,Curtis noted the appearance of dark lanes resemblingthe dust clouds in the Milky Way, as well as thesignificant Doppler shift.[37]

The matter was conclusively settled in the early 1920s.In 1922, astronomer Ernst Öpik gave a distancedetermination which supported the theory that theAndromeda Nebula is indeed a distant extra-galacticobject.[38] Using the new 100 inch Mt. Wilson

telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual starsand identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far toodistant to be part of the Milky Way.[39] In 1936 Hubble produced a classification system for galaxies that is used tothis day, the Hubble sequence.[40]

Modern research

Rotation curve of a typical spiral galaxy:predicted (A) and observed (B). The distance is

from the galactic core.

In 1944 Hendrik van de Hulst predicted microwave radiation at awavelength of 21 cm resulting from interstellar atomic hydrogengas;[41] this radiation was observed in 1951. The radiation allowed formuch improved study of the Milky Way Galaxy, since it is not affectedby dust absorption and its Doppler shift can be used to map the motionof the gas in the Galaxy. These observations led to the postulation of arotating bar structure in the center of the Galaxy.[42] With improvedradio telescopes, hydrogen gas could also be traced in other galaxies.

In the 1970s it was discovered in Vera Rubin's study of the rotationspeed of gas in galaxies that the total

Galaxy 5

The most distant galaxy: UDFy-38135539

visible mass (from the stars and gas) does not properlyaccount for the speed of the rotating gas. This galaxyrotation problem is thought to be explained by thepresence of large quantities of unseen dark matter.[43]

[44]

Beginning in the 1990s, the Hubble Space Telescopeyielded improved observations. Among other things, itestablished that the missing dark matter in our galaxycannot solely consist of inherently faint and smallstars.[45] The Hubble Deep Field, an extremely longexposure of a relatively empty part of the sky, providedevidence that there are about 125 billion (1.25×1011)galaxies in the universe.[46] Improved technology in detecting the spectra invisible to humans (radio telescopes,infrared cameras, and x-ray telescopes) allow detection of other galaxies that are not detected by Hubble.Particularly, galaxy surveys in the zone of avoidance (the region of the sky blocked by the Milky Way) haverevealed a number of new galaxies.[47]

The most distant galaxy as seen in the Hubble Ultra Deep Field is UDFy-38135539 approximately 13 billion ly fromEarth.

Types and morphology

Types of galaxies according to the Hubble classification scheme. An E indicates a type ofelliptical galaxy; an S is a spiral; and SB is a barred-spiral galaxy.[48]

Galaxies come in three main types:ellipticals, spirals, and irregulars. Aslightly more extensive description ofgalaxy types based on their appearanceis given by the Hubble sequence. Sincethe Hubble sequence is entirely basedupon visual morphological type, it maymiss certain important characteristicsof galaxies such as star formation rate(in starburst galaxies) and activity inthe core (in active galaxies).[6]

Ellipticals

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, beingnearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them anelliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically haverelatively little interstellar matter. Consequently these galaxies also have a low portion of open clusters and areduced rate of new star formation. Instead they are dominated by generally older, more evolved stars that areorbiting the common center of gravity in random directions. In this sense they have some similarity to the muchsmaller globular clusters.[49]

The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the interaction ofgalaxies, resulting in a collision and merger. They can grow to enormous sizes (compared to spiral galaxies, forexample), and giant elliptical galaxies are often found near the core of large galaxy clusters.[50] Starburst galaxies arethe result of such a galactic collision that can result in the formation of an elliptical galaxy.[49]

Galaxy 6

Spirals

The Whirlpool Galaxy (on left), an example of anunbarred spiral galaxy.

Spiral galaxies consist of a rotating disk of stars and interstellarmedium, along with a central bulge of generally older stars. Extendingoutward from the bulge are relatively bright arms. In the Hubbleclassification scheme, spiral galaxies are listed as type S, followed by aletter (a, b, or c) that indicates the degree of tightness of the spiral armsand the size of the central bulge. An Sa galaxy has tightly wound,poorly defined arms and possesses a relatively large core region. At theother extreme, an Sc galaxy has open, well-defined arms and a smallcore region.[51]

In spiral galaxies, the spiral arms do have the shape of approximatelogarithmic spirals, a pattern that can be theoretically shown to resultfrom a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, butthey do so with constant angular velocity. The spiral arms are thought to be areas of high density matter, or "densitywaves". As stars move through an arm, the space velocity of each stellar system is modified by the gravitationalforce of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) Thiseffect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible becausethe high density facilitates star formation, and therefore they harbor many bright and young stars.

NGC 1300, an example of a barred spiral galaxy.

A majority of spiral galaxies have a linear,bar-shaped band of stars that extendsoutward to either side of the core, thenmerges into the spiral arm structure.[52] Inthe Hubble classification scheme, these aredesignated by an SB, followed by alower-case letter (a, b or c) that indicates theform of the spiral arms (in the same manneras the categorization of normal spiralgalaxies). Bars are thought to be temporarystructures that can occur as a result of adensity wave radiating outward from thecore, or else due to a tidal interaction with

another galaxy.[53] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the corealong the arms.[54]

Our own galaxy is a large disk-shaped barred-spiral galaxy[55] about 30 kiloparsecs in diameter and a kiloparsec inthickness. It contains about two hundred billion (2×1011)[56] stars and has a total mass of about six hundred billion(6×1011) times the mass of the Sun.[57]

Galaxy 7

Other morphologies

Hoag's Object, an example of a ring galaxy.

Peculiar galaxies are galactic formations that develop unusualproperties due to tidal interactions with other galaxies. An example ofthis is the ring galaxy, which possesses a ring-like structure of stars andinterstellar medium surrounding a bare core. A ring galaxy is thoughtto occur when a smaller galaxy passes through the core of a spiralgalaxy.[58] Such an event may have affected the Andromeda Galaxy, asit displays a multi-ring-like structure when viewed in infraredradiation.[59]

A lenticular galaxy is an intermediate form that has properties of bothelliptical and spiral galaxies. These are categorized as Hubble type S0,and they possess ill-defined spiral arms with an elliptical halo ofstars.[60] (Barred lenticular galaxies receive Hubble classificationSB0.)

NGC 5866, an example of a lenticular galaxy. Credit:NASA/ESA.

In addition to the classifications mentioned above, there are anumber of galaxies that can not be readily classified into anelliptical or spiral morphology. These are categorized as irregulargalaxies. An Irr-I galaxy has some structure but does not aligncleanly with the Hubble classification scheme. Irr-II galaxies donot possess any structure that resembles a Hubble classification,and may have been disrupted.[61] Nearby examples of (dwarf)irregular galaxies include the Magellanic Clouds.

Dwarfs

Despite the prominence of large elliptical and spiral galaxies, mostgalaxies in the universe appear to be dwarf galaxies. Thesegalaxies are relatively small when compared with other galacticformations, being about one hundredth the size of the Milky Way,containing only a few billion stars. Ultra-compact dwarf galaxieshave recently been discovered that are only 100 parsecs across.[62]

Many dwarf galaxies may orbit a single larger galaxy; the MilkyWay has at least a dozen such satellites, with an estimated 300–500 yet to be discovered.[63] Dwarf galaxies mayalso be classified as elliptical, spiral, or irregular. Since small dwarf ellipticals bear little resemblance to largeellipticals, they are often called dwarf spheroidal galaxies instead.

A study of 27 Milky Way neighbors found that dwarf galaxies were all approximately 10 million solar masses,regardless of whether they have thousands or millions of stars. This has led to the suggestion that galaxies are largelyformed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable ofgravitational coalescence on a smaller scale.[64]

Galaxy 8

Unusual dynamics and activities

InteractingThe average separation between galaxies within a cluster is a little over an order of magnitude larger than theirdiameter. Hence interactions between these galaxies are relatively frequent, and play an important role in theirevolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause someexchange of gas and dust.[65] [66]

The Antennae Galaxies are undergoing a collision that will result in theireventual merger.

Collisions occur when two galaxies pass directlythrough each other and have sufficient relativemomentum not to merge. The stars within theseinteracting galaxies will typically pass straightthrough without colliding. However, the gas anddust within the two forms will interact. This cantrigger bursts of star formation as the interstellarmedium becomes disrupted and compressed. Acollision can severely distort the shape of one orboth galaxies, forming bars, rings or tail-likestructures.[65] [66]

At the extreme of interactions are galacticmergers. In this case the relative momentum ofthe two galaxies is insufficient to allow thegalaxies to pass through each other. Instead, theygradually merge together to form a single, largergalaxy. Mergers can result in significant changesto morphology, as compared to the originalgalaxies. In the case where one of the galaxies ismuch more massive, however, the result is

known as cannibalism. In this case the larger galaxy will remain relatively undisturbed by the merger, while thesmaller galaxy is torn apart. The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius DwarfElliptical Galaxy and the Canis Major Dwarf Galaxy.[65] [66]

Galaxy 9

Starburst

M82, the archetype starburst galaxy, has experienced a 10-fold increase[67]

in star formation rate as compared to a "normal" galaxy.

Stars are created within galaxies from a reserveof cold gas that forms into giant molecularclouds. Some galaxies have been observed toform stars at an exceptional rate, known as astarburst. Should they continue to do so,however, they would consume their reserve ofgas in a time frame lower than the lifespan of thegalaxy. Hence starburst activity usually lasts foronly about ten million years, a relatively briefperiod in the history of a galaxy. Starburstgalaxies were more common during the earlyhistory of the universe,[68] and, at present, stillcontribute an estimated 15% to the total starproduction rate.[69]

Starburst galaxies are characterized by dustyconcentrations of gas and the appearance ofnewly formed stars, including massive stars that ionize the surrounding clouds to create H II regions.[70] Thesemassive stars produce supernova explosions, resulting in expanding remnants that interact powerfully with thesurrounding gas. These outbursts trigger a chain reaction of star building that spreads throughout the gaseous region.Only when the available gas is nearly consumed or dispersed does the starburst activity come to an end.[68]

Starbursts are often associated with merging or interacting galaxies. The prototype example of such astarburst-forming interaction is M82, which experienced a close encounter with the larger M81. Irregular galaxiesoften exhibit spaced knots of starburst activity.[71]

Active nucleusA portion of the galaxies we can observe are classified as active. That is, a significant portion of the total energyoutput from the galaxy is emitted by a source other than the stars, dust and interstellar medium.The standard model for an active galactic nucleus is based upon an accretion disc that forms around a supermassiveblack hole (SMBH) at the core region. The radiation from an active galactic nucleus results from the gravitationalenergy of matter as it falls toward the black hole from the disc.[72] In about 10% of these objects, a diametricallyopposed pair of energetic jets ejects particles from the core at velocities close to the speed of light. The mechanismfor producing these jets is still not well understood.[73]

Galaxy 10

A jet of particles is being emitted from the core of the elliptical radio galaxyM87.

Active galaxies that emit high-energy radiationin the form of x-rays are classified as Seyfertgalaxies or quasars, depending on theluminosity. Blazars are believed to be an activegalaxy with a relativistic jet that is pointed in thedirection of the Earth. A radio galaxy emits radiofrequencies from relativistic jets. A unifiedmodel of these types of active galaxies explainstheir differences based on the viewing angle ofthe observer.[73]

Possibly related to active galactic nuclei (as wellas starburst regions) are low-ionization nuclearemission-line regions (LINERs). The emissionfrom LINER-type galaxies is dominated byweakly ionized elements.[74] Approximatelyone-third of nearby galaxies are classified ascontaining LINER nuclei.[72] [74] [75]

Formation and evolution

The study of galactic formation and evolutionattempts to answer questions regarding how galaxies formed and their evolutionary path over the history of theuniverse. Some theories in this field have now become widely accepted, but it is still an active area in astrophysics.

Formation

Artist's impression of a young galaxy accretingmaterial.

Current cosmological models of the early Universe are based on theBig Bang theory. About 300,000 years after this event, atoms ofhydrogen and helium began to form, in an event called recombination.Nearly all the hydrogen was neutral (non-ionized) and readily absorbedlight, and no stars had yet formed. As a result this period has beencalled the "Dark Ages". It was from density fluctuations (or anisotropicirregularities) in this primordial matter that larger structures began toappear. As a result, masses of baryonic matter started to condensewithin cold dark matter halos.[76] These primordial structures wouldeventually become the galaxies we see today.

Evidence for the early appearance of galaxies was found in 2006, whenit was discovered that the galaxy IOK-1 has an unusually high redshift of 6.96, corresponding to just 750 millionyears after the Big Bang and making it the most distant and primordial galaxy yet seen.[77] While some scientistshave claimed other objects (such as Abell 1835 IR1916) have higher redshifts (and therefore are seen in an earlierstage of the Universe's evolution), IOK-1's age and composition have been more reliably established. The existenceof such early protogalaxies suggests that they must have grown in the so-called "Dark Ages".[76]

The detailed process by which such early galaxy formation occurred is a major open question in astronomy. Theories could be divided into two categories: top-down and bottom-up. In top-down theories (such as the Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years.[78] In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as

Galaxy 11

globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.[79] Modern theoriesmust be modified to account for the probable presence of large dark matter halos.Once protogalaxies began to form and contract, the first halo stars (called Population III stars) appeared within them.These were composed almost entirely of hydrogen and helium, and may have been massive. If so, these huge starswould have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into theinterstellar medium.[80] This first generation of stars re-ionized the surrounding neutral hydrogen, creating expandingbubbles of space through which light could readily travel.[81]

Evolution

I Zwicky 18 (lower left) resembles a newly formed galaxy.[82] [83]

Within a billion years of a galaxy's formation,key structures begin to appear. Globular clusters,the central supermassive black hole, and agalactic bulge of metal-poor Population II starsform. The creation of a supermassive black holeappears to play a key role in actively regulatingthe growth of galaxies by limiting the totalamount of additional matter added.[84] Duringthis early epoch, galaxies undergo a major burstof star formation.[85]

During the following two billion years, theaccumulated matter settles into a galacticdisc.[86] A galaxy will continue to absorbinfalling material from high velocity clouds anddwarf galaxies throughout its life.[87] This matteris mostly hydrogen and helium. The cycle ofstellar birth and death slowly increases theabundance of heavy elements, eventuallyallowing the formation of planets.[88]

The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of galaxies werecommon during the early epoch, and the majority of galaxies were peculiar in morphology.[89] Given the distancesbetween the stars, the great majority of stellar systems in colliding galaxies will be unaffected. However,gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of starsknown as tidal tails. Examples of these formations can be seen in NGC 4676[90] or the Antennae Galaxies.[91]

As an example of such an interaction, the Milky Way galaxy and the nearby Andromeda Galaxy are moving towardeach other at about 130 km/s, and—depending upon the lateral movements—the two may collide in about five to sixbillion years. Although the Milky Way has never collided with a galaxy as large as Andromeda before, evidence ofpast collisions of the Milky Way with smaller dwarf galaxies is increasing.[92]

Such large-scale interactions are rare. As time passes, mergers of two systems of equal size become less common.Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of starformation probably also peaked approximately ten billion years ago.[93]

Galaxy 12

Future trendsAt present, most star formation occurs in smaller galaxies where cool gas is not so depleted.[89] Spiral galaxies, likethe Milky Way, only produce new generations of stars as long as they have dense molecular clouds of interstellarhydrogen in their spiral arms.[94] Elliptical galaxies are already largely devoid of this gas, and so form no newstars.[95] The supply of star-forming material is finite; once stars have converted the available supply of hydrogeninto heavier elements, new star formation will come to an end.[96]

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellarage" will wind down after about ten trillion to one hundred trillion years (1013–1014 years), as the smallest,longest-lived stars in our astrosphere, tiny red dwarfs, begin to fade. At the end of the stellar age, galaxies will becomposed of compact objects: brown dwarfs, white dwarfs that are cooling or cold ("black dwarfs"), neutron stars,and black holes. Eventually, as a result of gravitational relaxation, all stars will either fall into central supermassiveblack holes or be flung into intergalactic space as a result of collisions.[96] [97]

Larger-scale structuresDeep sky surveys show that galaxies are often found in relatively close association with other galaxies. Solitarygalaxies that have not significantly interacted with another galaxy of comparable mass during the past billion yearsare relatively scarce. Only about 5% of the galaxies surveyed have been found to be truly isolated; however, theseisolated formations may have interacted and even merged with other galaxies in the past, and may still be orbited bysmaller, satellite galaxies. Isolated galaxies[98] can produce stars at a higher rate than normal, as their gas is not beingstripped by other nearby galaxies.[99]

On the largest scale, the universe is continually expanding, resulting in an average increase in the separation betweenindividual galaxies (see Hubble's law). Associations of galaxies can overcome this expansion on a local scalethrough their mutual gravitational attraction. These associations formed early in the universe, as clumps of darkmatter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. Thison-going merger process (as well as an influx of infalling gas) heats the inter-galactic gas within a cluster to veryhigh temperatures, reaching 30–100 megakelvins.[100] About 70–80% of the mass in a cluster is in the form of darkmatter, with 10–30% consisting of this heated gas and the remaining few percent of the matter in the form ofgalaxies.[101]

Galaxy 13

Seyfert's Sextet is an example of a compact galaxy group.

Most galaxies in the universe are gravitationallybound to a number of other galaxies. These forma fractal-like hierarchy of clustered structures,with the smallest such associations being termedgroups. A group of galaxies is the most commontype of galactic cluster, and these formationscontain a majority of the galaxies (as well asmost of the baryonic mass) in the universe.[102]

[103] To remain gravitationally bound to such agroup, each member galaxy must have asufficiently low velocity to prevent it fromescaping (see Virial theorem). If there isinsufficient kinetic energy, however, the groupmay evolve into a smaller number of galaxiesthrough mergers.[104]

Larger structures containing many thousands ofgalaxies packed into an area a few megaparsecsacross are called clusters. Clusters of galaxies

are often dominated by a single giant elliptical galaxy, known as the brightest cluster galaxy, which, over time,tidally destroys its satellite galaxies and adds their mass to its own.[105]

Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually.At the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids.[106] Abovethis scale, the universe appears to be isotropic and homogeneous.[107]

The Milky Way galaxy is a member of an association named the Local Group, a relatively small group of galaxiesthat has a diameter of approximately one megaparsec. The Milky Way and the Andromeda Galaxy are the twobrightest galaxies within the group; many of the other member galaxies are dwarf companions of these twogalaxies.[108] The Local Group itself is a part of a cloud-like structure within the Virgo Supercluster, a large,extended structure of groups and clusters of galaxies centered around the Virgo Cluster.[109]

Multi-wavelength observationAfter galaxies external to the Milky Way were found to exist, initial observations were made mostly using visiblelight. The peak radiation of most stars lies here, so the observation of the stars that form galaxies has been a majorcomponent of optical astronomy. It is also a favorable portion of the spectrum for observing ionized H II regions,and for examining the distribution of dusty arms.The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared, which canbe used to observe the interior regions of giant molecular clouds and galactic cores in great detail.[110] Infrared isalso used to observe distant, red-shifted galaxies that were formed much earlier in the history of the universe. Watervapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude orspace-based telescopes are used for infrared astronomy.The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies. Theatmosphere is nearly transparent to radio between 5 MHz and 30 GHz. (The ionosphere blocks signals below thisrange.)[111] Large radio interferometers have been used to map the active jets emitted from active nuclei. Radiotelescopes can also be used to observe neutral hydrogen (via 21 centimetre radiation), including, potentially, thenon-ionized matter in the early universe that later collapsed to form galaxies.[112]

Galaxy 14

Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. An ultraviolet flare was observedwhen a star in a distant galaxy was torn apart from the tidal forces of a black hole.[113] The distribution of hot gas ingalactic clusters can be mapped by X-rays. The existence of super-massive black holes at the cores of galaxies wasconfirmed through X-ray astronomy.[114]

See also• Galactic orientation• List of galaxies• List of nearest galaxies• Luminous infrared galaxy• Supermassive black hole• Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure• Galaxy formation and evolution• Dark galaxy

Notes[1] Sparke, L. S.; Gallagher III, J. S. (2000). Galaxies in the Universe: An Introduction. Cambridge: Cambridge University Press.

ISBN 0-521-59704-4.[2] Hupp, E.; Roy, S.; Watzke, M. (2006-08-12). "NASA Finds Direct Proof of Dark Matter" (http:/ / www. nasa. gov/ home/ hqnews/ 2006/ aug/

HQ_06297_CHANDRA_Dark_Matter. html). NASA. . Retrieved 2007-04-17.[3] "Unveiling the Secret of a Virgo Dwarf Galaxy" (http:/ / www. eso. org/ outreach/ press-rel/ pr-2000/ pr-12-00. html). ESO. 2000-05-03. .

Retrieved 2007-01-03.[4] Wilford, John Noble (1990-10-26). "Sighting of Largest Galaxy Hints Clues on the Clustering of Matter" (http:/ / www. nytimes. com/ 1990/

10/ 26/ us/ sighting-of-largest-galaxy-hints-clues-on-the-clustering-of-matter. html). New York Times. . Retrieved 2010-05-06.[5] Hoover, Aaron (2003-06-16). "UF Astronomers: Universe Slightly Simpler Than Expected" (http:/ / web. archive. org/ web/

20070225064443/ http:/ / www. napa. ufl. edu/ 2003news/ galaxies. htm). Hubble News Desk. Archived from the original (http:/ / www. napa.ufl. edu/ 2003news/ galaxies. htm) on February 25, 2007. . Retrieved 2007-02-05.

[6] Jarrett, T. H.. "Near-Infrared Galaxy Morphology Atlas" (http:/ / www. ipac. caltech. edu/ 2mass/ gallery/ galmorph/ ). California Institute ofTechnology. . Retrieved 2007-01-09.

[7] Gott, J. Richard, III; et al. (May 2005). "A Map of the Universe". The Astrophysical Journal 624 (2): 463–484. doi:10.1086/428890.Bibcode: 2005ApJ...624..463G.

[8] Mackie, Glen (2002-02-01). "To see the Universe in a Grain of Taranaki Sand" (http:/ / astronomy. swin. edu. au/ ~gmackie/ billions. html).Swinburne University. . Retrieved 2006-12-20.

[9] "Hubble's Largest Galaxy Portrait Offers a New High-Definition View" (http:/ / www. nasa. gov/ mission_pages/ hubble/ science/hst_spiral_m10. html). NASA. 2006-02-28. . Retrieved 2007-01-03.

[10] Gilman, D.. "The Galaxies: Islands of Stars" (http:/ / www. hq. nasa. gov/ office/ pao/ History/ EP-177/ ch4-7. html). NASA WMAP. .Retrieved 2006-08-10.

[11] "Galaxy Clusters and Large-Scale Structure" (http:/ / www. damtp. cam. ac. uk/ user/ gr/ public/ gal_lss. html). University of Cambridge. .Retrieved 2007-01-15.

[12] Finley, D.; Aguilar, D. (2005-11-02). "Astronomers Get Closest Look Yet At Milky Way's Mysterious Core" (http:/ / www. nrao. edu/ pr/2005/ sagastar/ ). National Radio Astronomy Observatory. . Retrieved 2006-08-10.

[13] Koneãn˘, Lubomír. "Emblematics, Agriculture, and Mythography in The Origin of the Milky Way" (http:/ / web. archive. org/ web/20060720204104/ http:/ / www. udu. cas. cz/ collegium/ tintoretto. pdf) (PDF). Academy of Sciences of the Czech Republic. Archived fromthe original (http:/ / www. udu. cas. cz/ collegium/ tintoretto. pdf) on July 20, 2006. . Retrieved 2007-01-05.

[14] "[[Online Etymology Dictionary (http:/ / www. etymonline. com/ index. php?term=galaxy)]"]. . Retrieved 2007-01-03.[15] "Explore the Archer's Realm" (http:/ / www. space. com/ spacewatch/ 050902_teapot. html). space.com. 2005-09-02. . Retrieved

2007-01-03.[16] Burns, Tom (2007-07-31). "Constellations reflect heroes, beasts, star-crossed lovers" (http:/ / www. dispatch. com/ live/ content/ now/

stories/ 2007/ 07/ stars. html). The Dispatch. . Retrieved 2008-03-18.[17] Montada, Josep Puig (September 28, 2007). "Ibn Bajja" (http:/ / plato. stanford. edu/ entries/ ibn-bajja). Stanford Encyclopedia of

Philosophy. . Retrieved 2008-07-11.[18] Heidarzadeh, Tofigh (2008). A history of physical theories of comets, from Aristotle to Whipple. Springer. pp. 23–25. ISBN 140208322X.[19] Mohamed, Mohaini (2000). Great Muslim Mathematicians. Penerbit UTM. pp. 49–50. ISBN 9835201579. OCLC 48759017.

Galaxy 15

[20] Bouali, Hamid-Eddine; Zghal, Mourad; Lakhdar, Zohra Ben (2005). "Popularisation of Optical Phenomena: Establishing the First IbnAl-Haytham Workshop on Photography" (http:/ / spie. org/ etop/ ETOP2005_080. pdf) (PDF). The Education and Training in Optics andPhotonics Conference. . Retrieved 2008-07-08.

[21] O'Connor, John J.; Robertson, Edmund F., "Abu Rayhan Muhammad ibn Ahmad al-Biruni" (http:/ / www-history. mcs. st-andrews. ac. uk/Biographies/ Al-Biruni. html), MacTutor History of Mathematics archive, University of St Andrews, .

[22] Heidarzadeh (2008) Table 2.1, p. 25.[23] Livingston, John W. (1971). "Ibn Qayyim al-Jawziyyah: A Fourteenth Century Defense against Astrological Divination and Alchemical

Transmutation" (http:/ / jstor. org/ stable/ 600445). Journal of the American Oriental Society 91 (1): 96–103 [99]. doi:10.2307/600445. .[24] O'Connor, J. J.; Robertson, E. F. (November 2002). "Galileo Galilei" (http:/ / www-gap. dcs. st-and. ac. uk/ ~history/ Biographies/ Galileo.

html). University of St. Andrews. . Retrieved 2007-01-08.[25] Evans, J. C. (1998-11-24). "Our Galaxy" (http:/ / physics. gmu. edu/ ~jevans/ astr103/ CourseNotes/ ECText/ ch20_txt. htm). George Mason

University. . Retrieved 2007-01-04.[26] Marschall, Laurence A. (1999-10-21). "How did scientists determine our location within the Milky Way galaxy--in other words, how do we

know that our solar system is in the arm of a spiral galaxy, far from the galaxy's center?" (http:/ / www. sciam. com/ space/ article/ id/how-did-scientists-determ/ topicID/ 2/ catID/ 3). Scientific American. . Retrieved 2007-12-13.

[27] Kuhn, Karl F.; Koupelis, Theo (2004). In Quest of the Universe. Jones and Bartlett Publishers. ISBN 0763708100. OCLC 148117843.[28] Trimble, V. (1999). "Robert Trumpler and the (Non)transparency of Space" (http:/ / adsabs. harvard. edu/ abs/ 1999AAS. . . 195. 7409T).

Bulletin of the American Astronomical Society (31): 1479. . Retrieved 2007-01-08.[29] Kepple, George Robert; Glen W. Sanner (1998). The Night Sky Observer's Guide, Volume 1. Willmann-Bell, Inc.. pp. 18.

ISBN 0-943396-58-1.[30] "Observatoire de Paris (Abd-al-Rahman Al Sufi)" (http:/ / messier. obspm. fr/ xtra/ Bios/ alsufi. html). . Retrieved 2007-04-19.[31] "Observatoire de Paris (LMC)" (http:/ / messier. obspm. fr/ xtra/ ngc/ lmc. html). . Retrieved 2007-04-19.[32] See text quoted from Wright's An original theory or new hypothesis of the Universe in Dyson, Freeman (1979). Disturbing the Universe.

London: Pan. p. 245. ISBN 0330263242.[33] Abbey, Lenny. "The Earl of Rosse and the Leviathan of Parsontown" (http:/ / labbey. com/ Telescopes/ Parsontown. html). The Compleat

Amateur Astronomer. . Retrieved 2007-01-04.[34] Slipher, V. M. (1913). "The radial velocity of the Andromeda Nebula". Lowell Observatory Bulletin 1: 56–57.

Bibcode: 1913LowOB...2...56S.[35] Slipher, V. M. (January 1915). "Spectrographic Observations of Nebulae". Popular Astronomy 23: 21–24. Bibcode: 1915PA.....23...21S.[36] Curtis, Heber D. (1988). "Novae in Spiral Nebulae and the Island Universe Theory" (http:/ / adsabs. harvard. edu/ abs/ 1988PASP. . 100. . . .

6C). Publications of the Astronomical Society of the Pacific 100: 6. doi:10.1086/132128. .[37] Weaver, Harold F.. "Robert Julius Trumpler" (http:/ / www. nap. edu/ readingroom/ books/ biomems/ rtrumpler. html). National Academy of

Sciences. . Retrieved 2007-01-05.[38] Öpik, E. (1922). "An estimate of the distance of the Andromeda Nebula" (http:/ / adsabs. harvard. edu/ abs/ 1922ApJ. . . . 55. . 406O).

Astrophysical Journal 55: 406\u2013410. doi:10.1086/142680. .[39] Hubble, E. P. (1929). "A spiral nebula as a stellar system, Messier 31" (http:/ / adsabs. harvard. edu/ cgi-bin/ bib_query?1929ApJ. . . . 69. .

103H). Astrophysical Journal 69: 103–158. doi:10.1086/143167. .[40] Sandage, Allan (1989). "Edwin Hubble, 1889–1953" (http:/ / antwrp. gsfc. nasa. gov/ diamond_jubilee/ 1996/ sandage_hubble. html).

Journal of the Royal Astronomical Society of Canada 83 (6). . Retrieved 2007-01-08.[41] Tenn, Joe. "Hendrik Christoffel van de Hulst" (http:/ / www. phys-astro. sonoma. edu/ BruceMedalists/ vandeHulst/ ). Sonoma State

University. . Retrieved 2007-01-05.[42] López-Corredoira, M.; Hammersley, P. L.; Garzón, F.; Cabrera-Lavers, A.; Castro-Rodríguez, N.; Schultheis, M.; Mahoney, T. J. (2001).

"Searching for the in-plane Galactic bar and ring in DENIS" (http:/ / adsabs. harvard. edu/ cgi-bin/ nph-bib_query?bibcode=2001A& A. . .373. . 139L). Astronomy and Astrophysics 373: 139–152. doi:10.1051/0004-6361:20010560. . Retrieved 2007-01-08.

[43] Rubin, V. C. (Jone 1983). "Dark matter in spiral galaxies". Scientific American 248: 96–106. doi:10.1038/scientificamerican0683-96.Bibcode: 1983SciAm.248...96R.

[44] Rubin, Vera C. (June 2000). "One Hundred Years of Rotating Galaxies". The Publications of the Astronomical Society of the Pacific 112(772): 747–750. doi:10.1086/316573. Bibcode: 2000PASP..112..747R.

[45] "Hubble Rules Out a Leading Explanation for Dark Matter" (http:/ / hubblesite. org/ newscenter/ archive/ releases/ 1994/ 41/ text/ ). HubbleNews Desk. 1994-10-17. . Retrieved 2007-01-08.

[46] "How many galaxies are there?" (http:/ / imagine. gsfc. nasa. gov/ docs/ ask_astro/ answers/ 021127a. html). NASA. 2002-11-27. . Retrieved2007-01-08.

[47] Kraan-Korteweg, R. C.; Juraszek, S. (2000). "Mapping the hidden universe: The galaxy distribution in the Zone of Avoidance" (http:/ /adsabs. harvard. edu/ abs/ 1999astro. ph. 10572K). Publications of the Astronomical Society of Australia 17 (1): 6–12. . Retrieved 2008-11-01.

[48] Galaxies to the left side of the Hubble classification scheme are sometimes referred to as "early-type", while those to the right are"late-type".

[49] Barstow, M. A. (2005). "Elliptical Galaxies" (http:/ / www. star. le. ac. uk/ edu/ Elliptical. shtml). Leicester University Physics Department. .Retrieved 2006-06-08.

[50] "Galaxies" (http:/ / curious. astro. cornell. edu/ galaxies. php). Cornell University. 2005-10-20. . Retrieved 2006-08-10.

Galaxy 16

[51] Smith, Gene (2000-03-06). "Galaxies — The Spiral Nebulae" (http:/ / casswww. ucsd. edu/ public/ tutorial/ Galaxies. html). University ofCalifornia, San Diego Center for Astrophysics & Space Sciences. . Retrieved 2006-11-30.

[52] Eskridge, P. B.; Frogel, J. A. (1999). "What is the True Fraction of Barred Spiral Galaxies?" (http:/ / adsabs. harvard. edu/ abs/ 1999Ap&SS. 269. . 427E). Astrophysics and Space Science 269/270: 427–430. doi:10.1023/A:1017025820201. .

[53] Bournaud, F.; Combes, F. (2002). "Gas accretion on spiral galaxies: Bar formation and renewal" (http:/ / adsabs. harvard. edu/ abs/ 2002A&A. . . 392. . . 83B). Astronomy and Astrophysics 392: 83–102. doi:10.1051/0004-6361:20020920. .

[54] Knapen, J. H.; Pérez-Ramírez, D.; Laine, S. (2002). "Circumnuclear regions in barred spiral galaxies — II. Relations to host galaxies" (http:// adsabs. harvard. edu/ abs/ 2002MNRAS. 337. . 808K). Monthly Notices of the Royal Astronomical Society 337 (3): 808–828.doi:10.1046/j.1365-8711.2002.05840.x. . Retrieved 2008-11-01.

[55] Alard, Alard (2001). "Another bar in the Bulge" (http:/ / adsabs. harvard. edu/ abs/ 2001A& A. . . 379L. . 44A). Astronomy and Astrophysics379 (2): L44–L47. doi:10.1051/0004-6361:20011487. . Retrieved 2010-03-05.

[56] Sanders, Robert (2006-01-09). "Milky Way galaxy is warped and vibrating like a drum" (http:/ / www. berkeley. edu/ news/ media/ releases/2006/ 01/ 09_warp. shtml). UCBerkeley News. . Retrieved 2006-05-24.

[57] Bell, G. R.; Levine, S. E. (1997). "Mass of the Milky Way and Dwarf Spheroidal Stream Membership" (http:/ / adsabs. harvard. edu/ abs/1997AAS. . . 19110806B). Bulletin of the American Astronomical Society 29 (2): 1384. . Retrieved 2008-11-01.

[58] Gerber, R. A.; Lamb, S. A.; Balsara, D. S. (1994). "Ring Galaxy Evolution as a Function of "Intruder" Mass" (http:/ / adsabs. harvard. edu/abs/ 1994AAS. . . 184. 3204G). Bulletin of the American Astronomical Society 26: 911. . Retrieved 2008-11-01.

[59] Esa Science News (1998-10-14). "ISO unveils the hidden rings of Andromeda" (http:/ / www. iso. vilspa. esa. es/ outreach/ esa_pr/andromed. htm). Press release. . Retrieved 2006-05-24.

[60] "Spitzer Reveals What Edwin Hubble Missed" (http:/ / www. cfa. harvard. edu/ press/ pr0419. html). Harvard-Smithsonian Center forAstrophysics. 2004-05-31. . Retrieved 2006-12-06.

[61] Barstow, M. A. (2005). "Irregular Galaxies" (http:/ / www. star. le. ac. uk/ edu/ Irregular. shtml). University of Leicester. . Retrieved2006-12-05.

[62] Phillipps, S.; Drinkwater, M. J.; Gregg, M. D.; Jones, J. B. (2001). "Ultracompact Dwarf Galaxies in the Fornax Cluster" (http:/ / adsabs.harvard. edu/ abs/ 2001ApJ. . . 560. . 201P). The Astrophysical Journal 560 (1): 201–206. doi:10.1086/322517. .

[63] Groshong, Kimm (2006-04-24). "Strange satellite galaxies revealed around Milky Way" (http:/ / space. newscientist. com/ article/ dn9043).New Scientist. . Retrieved 2007-01-10.

[64] Schirber, Michael (2008-08-27). "No Slimming Down for Dwarf Galaxies" (http:/ / sciencenow. sciencemag. org/ cgi/ content/ full/ 2008/827/ 3). ScienceNOW Daily News. . Retrieved 2008-08-27.

[65] "Galaxy Interactions" (http:/ / web. archive. org/ web/ 20060509074300/ http:/ / www. astro. umd. edu/ education/ astro/ gal/ interact. html).University of Maryland Department of Astronomy. Archived from the original (http:/ / www. astro. umd. edu/ education/ astro/ gal/ interact.html) on May 9, 2006. . Retrieved 2006-12-19.

[66] "Interacting Galaxies" (http:/ / cosmos. swin. edu. au/ entries/ interactinggalaxies/ interactinggalaxies. html?e=1). Swinburne University. .Retrieved 2006-12-19.

[67] "Happy Sweet Sixteen, Hubble Telescope!" (http:/ / hubblesite. org/ newscenter/ archive/ releases/ 2006/ 14/ image/ a). NASA. 2006-04-24.. Retrieved 2006-08-10.

[68] "Starburst Galaxies" (http:/ / chandra. harvard. edu/ xray_sources/ starburst. html). Harvard-Smithsonian Center for Astrophysics.2006-08-29. . Retrieved 2006-08-10.

[69] Kennicutt Jr., R. C.; Lee, J. C.; Funes, J. G.; Shoko, S.; Akiyama, S. (September 6–10, 2004). "Demographics and Host Galaxies ofStarbursts" (http:/ / adsabs. harvard. edu/ abs/ 2005sdlb. proc. . 187K). . Cambridge, UK: Dordrecht: Springer. p. 187. . Retrieved 2006-12-11.

[70] Smith, Gene (2006-07-13). "Starbursts & Colliding Galaxies" (http:/ / casswww. ucsd. edu/ public/ tutorial/ Starbursts. html). University ofCalifornia, San Diego Center for Astrophysics & Space Sciences. . Retrieved 2006-08-10.

[71] Keel, Bill (September 2006). "Starburst Galaxies" (http:/ / www. astr. ua. edu/ keel/ galaxies/ starburst. html). University of Alabama. .Retrieved 2006-12-11.

[72] Keel, William C. (2000). "Introducing Active Galactic Nuclei" (http:/ / www. astr. ua. edu/ keel/ galaxies/ agnintro. html). The University ofAlabama. . Retrieved 2006-12-06.

[73] Lochner, J.; Gibb, M.. "A Monster in the Middle" (http:/ / imagine. gsfc. nasa. gov/ docs/ science/ know_l2/ active_galaxies. html). NASA. .Retrieved 2006-12-20.

[74] Heckman, T. M. (1980). "An optical and radio survey of the nuclei of bright galaxies — Activity in normal galactic nuclei" (http:/ / adsabs.harvard. edu/ abs/ 1980A& A. . . . 87. . 152H). Astronomy and Astrophysics 87: 152–164. . Retrieved 2008-11-01.

[75] Ho, L. C.; Filippenko, A. V.; Sargent, W. L. W. (1997). "A Search for "Dwarf" Seyfert Nuclei. V. Demographics of Nuclear Activity inNearby Galaxies" (http:/ / adsabs. harvard. edu/ abs/ 1997ApJ. . . 487. . 568H). Astrophysical Journal 487: 568–578. doi:10.1086/304638. .

[76] "Search for Submillimeter Protogalaxies" (http:/ / cfa-www. harvard. edu/ ~aas/ tenmeter/ proto. htm). Harvard-Smithsonian Center forAstrophysics. 1999-11-18. . Retrieved 2007-01-10.

[77] McMahon, R. (2006). "Journey to the birth of the Universe". Nature 443 (7108): 151. doi:10.1038/443151a. PMID 16971933.[78] Eggen, O. J.; Lynden-Bell, D.; Sandage, A. R. (1962). "Evidence from the motions of old stars that the Galaxy collapsed" (http:/ / adsabs.

harvard. edu/ abs/ 1962ApJ. . . 136. . 748E). Reports on Progress in Physics 136: 748. doi:10.1086/147433. . Retrieved 2008-11-01.[79] Searle, L.; Zinn, R. (1978). "Compositions of halo clusters and the formation of the galactic halo" (http:/ / adsabs. harvard. edu/ abs/

1978ApJ. . . 225. . 357S). Astrophysical Journal 225 (1): 357–379. doi:10.1086/156499. .

Galaxy 17

[80] Heger, A.; Woosley, S. E. (2002). "The Nucleosynthetic Signature of Population III" (http:/ / adsabs. harvard. edu/ cgi-bin/nph-bib_query?bibcode=2002ApJ. . . 567. . 532H). Astrophysical Journal 567 (1): 532–543. doi:10.1086/338487. .

[81] Barkana, R.; Loeb, A. (1999). "In the beginning: the first sources of light and the reionization of the universe" (http:/ / adsabs. harvard. edu/abs/ 2000astro. ph. 10468B). Physics Reports 349 (2): 125–238. doi:10.1016/S0370-1573(01)00019-9. .

[82] Villard, R.; Samarrai, F.; Thuan, T.; Ostlin, G. (2004-12-01). "Hubble Uncovers a Baby Galaxy in a Grown-Up Universe" (http:/ /hubblesite. org/ newscenter/ archive/ releases/ 2004/ 35/ text/ ). HubbleSite News Center. . Retrieved 2007-01-11.

[83] Weaver, D.; Villard, R. (2007-10-16). "Hubble Finds 'Dorian Gray' Galaxy" (http:/ / hubblesite. org/ newscenter/ archive/ releases/ 2007/ 35/full/ ). HubbleSite News Center. . Retrieved 2007-10-16.

[84] "Simulations Show How Growing Black Holes Regulate Galaxy Formation" (http:/ / www. cmu. edu/ PR/ releases05/ 050209_blackhole.html). Carnegie Mellon University. 2005-02-09. . Retrieved 2007-01-07.

[85] Massey, Robert (2007-04-21). "Caught in the act; forming galaxies captured in the young universe" (http:/ / www. ras. org. uk/ index.php?option=com_content& task=view& id=1190& Itemid=2). Royal Astronomical Society. . Retrieved 2007-04-20.

[86] Noguchi, Masafumi (1999). "Early Evolution of Disk Galaxies: Formation of Bulges in Clumpy Young Galactic Disks" (http:/ / adsabs.harvard. edu/ abs/ 1999ApJ. . . 514. . . 77N). Astrophysical Journal 514 (1): 77–95. doi:10.1086/306932. . Retrieved 2007-01-16.

[87] Baugh, C.; Frenk, C. (May 1999). "How are galaxies made?" (http:/ / physicsweb. org/ articles/ world/ 12/ 5/ 9). Physics Web. . Retrieved2007-01-16.

[88] Gonzalez, G. (1998). "The Stellar Metallicity — Planet Connection" (http:/ / adsabs. harvard. edu/ abs/ 1998bdep. conf. . 431G). . Puerto dela Cruz, Tenerife, Spain. pp. 431. . Retrieved 2007-01-16.

[89] Conselice, Christopher J. (February 2007). "The Universe's Invisible Hand". Scientific American 296 (2): 35–41.[90] Ford, H. et al. (2002-04-30). "Hubble's New Camera Delivers Breathtaking Views of the Universe" (http:/ / hubblesite. org/ newscenter/

archive/ releases/ 2002/ 11/ image/ d). Hubble News Desk. . Retrieved 2007-05-08.[91] Struck, Curtis (1999). "Galaxy Collisions" (http:/ / xxx. lanl. gov/ html/ astro-ph/ 9908269/ homepage. html). Galaxy Collisions 321. .[92] Wong, Janet (2000-04-14). "Astrophysicist maps out our own galaxy's end" (http:/ / web. archive. org/ web/ 20070108183824/ http:/ / www.

news. utoronto. ca/ bin/ 000414b. asp). University of Toronto. Archived from the original (http:/ / www. news. utoronto. ca/ bin/ 000414b.asp) on January 8, 2007. . Retrieved 2007-01-11.

[93] Panter, Ben; Jimenez, Raul; Heavens, Alan F.; Charlot, Stephane (2007). "The star formation histories of galaxies in the Sloan Digital SkySurvey" (http:/ / arxiv. org/ abs/ astro-ph/ 0608531). Monthly Notices of the Royal Astronomical Society 378 (4): 1550–1564.doi:10.1111/j.1365-2966.2007.11909.x. . Retrieved 2008-06-04.

[94] Kennicutt Jr., R. C.; Tamblyn, P.; Congdon, C. E. (1994). "Past and future star formation in disk galaxies" (http:/ / adsabs. harvard. edu/ abs/1994ApJ. . . 435. . . 22K). Astrophysical Journal 435 (1): 22–36. doi:10.1086/174790. .

[95] Knapp, G. R. (1999). Star Formation in Early Type Galaxies (http:/ / adsabs. harvard. edu/ abs/ 1998astro. ph. . 8266K). San Francisco,Calif.: Astronomical Society of the Pacific. ISBN 1-886733-84-8. OCLC 41302839. .

[96] Adams, Fred; Laughlin, Greg (2006-07-13). "The Great Cosmic Battle" (http:/ / www. astrosociety. org/ pubs/ mercury/ 0001/ cosmic.html). Astronomical Society of the Pacific. . Retrieved 2007-01-16.

[97] Pobojewski, Sally (1997-01-21). "Physics offers glimpse into the dark side of the universe" (http:/ / www. umich. edu/ ~urecord/ 9697/Jan21_97/ artcl17. htm). University of Michigan. . Retrieved 2007-01-13.

[98] The term "field galaxy" is sometimes used to mean an isolated galaxy, although the same term is also used to describe galaxies that do notbelong to a cluster but may be a member of a group of galaxies.

[99] McKee, Maggie (2005-06-07). "Galactic loners produce more stars" (http:/ / www. newscientist. com/ article. ns?id=dn7478). New Scientist.. Retrieved 2007-01-15.

[100] "Groups & Clusters of Galaxies" (http:/ / chandra. harvard. edu/ xray_sources/ galaxy_clusters. html). NASA Chandra. . Retrieved2007-01-15.

[101] Ricker, Paul. "When Galaxy Clusters Collide" (http:/ / www. sdsc. edu/ pub/ envision/ v15. 2/ ricker. html). San Diego SupercomputerCenter. . Retrieved 2008-08-27.

[102] Dahlem, Michael (2006-11-24). "Optical and radio survey of Southern Compact Groups of galaxies" (http:/ / web. archive. org/ web/20070613151936/ http:/ / www. atnf. csiro. au/ people/ mdahlem/ sci/ SCGs. html). University of Birmingham Astrophysics and SpaceResearch Group. Archived from the original (http:/ / www. atnf. csiro. au/ people/ mdahlem/ sci/ SCGs. html) on June 13, 2007. . Retrieved2007-01-15.

[103] Ponman, Trevor (2005-02-25). "Galaxy Systems: Groups" (http:/ / www. sr. bham. ac. uk/ research/ groups. html). University ofBirmingham Astrophysics and Space Research Group. . Retrieved 2007-01-15.

[104] Girardi, M.; Giuricin, G. (2000). "The Observational Mass Function of Loose Galaxy Groups" (http:/ / adsabs. harvard. edu/ abs/ 2000ApJ.. . 540. . . 45G). The Astrophysical Journal 540 (1): 45–56. doi:10.1086/309314. .

[105] Dubinski, John (1998). "The Origin of the Brightest Cluster Galaxies" (http:/ / www. cita. utoronto. ca/ ~dubinski/ bcg/ ). AstrophysicalJournal 502 (2): 141–149. doi:10.1086/305901. .

[106] Bahcall, Neta A. (1988). "Large-scale structure in the universe indicated by galaxy clusters" (http:/ / adsabs. harvard. edu/ abs/ 1988ARA&A. . 26. . 631B). Annual review of astronomy and astrophysics 26: 631–686. doi:10.1146/annurev.aa.26.090188.003215. .

[107] Mandolesi, N.; Calzolari, P.; Cortiglioni, S.; Delpino, F.; Sironi, G. (1986). "Large-scale homogeneity of the Universe measured by themicrowave background" (http:/ / www. nature. com/ nature/ journal/ v319/ n6056/ abs/ 319751a0. html). Letters to Nature 319: 751–753.doi:10.1038/319751a0. .

Galaxy 18

[108] van den Bergh, Sidney (2000). "Updated Information on the Local Group" (http:/ / adsabs. harvard. edu/ abs/ 2000astro. ph. . 1040V). ThePublications of the Astronomical Society of the Pacific 112 (770): 529–536. doi:10.1086/316548. .

[109] Tully, R. B. (1982). "The Local Supercluster" (http:/ / adsabs. harvard. edu/ abs/ 1982ApJ. . . 257. . 389T). Astrophysical Journal 257:389–422. doi:10.1086/159999. .

[110] "Near, Mid & Far Infrared" (http:/ / www. ipac. caltech. edu/ Outreach/ Edu/ Regions/ irregions. html). IPAC/NASA. . Retrieved2007-01-02.

[111] "The Effects of Earth's Upper Atmosphere on Radio Signals" (http:/ / radiojove. gsfc. nasa. gov/ education/ educ/ radio/ tran-rec/ exerc/iono. htm). NASA. . Retrieved 2006-08-10.

[112] "Giant Radio Telescope Imaging Could Make Dark Matter Visible" (http:/ / www. sciencedaily. com/ releases/ 2006/ 12/ 061214135537.htm). ScienceDaily. 2006-12-14. . Retrieved 2007-01-02.

[113] "NASA Telescope Sees Black Hole Munch on a Star" (http:/ / www. nasa. gov/ mission_pages/ galex/ galex-20061205. html). NASA.2006-12-05. . Retrieved 2007-01-02.

[114] Dunn, Robert. "An Introduction to X-ray Astronomy" (http:/ / www-xray. ast. cam. ac. uk/ xray_introduction/ ). Institute of AstronomyX-Ray Group. . Retrieved 2007-01-02.

References

Bibliography• Dickinson, Terence (2004). The Universe and Beyond (4th ed.). Firefly Books Ltd.. ISBN 1552979016.

OCLC 55596414.• James Binney, Michael Merrifield (1998). Galactic Astronomy. Princeton University Press. ISBN 0691004021.

OCLC 39108765.

External links• Galaxies, SEDS Messier pages (http:/ / www. seds. org/ messier/ galaxy. html)• An Atlas of The Universe (http:/ / www. atlasoftheuniverse. com/ )• Galaxies — Information and amateur observations (http:/ / www. nightskyinfo. com/ galaxies)• The Oldest Galaxy Yet Found (http:/ / science. nasa. gov/ headlines/ y2002/ 08feb_gravlens. htm)• Galaxies — discussed on BBC Radio 4's "In Our Time" programme (http:/ / www. bbc. co. uk/ radio4/ history/

inourtime/ inourtime_20060629. shtml)• Galaxy classification project, harnessing the power of the internet and the human brain (http:/ / www. galaxyzoo.

org)• How many galaxies are in our universe? (http:/ / www. physics. org/ facts/ sand-galaxies. asp)

Galaxy formation and evolution 19

Galaxy formation and evolutionThe study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universefrom a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and theprocesses that have generated the variety of structures observed in nearby galaxies. It is one of the most activeresearch areas in astrophysics.Galaxy formation is hypothesized to occur, from structure formation theories, as a result of tiny quantum fluctuationsin the aftermath of the Big Bang. The simplest model for this that is in general agreement with observed phenomenais the Cold Dark Matter cosmology; that is to say that clustering and merging is how galaxies gain in mass, andcan also determine their shape and structure.

Formation of the first galaxiesAfter the Big Bang, the universe, for a time, was remarkably homogeneous, as can be observed in the CosmicMicrowave Background or CMB (the fluctuations of which are less than one part in one hundred thousand). Therewas little-to-no structure in the universe, and thus no galaxies. Thus we must ask how the smoothly distributeduniverse of the CMB became the clumpy universe we see today.The most accepted theory of how these structures came to be is that all the structure we observe today was formed asa consequence of the growth of the primordial fluctuations, which are small changes in the density of the universe ina confined region. As the universe cooled clumps of dark matter began to condense, and within them gas began tocondense. The primordial fluctuations gravitationally attracted gas and dark matter to the denser areas, and thus theseeds that would later become galaxies were formed. These structures constituted the first galaxies. At this point theuniverse was almost exclusively composed of hydrogen, helium, and dark matter. Soon after the first proto-galaxiesformed the hydrogen and helium gas within them began to condense and make the first stars. Thus the first galaxieswere then formed. In 2007 the Keck telescope, a team from California Institute of Technology found six star forminggalaxies about 13.2 billion light years (light travel distance) away and therefore created when the universe was only500 million years old.[1]

The universe was very violent in its early epochs, and galaxies grew quickly, evolving by accretion of smaller massgalaxies. The result of this process is left imprinted on the distribution of galaxies in the nearby universe (see imageof 2dF Galaxy Redshift Survey). Galaxies are not isolated objects in space, but rather galaxies in the universe aredistributed in a great cosmic web of filaments. The locations where the filaments meet are dense clusters of galaxies,that began as the small fluctuations to the universe. Hence the distribution of galaxies is closely related to the physicsof the early universe.Despite its many successes, this picture is not sufficient to explain the variety of structure we see in galaxies.Galaxies come in a variety of shapes, from round featureless elliptical galaxies to the pancake-flat spiral galaxies.

Galaxy formation and evolution 20

Commonly observed properties of galaxies

NGC 891, a very thin disk galaxy.

Hubble tuning fork diagram of galaxy morphology

Some notable observed features of galaxy structure (includingour own Milky Way) that astronomers wish to explain withgalactic formation theories include (but are certainly not limitedto) the following:

• Spiral galaxies and the Galactic disk are quite thin, dense,and rotate very fast. The Milky Way disk is 100 times longerthan it is thick.

• The majority of mass in galaxies is made up of dark matter, asubstance which is not directly observable, and does notinteract through any means except gravity.

• Halo stars are typically much older and have much lowermetallicities (that is to say they are almost exclusivelycomposed of hydrogen and helium) than disk stars.

• Many disk galaxies have a puffed up outer disk (often calledthe "thick disk") that is composed of old stars.

• Globular clusters are typically old and metal-poor as well, butthere are a few which are not nearly as metal-poor as most,and/or have some younger stars. Some stars in globularclusters appear to be as old as the universe itself (by entirelydifferent measurement and analysis methods).

• High Velocity Clouds, clouds of neutral hydrogen are"raining" down on the galaxy, and presumably have beenfrom the beginning (these would be the necessary source of agas disk from which the disk stars formed).

• Galaxies come in a great variety of shapes and sizes (see the Hubble Sequence) from giant featureless blobs of oldstars (called elliptical galaxies) to thin disks with gas and stars arranged in highly ordered spirals.

• The majority of giant galaxies contain a supermassive black hole in their centers, ranging in mass from millions tobillions of times the mass of our sun. The black hole mass is tied to properties of the galaxy that hosts it.

• Many of the properties of galaxies (including the galaxy color-magnitude diagram) indicate that there arefundamentally two types of galaxies. These groups divide into blue-star forming galaxies that are more like spiraltypes, and red-nonstar forming galaxies that are more like elliptical galaxies.

Galaxy formation and evolution 21

The formation of disk galaxies

An image of Messier 101 a prototypical spiral galaxy seenface-on.

A spiral galaxy, ESO 510-G13, was warped as a result ofcolliding with another galaxy. After the other galaxy iscompletely absorbed, the distortion will disappear. Theprocess typically takes millions if not billions of years.

The key properties of disk galaxies, which are also commonlycalled spiral galaxies, is that they are very thin, rotate rapidly,and often show spiral structure. One of the main challenges togalaxy formation is the great number of thin disk galaxies inthe local universe. The problem is that disks are very fragile,and mergers with other galaxies can quickly destroy thindisks.

Olin Eggen, Donald Lynden-Bell, and Allan Sandage[2] in1962, proposed a theory that disk galaxies form through amonolithic collapse of a large gas cloud. As the cloudcollapses the gas settles into a rapidly rotating disk. Known asa top-down formation scenario, this theory is quite simple yetno longer widely accepted because observations of the earlyuniverse strongly suggest that objects grow from bottom-up(i.e. smaller objects merging to form larger ones). It was firstproposed by Leonard Searle and Robert Zinn[3] that galaxiesform by the coalescence of smaller progenitors.

More recent theories include the clustering of dark matterhalos in the bottom-up process. Essentially early on in theuniverse galaxies were composed mostly of gas and darkmatter, and thus, there were fewer stars. As a galaxy gainedmass (by accreting smaller galaxies) the dark matter staysmostly on the outer parts of the galaxy. This is because thedark matter can only interact gravitationally, and thus will notdissipate. The gas, however, can quickly contract, and as itdoes so it rotates faster, until the final result is a very thin, very rapidly rotating disk.Astronomers do not currently know what process stops the contraction. In fact, theories of disk galaxy formation arenot successful at producing the rotation speed and size of disk galaxies. It has been suggested that the radiation frombright newly formed stars, or from an active galactic nuclei can slow the contraction of a forming disk. It has alsobeen suggested that the dark matter halo can pull the galaxy, thus stopping disk contraction.

In recent years, a great deal of focus has been put on understanding merger events in the evolution of galaxies. Ourown galaxy (the Milky Way) has a tiny satellite galaxy (the Sagittarius Dwarf Elliptical Galaxy) which is currentlygradually being ripped up and "eaten" by the Milky Way. It is thought these kinds of events may be quite common inthe evolution of large galaxies. The Sagittarius dwarf galaxy is orbiting our galaxy at almost a right angle to the disk.It is currently passing through the disk; stars are being stripped off of it with each pass and joining the halo of ourgalaxy. There are other examples of these minor accretion events, and it is likely a continual process for manygalaxies. Such mergers provide "new" gas stars and dark matter to galaxies. Evidence for this process is oftenobservable as warps or streams coming out of galaxies.The Lambda-CDM model of galaxy formation underestimates the number of thin disk galaxies in the universe.[4]

The reason is that these galaxy formation models predict a large number of mergers. If disk galaxies merge withanother galaxy of comparable mass (at least 15 percent of its mass) the merger will likely destroy, or at a minimumgreatly disrupt the disk, yet the resulting galaxy is not expected to be a disk galaxy. While this remains an unsolvedproblem for astronomers, it does not necessarily mean that the Lambda-CDM model is completely wrong, but ratherthat it requires further refinement to accurately reproduce the population of galaxies in the universe.

Galaxy formation and evolution 22

Galaxy mergers and the formation of elliptical galaxies

ESO 325-G004, a typical elliptical galaxy.

An image of NGC 4676 (also called the Mice Galaxies) is anexample of a present merger.

The most massive galaxies in the sky are giant ellipticalgalaxies. Their stars are on orbits that are randomly orientedwithin the galaxy (i.e. they are not rotating like disk galaxies).They are composed of old stars and have little to no dust. Allelliptical galaxies probed so far have supermassive blackholes in their center, and the mass of these black holes iscorrelated with the mass of the elliptical galaxy. They are alsocorrelated to a property called sigma which is the speed of thestars at the far edge of the elliptical galaxies. Ellipticalgalaxies do not have disks around them, although some bulgesof disk galaxies look similar to elliptical galaxies. One is morelikely to find elliptical galaxies in more crowded regions ofthe universe (such as galaxy clusters).

Astronomers now see elliptical galaxies as some of the mostevolved systems in the universe. It is widely accepted that themain driving force for the evolution of elliptical galaxies ismergers of smaller galaxies. These mergers can be extremelyviolent; galaxies often collide at speeds of 500 kilometers persecond.Many galaxies in the universe are gravitationally bound toother galaxies, that is to say they will never escape the pull ofthe other galaxy. If the galaxies are of similar size, theresultant galaxy will appear similar to neither of the twogalaxies merging.[5] An image of an ongoing merger of equalsized disk galaxies is shown left. During the merger, stars anddark matter in each galaxy become affected by the

approaching galaxy. Toward the late stages of the merger, the gravitational potential, the shape of galaxy, beginschanging so quickly that star orbits are greatly affected, and lose any memory of their previous orbit. This process iscalled violent relaxation.[6] Thus if two disk galaxies collide, they begin with their stars in an orderly rotation in theplane of the disk. During the merger, the ordered motion is transformed into random energy. And the resultantgalaxy is dominated by stars that orbit the galaxy in a complex, and random, web of orbits. And this is what we seein elliptical galaxies, stars on random unordered orbits.

Galaxy formation and evolution 23

The Antennae Galaxies are a dramatic pair of collidinggalaxies. In such a collision, the stars within eachgalaxy will pass by each other (virtually) without

incident. This is due to the relatively large interstellardistances compared to the relatively small size of anindividual star. Diffuse gas clouds, however, readily

collide to produce shocks which in turn stimulatebursts of star formation. The bright, blue knots indicate

the hot, young stars that have recently ignited as aresult of the merger.

Mergers are also locations of extreme amounts of star formation.[7]

During a merger some galaxies can make thousands of solarmasses of new stars each year, which is large compared to ourgalaxy which makes about 10 new stars each year. Though starsalmost never get close enough to actually collide in galaxymergers, giant molecular clouds rapidly fall to the center of thegalaxy where they collide with other molecular clouds. Thesecollisions then induce condensations of these clouds into newstars. We can see this phenomenon in merging galaxies in thenearby universe. Yet, this process was more pronounced duringthe mergers that formed most elliptical galaxies we see today,which likely occurred 1-10 billion years ago, when there wasmuch more gas (and thus more molecular clouds) in galaxies.Also, away from the center of the galaxy gas clouds will run intoeach other producing shocks which stimulate the formation of newstars in gas clouds. The result of all this violence is that galaxiestend to have little gas available to form new stars after they merge.Thus if a galaxy is involved in a major merger, and then a fewbillion years pass, the galaxy will have very few young stars (seeStellar evolution) left. This is what we see in today's ellipticalgalaxies, very little molecular gas and very few young stars. It isthought that this is because elliptical galaxies are the end productsof major mergers which use up the majority of gas during the merger, and thus further star formation after the mergeris quenched.

In the Local Group, the Milky Way and M31 (the Andromeda Galaxy) are gravitationally bound, and currentlyapproaching each other at high speed. If the two galaxies do meet they will pass through each other, with gravitydistorting both galaxies severely and ejecting some gas, dust and stars into intergalactic space. They will travel apart,slow down, and then again be drawn towards each other, and again collide. Eventually both galaxies will havemerged completely, streams of gas and dust will be flying through the space near the newly formed giant ellipticalgalaxy. M31 is actually already distorted: the edges are warped. This is probably because of interactions with its owngalactic companions, as well as possible mergers with dwarf spheroidal galaxies in the recent past - the remnants ofwhich are still visible in the disk populations.

In our epoch, large concentrations of galaxies (clusters and superclusters) are still assembling.While scientists have learned a great deal about ours and other galaxies, the most fundamental questions aboutformation and evolution remain only tentatively answered.

Galaxy formation and evolution 24

See also• Bulge (astronomy)• Disc (galaxy)• Galactic coordinate system• Galactic corona• Galactic halo• Galaxy rotation problem• Pea galaxy• Zeldovich pancake

References[1] "New Scientist" 14th July 2007[2] Eggen, O.J.; Lynden-Bell, D.; Sandage, A. R. (1962). "Evidence from the motions of old stars that the Galaxy collapsed" (http:/ / adsabs.

harvard. edu/ abs/ 1962ApJ. . . 136. . 748E). The Astrophysical Journal 136: 748. doi:10.1086/147433. .[3] Searle, L.; Zinn, R. (1978). "Compositions of halo clusters and the formation of the galactic halo" (http:/ / adsabs. harvard. edu/ abs/

1978ApJ. . . 225. . 357S). The Astrophysical Journal 225: 357–379. doi:10.1086/156499. .[4] Steinmetz, M.; Navarro, J.F. (2002). "The hierarchical origin of galaxy morphologies" (http:/ / arxiv. org/ abs/ astro-ph/ 0202466v1). New

Astronomy 7 (4): 155–160. doi:10.1016/S1384-1076(02)00102-1. .[5] Barnes,J. Nature, vol. 338, March 9, 1989, p. 123-126[6] van Albada, T. S. 1982 Royal Astronomical Society, Monthly Notices, vol. 201 p.939[7] Schweizer, F. Starbursts: From 30 Doradus to Lyman Break Galaxies, Held in Cambridge, UK, 6–10 September 2004. Edited by R. de Grijs

and R.M. González Delgado. Astrophysics & Space Science Library, Vol. 329. Dordrecht: Springer, 2005, p.143

External links• NOAO gallery of galaxy images (http:/ / www. noao. edu/ image_gallery/ galaxies. html)

• Image of Andromeda galaxy (M31) (http:/ / www. noao. edu/ image_gallery/ html/ im0685. html)• Javascript passive evolution calculator (http:/ / www. astro. yale. edu/ dokkum/ evocalc/ ) for early type

(elliptical) galaxies• Video on the evolution of galaxies by Canadian astrophysicist Doctor P (http:/ / spacegeek. org/ ep4_flash. shtml)

Galaxy merger 25

Galaxy mergerGalaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxyinteraction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distancesbetween stars in most circumstances, the gravitational interactions between galaxies and the friction between the gasand dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety ofparameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active areaof research. There are some generally accepted results, however:• When one of the galaxies is significantly larger than the other, the larger will often "eat" the smaller, absorbing

most of its gas and stars with little other major effect on the larger galaxy. Our home galaxy, the Milky Way, isthought to be currently absorbing smaller galaxies in this fashion, such as the Canis Major Dwarf Galaxy, andpossibly the Magellanic Clouds. The Virgo Stellar Stream is thought to be the remains of a dwarf galaxy that hasbeen mostly merged with the Milky Way.

• If two spiral galaxies that are approximately the same size collide at appropriate angles and speeds, they willlikely merge in a fashion that drives away much of the dust and gas through a variety of feedback mechanismsthat often include a stage in which there are active galactic nuclei. This is thought to be the driving force behindmany quasars. The end result is an elliptical galaxy, and many astronomers hypothesize that this is the primarymechanism that creates ellipticals.

Note that the Milky Way and the Andromeda Galaxy will probably collide in about 4.5 billion years. If thesegalaxies merged, the result would quite possibly be an elliptical galaxy as described above.One of the largest galaxy mergers ever observed consisted of four elliptical galaxies in the cluster CL0958+4702. Itmay form one of the largest galaxies in the Universe.[1]

Galaxy mergers can be simulated in computers, to learn more about galaxy formation. Galaxy pairs initially of anymorphological type can be followed, taking into account all gravitational forces, and also the hydrodynamics anddissipation of the interstellar gas, the star formation out of the gas, and the energy and mass released back in theinterstellar medium by supernovae. Such a library of galaxy merger simulations can be found on the GALMERwebsite [2]

ExamplesSome galaxies that are suspected to be in the process of merging:• Antennae Galaxies• Mice Galaxies• Centaurus A• NGC 7318

Galaxy merger 26

See also• Andromeda-Milky Way collision• Bulge (astronomy)• Galaxy formation and evolution• Interacting galaxies• Pea galaxy

References[1] "Galaxies clash in four-way merger" (http:/ / news. bbc. co. uk/ 1/ hi/ sci/ tech/ 6933566. stm). BBC News. August 6, 2007. . Retrieved

2007-08-07.[2] Galaxy merger library (http:/ / galmer. obspm. fr), March 27, 2010, , retrieved 2010-03-27

External links• "Andromeda involved in galactic collision" (http:/ / www. msnbc. msn. com/ id/ 16872449/ )• "GALMER: Galaxy Merger Simulations" (http:/ / galmer. obspm. fr)

Galaxy morphological classification

Artist's concept illustrating bulge & no bulgespiral galaxies.

Galaxy morphological classification is a system used by astronomersto divide galaxies into groups based on their visual appearance. Thereare several schemes in use by which galaxies can be classifiedaccording to their morphologies, the most famous being the Hubblesequence, devised by Edwin Hubble and later expanded by Gérard deVaucouleurs and Allan Sandage.

Hubble sequence

The Hubble sequence is a morphological classification scheme forgalaxies invented by Edwin Hubble in 1936.[1] It is often knowncolloquially as the “Hubble tuning-fork” because of the shape in whichit is traditionally represented. Hubble’s scheme divides galaxies into 3 broad classes based on their visual appearance(originally on photographic plates):

Galaxy morphological classification 27

Tuning-fork style diagram of the Hubble sequence

• Elliptical galaxies have smooth,featureless light distributions and appearas ellipses in images. They are denotedby the letter E, followed by an integer representing their degree of ellipticity onthe sky.

• Spiral galaxies consist of a flatteneddisk, with stars forming a (usuallytwo-armed) spiral structure, and a centralconcentration of stars known as thebulge, which is similar in appearance toan elliptical galaxy. They are given thesymbol S. Roughly half of all spirals arealso observed to have a bar-like structure,extending from the central bulge. Thesebarred spirals are given the symbol SB.

• Lenticular galaxies (designated S0) alsoconsist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, thedisks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significantquantity.

These broad classes can be extended to enable finer distinctions of appearance and to encompass other types ofgalaxy, such as irregular galaxies, which have no obvious regular structure (either disk-like or ellipsoidal).The Hubble sequence is often represented in the form of a two-pronged fork, with the ellipticals on the left (with thedegree of ellipticity increasing from left to right) and the barred and unbarred spirals forming the two parallel prongsof the fork. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two prongsmeet the “handle”.To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professionalastronomical research and in amateur astronomy.

Galaxy morphological classification 28

De Vaucouleurs system

NGC 6782: a spiral galaxy (type SB(r)0/a) withthree rings of different radii, as well as a bar.

The de Vaucouleurs system for classifying galaxies is awidely used extension to the Hubble sequence, firstdescribed by Gérard de Vaucouleurs in 1959.[2] DeVaucouleurs argued that Hubble's two-dimensionalclassification of spiral galaxies—based on the tightnessof the spiral arms and the presence or absence of abar—did not adequately describe the full range ofobserved galaxy morphologies. In particular, he arguedthat rings and lenses were important structuralcomponents of spiral galaxies.[3]

The de Vaucouleurs system retains Hubble’s basicdivision of galaxies into ellipticals, lenticulars, spiralsand irregulars. To complement Hubble’s scheme, deVaucouleurs introduced a more elaborate classificationsystem for spiral galaxies, based on threemorphological characteristics:

• Bars. Galaxies are divided on the basis of thepresence or absence of a nuclear bar. DeVaucouleurs introduced the notation SA to denotespiral galaxies without bars, complementingHubble’s use of SB for barred spirals. He alsoallowed for an intermediate class, denoted SAB,containing weakly barred spirals. Lenticular galaxiesare also classified as unbarred (SA0) or barred(SB0), with the notation S0 reserved for thosegalaxies for which it is impossible to tell if a bar ispresent or not (usually because they are edge-on tothe line-of-sight).

• Rings. Galaxies are divided into those possessingring-like structures (denoted ‘(r)’) and those withoutrings (denoted ‘(s)’). So-called ‘transition’ galaxiesare given the symbol (rs).

• Spiral arms. As in Hubble’s original scheme, spiral galaxies are assigned to a class based primarily on thetightness of their spiral arms. The de Vaucouleurs scheme extends the arms of Hubble’s tuning fork to includeseveral additional spiral classes:

• Sd (SBd) - diffuse, broken arms made up of individual stellar clusters and nebulae; very faint central bulge• Sm (SBm) - irregular in appearance; no bulge component• Im - highly irregular galaxy

Galaxy morphological classification 29

NGC 7793: a spiral galaxy of type SA(s)d.

The Large Magellanic Cloud: a type SBm galaxy.

Most galaxies in these three classes were classified as Irr I inHubble’s original scheme. In addition, the Sd class contains somegalaxies from Hubble’s Sc class. Galaxies in the classes Sm and Imare termed the “Magellanic” spirals and irregulars, respectively,after the Magellanic Clouds. The Large Magellanic Cloud is of typeSBm, while the Small Magellanic Cloud is an irregular (Im).

The different elements of the classification scheme are combined - inthe order in which they are listed - to give the complete classificationof a galaxy. For example, a weakly-barred spiral galaxy withloosely-wound arms and a ring is denoted SAB(r)c.Visually, the de Vaucouleurs system can be represented as athree-dimensional version [4] of Hubble’s tuning fork, with stage(spiralness) on the x-axis, family (barredness) on the y-axis, andvariety (ringedness) on the z-axis.[5]

Numerical Hubble stage

De Vaucouleurs also assigned numerical values to each class of galaxyin his scheme. Values of the numerical Hubble stage T run from -6 to+10, with negative numbers corresponding to early-type galaxies(ellipticals and lenticulars) and positive numbers to late types (spiralsand irregulars). Elliptical galaxies are divided into three 'stages':compact ellipticals (cE), normal ellipticals (E) and transition types(E+). Lenticulars are similarly subdivided into early (S-), intermediate(S0) and late (S+) types.

Numerical Hubble stage

Hubble stage -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10

de Vaucouleurs class[5] cE E E+ S0- S00 S0+ S0/a Sa Sab Sb Sbc Sc Scd Sd Sdm Sm Im

approximate Hubble class[6] E S0 S0/a Sa Sa-b Sb Sb-c Sc Sc-Irr Irr I

The use of numerical stages allows for more quantitative studies of galaxy morphology.

The Yerkes (or Morgan) schemeCreated by American astronomer William Wilson Morgan. Together with Philip Keenan, Morgan developed the MKsystem for the classification of stars through their spectra. The Yerkes scheme uses the spectra of stars in the galaxy;the shape, real and apparent; and the degree of the central concentration to classify galaxies.

Galaxy morphological classification 30

Spectral Type Explanation

a Prominent A stars

af Prominent A-F stars

f Prominent F stars

fg Prominent F-G stars

g Prominent G stars

gk Prominent G-K stars

k Prominent K stars

Galactic Shape Explanation

B Barred spiral

D Rotational symmetry without pronounced spiral or elliptical structure

E Elliptical

Ep Elliptical with dust absorption

I Irregular

L Low surface brightness

N small bright nucleus

S Spiral

Inclination Explanation

1 Galaxy is "Face-on"

2

3

4

5

6

7 Galaxy is "Edge-on"

So, for example, the Andromeda Galaxy is classified as kS5.

See also• Morphological Catalogue of Galaxies• Galaxy color-magnitude diagram• Galaxy Zoo• William Wilson Morgan• Fritz Zwicky

Galaxy morphological classification 31

References[1] Hubble, E. P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. ISBN 36018182.[2] De Vaucouleurs, G. (1959). "Classification and Morphology of External Galaxies". Handbuch der Physik 53: 275.[3] Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 9780691025650.[4] http:/ / www. astr. ua. edu/ keel/ galaxies/ classify. html[5] De Vaucouleurs, G. (1994) (PostScript). Global Physical Parameters of Galaxies (http:/ / www. stsci. edu/ institute/ conference/

galaxy-morphology/ devaucouleurs. ps). . Retrieved 2008-01-02.[6] Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 9780691025650.

External links• Galaxies and the Universe (http:/ / www. astr. ua. edu/ keel/ galaxies/ classify. html) - an introduction to galaxy

classification• Near-Infrared Galaxy Morphology Atlas (http:/ / www. ipac. caltech. edu/ 2mass/ gallery/ galmorph/ ), T.H.

Jarrett• The Spitzer Infrared Nearby Galaxies Survey (SINGS) Hubble Tuning-Fork (http:/ / sings. stsci. edu/

Publications/ sings_poster. html), SINGS (http:/ / sings. stsci. edu/ ) Spitzer Space Telescope Legacy ScienceProject

• Go to GalaxyZoo.org (http:/ / www. galaxyzoo. org) to try your hand at classifying galaxies as part of an OxfordUniversity open community project

Hubble sequenceThe Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926.[1]

[2] [3] [4] It is often known colloquially as the Hubble tuning-fork diagram because of the shape in which it istraditionally represented.

Tuning-fork style diagram of the Hubble sequence

Hubble’s scheme divides regulargalaxies into 3 broad classes -ellipticals, lenticulars and spirals -based on their visual appearance(originally on photographic plates). Afourth class contains galaxies with anirregular appearance. To this day, theHubble sequence is the mostcommonly used system for classifyinggalaxies, both in professionalastronomical research and in amateurastronomy.

Classes of galaxies

Hubble sequence 32

Ellipticals

The giant elliptical galaxy ESO 325-G004.

On the left (in the sense that the sequence is usually drawn) lie the ellipticals. Elliptical galaxies have smooth,featureless light distributions and appear as ellipses in photographic images. They are denoted by the letter E,followed by an integer representing their degree of ellipticity on the sky. By convention, is ten times theellipticity of the galaxy, rounded to the nearest integer, where the ellipticity is defined as for an ellipsewith semi-major and semi-minor axes of lengths and respectively.[5] The ellipticity increases from left to righton the Hubble diagram, with near-circular (E0) galaxies situated on the very left of the diagram. It is important tonote that the ellipticity of a galaxy on the sky is only indirectly related to the true 3-dimensional shape (for example,a flattened, discus-shaped galaxy can appear almost round if viewed face-on or elliptical if viewed at an angle).Observationally, the most flattened elliptical galaxies have ellipticities e=0.7 (denoted E7). This is consistent withtheir being truly ellipsoidal structures rather than disks viewed at a range of angles.Examples of elliptical galaxies: M49, M59, M60, M87, NGC 4125.

Spirals

The Pinwheel Galaxy (Messier 101/NGC 5457):a spiral galaxy classified as type Scd on the

Hubble sequence

On the right of the Hubble sequence diagram are two parallel branchesencompassing the spiral galaxies. A spiral galaxy consists of aflattened disk, with stars forming a (usually two-armed) spiralstructure, and a central concentration of stars known as the bulge.Roughly half of all spirals are also observed to have a bar-likestructure, extending from the central bulge, at the ends of which thespiral arms begin. In the tuning-fork diagram, the regular spiralsoccupy the upper branch and are denoted by the letter S, while thelower branch contains the barred spirals, given the symbol SB. Bothtype of spirals are further subdivided according to the detailedappearance of their spiral structures. Membership of one of thesesubdivisions is indicated by adding a lower-case letter to themorphological type, as follows:

• Sa (SBa) - tightly-wound, smooth arms; large, bright central bulge• Sb (SBb) - less tightly-wound spiral arms than Sa (SBa); somewhat fainter bulge

Hubble sequence 33

The barred spiral galaxy NGC 1300: a type SBbc

• Sc (SBc) - loosely wound spiral arms, clearly resolved intoindividual stellar clusters and nebulae; smaller, fainter bulge

Hubble originally described three classes of spiral galaxy. This wasextended by de Vaucouleurs[6] to include a fourth class:

• Sd (SBd) - very loosely-wound, fragmentary arms; most of theluminosity is in the arms and not the bulge

Although strictly part of the de Vaucouleurs system of classification,the Sd class is often included in the Hubble sequence. The basic spiraltypes can be extended to enable finer distinctions of appearance. For example, spiral galaxies whose appearance isintermediate between two of the above classes are often identified by appending 2 lower-case letters to the maingalaxy type (for example Sbc for a galaxy that is intermediate between an Sb and an Sc).

Our own Milky Way is generally classed as SBb, making it a barred spiral with well-defined arms. However, thisclassification is somewhat uncertain since we can only infer how our galaxy would appear to an outside observer.Examples of regular spiral galaxies: M31 (Andromeda Galaxy), M74, M81, M104 (Sombrero Galaxy), M51a(Whirlpool Galaxy), NGC 300, NGC 772.Examples of barred spiral galaxies: M91, M95, NGC 1097, NGC 1300, NGC1672, NGC 2536, NGC 2903.

Lenticulars

The Spindle Galaxy (NGC 5866), a lenticulargalaxy with a prominent dust lane in the

constellation of Draco.

At the centre of the Hubble tuning fork, where the two spiral armsmeet the elliptical branch lies an intermediate class of galaxies knownas lenticulars and given the symbol S0. These galaxies consist of abright central bulge, similar in appearance to an elliptical galaxy,surrounded by an extended, disk-like structure. Unlike spiral galaxies,the disks of lenticular galaxies have no visible spiral structure and arenot actively forming stars in any significant quantity. The bulgecomponent is often the dominant source of light in a lenticulargalaxy.[7]

Face-on lenticulars are difficult to distinguish from ellipticals of typeE0, making the classification of many such galaxies uncertain. Whenviewed edge-on, prominent dust-lanes are sometimes visible inabsorption against the light of stars in the disk.

At the time of the initial publication of Hubble's galaxy classification scheme, the existence of lenticular galaxieswas purely hypothetical. Hubble believed that they were necessary as an intermediate stage between thehighly-flattened ellipticals and spirals. Later observations (by Hubble himself, among others) showed Hubble's beliefto be correct and the S0 class was included in the definitive exposition of the Hubble sequence by Allan Sandage.[8]

Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies.Examples of lenticular galaxies: M85, M86, NGC 1316, NGC 2787, NGC 5866 (Spindle Galaxy), Centaurus A.

Hubble sequence 34

Irregulars

The Large Magellanic Cloud (LMC) - a dwarfirregular galaxy

Galaxies that do not fit into the Hubble sequence, because they have noregular structure (either disk-like or ellipsoidal), are termed irregulargalaxies. Hubble defined two classes of irregular galaxy:[9]

• Irr I galaxies have asymmetric profiles and lack a central bulge orobvious spiral structure; instead they contain many individualclusters of young stars

• Irr II galaxies have smoother, asymmetric appearances and are notclearly resolved into individual stars or stellar clusters

In his extension to the Hubble sequence, de Vaucouleurs called the Irr Igalaxies 'Magellanic irregulars', after the Magellanic Clouds - twosatellites of the Milky Way which Hubble classified as Irr I. The discovery of a faint spiral structure[10] in the LargeMagellanic Cloud led de Vaucouleurs to further divide the irregular galaxies into those that, like the LMC, showsome evidence for spiral structure (these are given the symbol Sm) and those that have no obvious structure, such asthe Small Magellanic Cloud (denoted Im). In the extended Hubble sequence, the Magellanic irregulars are usuallyplaced at the end of the spiral branch of the Hubble tuning fork.

Examples of irregular galaxies: M82, NGC 1427A, Large Magellanic Cloud, Small Magellanic Cloud.

Physical significanceElliptical and lenticular galaxies are commonly referred to together as “early-type” galaxies, while spirals andirregular galaxies are referred to as “late types”. This nomenclature is the source of the common,[11] but erroneous,belief that the Hubble sequence was intended to reflect a supposed evolutionary sequence, from elliptical galaxiesthrough lenticulars to either barred or regular spirals. In fact, Hubble was clear from the beginning that no suchinterpretation was implied:

The nomenclature, it is emphasized, refers to position in the sequence, and temporal connotations aremade at one's peril. The entire classification is purely empirical and without prejudice to theories ofevolution...[3]

The evolutionary picture appears to be lent weight by the fact that the disks of spiral galaxies are observed to behome to many young stars and regions of active star formation, while elliptical galaxies are composed ofpredominantly old stellar populations. In fact, current evidence suggests the opposite: the early Universe appears tobe dominated by spiral and irregular galaxies. In the currently favored picture of galaxy formation, present-dayellipticals formed as a result of mergers between these earlier building blocks. Lenticular galaxies may also beevolved spiral galaxies, whose gas has been stripped away leaving no fuel for continued star formation.

ShortcomingsA common criticism of the Hubble scheme is that the criteria for assigning galaxies to classes are subjective, leading to different observers assigning galaxies to different classes (although experienced observers usually agree to within less than a single Hubble type [12] ). The different classification criteria can also be at odds with each other: for example, a more dominant bulge component does not always go hand-in-hand with more loosely-wound spiral arms. Another criticism of the Hubble classification scheme is that, being based on the appearance of a galaxy in a two-dimensional image, the classes are only indirectly related to the true physical properties of galaxies. In particular, problems arise because of orientation effects (the same galaxy looks very different when viewed edge-on, as opposed to face-on), because visual classifications are less reliable for faint or distant galaxies, and because the appearance of galaxies changes depending on the wavelength of light in which they are observed. Nevertheless, the

Hubble sequence 35

Hubble sequence is still commonly used in the field of extragalactic astronomy and Hubble types are known tocorrelate with many physically relevant properties of galaxies, such as luminosities, colours, masses (of stars andgas) and star formation rates.[13]

See also• Edwin Hubble• Gérard de Vaucouleurs• Galaxy color-magnitude diagram• Galaxy morphological classification

References[1] Hubble, E. P. (1926). "Extra-galactic nebulae". Contributions from the Mount Wilson Observatory / Carnegie Institution of Washington 324:

1–49.[2] Hubble, E. P. (1926). "Extra-galactic nebulae". Astrophysical Journal 64: 321–369.[3] Hubble, E. P. (1927). "The Classification of Spiral Nebulae". The Observatory 50: 276.[4] Hubble, E. P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. ISBN 36018182.[5] Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 9780691025650.[6] de Vaucouleurs, G.; Oemler, Augustus, Jr.; Butcher, Harvey R.; Gunn, James E. (1959). "Classification and Morphology of External

Galaxies". Handbuch der Physik 53: 275. doi:10.1086/174386.[7] Graham, A.; Worley, C. (August 2008). "Inclination- and dust-corrected galaxy parameters: bulge-to-disc ratios and size-luminosity

relations" (http:/ / adsabs. harvard. edu/ abs/ 2008MNRAS. 388. 1708G). Monthly Notices of the Royal Astronomical Society 388: 1708–1728.doi:10.1111/j.1365-2966.2008.13506.x. . Retrieved 2008-10-23.

[8] Sandage, A. (1975). "Classification and Stellar Content of Galaxies Obtained from Direct Photography" (http:/ / nedwww. ipac. caltech. edu/level5/ Sandage/ frames. html). In A. Sandage. . M. Sandage and J. Kristian. . Retrieved 2007-11-20.

[9] Longair, M. S. (1998). Galaxy Formation. New York: Springer. ISBN 3540637850.[10] de Vaucouleurs, G.; Oemler, Augustus, Jr.; Butcher, Harvey R.; Gunn, James E. (1955). "Studies of Magellanic Clouds. I. Dimensions and

structure of the Large Cloud" (http:/ / articles. adsabs. harvard. edu/ full/ 1955AJ. . . . . 60. . 126D). The Astronomical Journal 160: 126–140.doi:10.1086/174386. . Retrieved 2007-11-18.

[11] Baldry, I. K. (2008). "Hubble's Galaxy Nomenclature". Astronomy & Geophysics 49: 5.25.[12] Dressler, A.; Oemler, A., Jr.; Butcher, H. R.; Gunn, J.E. (July 1994). "The morphology of distant cluster galaxies. 1: HST observations of

CL 0939+4713" (http:/ / articles. adsabs. harvard. edu/ full/ 1994ApJ. . . 430. . 107D). The Astrophysical Journal 430 (1): 107–120.doi:10.1086/174386. . Retrieved 2007-09-15.

[13] Roberts, M. S.; Haynes, M. P. (1994). "Physical Parameters along the Hubble Sequence" (http:/ / articles. adsabs. harvard. edu/ full/1994ARA& A. . 32. . 115R). Annual Reviews of Astronomy & Astrophysics 32: 115–152. doi:10.1146/annurev.aa.32.090194.000555. .Retrieved 2007-09-15.

External links• Galaxies and the Universe (http:/ / www. astr. ua. edu/ keel/ galaxies/ classify. html) - an introduction to galaxy

classification• Near-Infrared Galaxy Morphology Atlas (http:/ / www. ipac. caltech. edu/ 2mass/ gallery/ galmorph/ ), T.H.

Jarrett• The Spitzer Infrared Nearby Galaxies Survey (SINGS) Hubble Tuning-Fork (http:/ / sings. stsci. edu/

Publications/ sings_poster. html), SINGS (http:/ / sings. stsci. edu/ ) Spitzer Space Telescope Legacy ScienceProject

• Galaxy Zoo (http:/ / www. galaxyzoo. org/ ) - Galaxy classification participation project.

36

Short-scale structure

Dark matter halo

Simulated dark matter halo from a cosmological N-bodysimulation

A dark matter halo is a hypothetical component of agalaxy, which extends beyond the edge of the visiblegalaxy and dominates the total mass. Since they consist ofdark matter, haloes cannot be observed directly, but theirexistence is inferred through their effects on the motionsof stars and gas in galaxies. Dark matter halos play a keyrole in current models of galaxy formation and evolution.

Galaxy rotation curve for the Milky Way. Vertical axis is speed of rotation aboutthe galactic center. Horizontal axis is distance from the galactic center. The sun ismarked with a yellow ball. The observed curve of speed of rotation is blue. The

predicted curve based upon stellar mass and gas in the Milky Way is red. Scatter inobservations roughly indicated by gray bars. The difference is due to dark matter or

perhaps a modification of the law of gravity.[1] [2] [3]

Rotation curves as evidenceof a dark matter halo

The presence of dark matter in the halo isdemonstrated by its gravitational effect on aspiral galaxy's rotation curve. Without largeamounts of mass in the extended halo, therotational velocity of the galaxy shoulddecrease at large distance from the galacticcore. However, observations of spiralgalaxies, particularly radio observations ofline emission from neutral atomic hydrogen(known, in astronomical parlance, as HI),show that the rotation curve of most spiralgalaxies remains flat far beyond the visiblematter. The absence of any visible matter toaccount for these observations implies thepresence of unobserved (i.e. dark) matter.Asserting that this dark matter does not existwould mean that the accepted theory of gravitation (General Relativity) is incomplete, and while that could be

Dark matter halo 37

possible, most scientists would require extensive amounts of compelling evidence before considering it. This isbecause if standard model calculations do not match observations, then the burden of proof is not on the proponentsof the model, but on the critics.The Navarro-Frenk-White profile:[4]

is often used to model the distribution of mass in dark matter halos. Theoretical dark matter halos produced incomputer simulations are best described by the Einasto profile:[5]

Theories about the nature of dark matterThe nature of dark matter in the galactic halo of spiral galaxies is still undetermined, but there are two populartheories: either the halo is composed of weakly-interacting elementary particles known as WIMPs, or it is home tolarge numbers of small, dark bodies known as MACHOs. It seems unlikely that the halo is composed of largequantities of gas and dust, because both ought to be detectable through observations. Searches for gravitationalmicrolensing events in the halo of the Milky Way show that the number of MACHOs is likely not sufficient toaccount for the required mass.

Milky Way dark matter haloThe dark matter halo is the single largest part of the Galaxy as it covers the space between 100,000 light-years to300,000 light-years from the galactic center. It is also the most mysterious part of the Galaxy. It is now believed thatabout 95% of the Galaxy is composed of dark matter, a type of matter that does not seem to interact with the rest ofthe Galaxy's matter and energy in any way except through gravity. The dark matter halo is the location of nearly allof the Galaxy's dark matter, which is more than ten times as much mass as all of the visible stars, gas, and dust in therest of the Galaxy. The luminous matter makes up approximately 90,000,000,000 (9 x 1010) solar masses. The darkmatter halo is likely to include around 600,000,000,000 (6 x 1011) to 3,000,000,000,000 (3 x 1012) solar masses ofdark matter.[6]

See also• Galaxy formation and evolution• Galactic coordinate system• Disc (galaxy)• Bulge (astronomy)• Galactic halo• Spiral arm• Dark matter• Dark galaxy

Dark matter halo 38

References[1] Peter Schneider (2006). Extragalactic Astronomy and Cosmology (http:/ / books. google. com/ books?id=uP1Hz-6sHaMC& pg=PA100&

dq=rotation+ Milky+ way& lr=& as_brr=0& as_pt=ALLTYPES#PPA5,M1). Springer. p. 4, Figure 1.4. ISBN 3540331743. .[2] Theo Koupelis, Karl F Kuhn (2007). In Quest of the Universe (http:/ / books. google. com/ books?id=6rTttN4ZdyoC& pg=PA491&

dq=Milky+ Way+ "rotation+ curve"& lr=& as_brr=0& as_pt=ALLTYPES#PPA492,M1). Jones & Bartlett Publishers. p. 492; Figure 16-13.ISBN 0763743879. .

[3] Mark H. Jones, Robert J. Lambourne, David John Adams (2004). An Introduction to Galaxies and Cosmology (http:/ / books. google. com/books?id=36K1PfetZegC& pg=PA20& dq=Milky+ Way+ "rotation+ curve"& lr=& as_brr=0& as_pt=ALLTYPES#PPA21,M1). CambridgeUniversity Press. p. 21; Figure 1.13. ISBN 0521546230. .

[4] Navarro, J. et al. (1997), A Universal Density Profile from Hierarchical Clustering (http:/ / adsabs. harvard. edu/ abs/ 1997ApJ. . . 490. .493N)

[5] Merritt, D. et al. (2006), Empirical Models for Dark Matter Halos. I. Nonparametric Construction of Density Profiles and Comparison withParametric Models (http:/ / adsabs. harvard. edu/ abs/ 2006AJ. . . . 132. 2685M)

[6] Battaglia et al. (2005, The radial velocity dispersion profile of the Galactic halo: constraining the density profile of the dark halo of the MilkyWay (http:/ / adsabs. harvard. edu/ abs/ 2005MNRAS. 364. . 433B)

Further reading• Bertone, Gianfranco (2010). Particle Dark Matter: Observations, Models and Searches. Cambridge University

Press. pp. 762. ISBN 13: 9780521763684.

External links• Rare Blob Unveiled: Evidence For Hydrogen Gas Falling Onto A Dark Matter Clump? (http:/ / www.

sciencedaily. com/ releases/ 2006/ 07/ 060703163148. htm) European Southern Observatory (ScienceDaily) July3, 2006

• Dark Matter Search Experiment , PICASSO Experiment (http:/ / www. picassoexperiment. ca/ )

Galactic bulge 39

Galactic bulgeIn astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refersto the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxiesthat happen to have a disk of stars around them. Yet, high resolution images, using Hubble Space Telescope revealthat many bulges have properties that are more like spiral galaxies. It is now thought that there are at least two typesof bulges, bulges that are like ellipticals and bulges that are like spiral galaxies.

Classical Bulges

An image of Messier 81, a galaxy with a classicalbulge. Notice that the spiral structure ends at the onset

of the bulge.

Bulges that have properties similar to elliptical galaxies[1] areoften called classical bulges due to their similarity to the historicview of bulges. These bulges are composed primarily of stars thatare older Population II, and hence redder (see stellar evolution).They are also in orbits that are essentially random compared to theplane of the galaxy, whence the round shape arises. Furthermore,they have very little dust and gas compared to the disk portion ofthe galaxy, explaining why there are so few young stars (that is,there is little material left from which to form stars). Thedistribution of light is well described by de Vaucouleurs' law. Atright, we show an example of a galaxy that harbors a bulge withproperties similar to an elliptical galaxy, Messier object 81. Noticethat the bulge is devoid of spiral structure, and the blue stars (indicating younger stars) are mainly in the outer disksurrounding the bulge.

It is this set of properties, that leads many astronomers to conclude that classical bulges are a product of the galacticmerging process. It is thought that classical bulges are the result of the coalescences of smaller structures. This is aviolent process, and thus disrupts the path of the stars, result in the randomness of bulge orbits. Also during themerger, gas clouds are more likely to be converted into stars, due to the shocks from the mergers. Thus the majorityof the gas is converted into stars. A bulge may be the end result of many mergers. This process is more likely in thedistant past, when the mergers were more common. Thus most classical bulges are old today, and have not evolvedsignificantly in the past 10 billion years. Then the remaining gas and stars, that did not participate in the merger,could settle around the bulge, thus making the outer disk.

Galactic bulge 40

Disk-like Bulges

An image of Messier 63, a galaxy with anon-classical bulge. Notice that the spiral

structure goes all the way to the center of thegalaxy.

A Hubble Space Telescope image of the central regionof NGC 4314, a galaxy with a star-forming nuclear

ring.

Many bulges have properties more similar to spiral galaxies thanelliptical galaxies.[2] [3] [4] They are often referred to aspseudobulges or disky-bulges. It was first discovered that the starsin some bulges orbit around the galaxy like disk stars. Thesebulges have stars that are not orbiting randomly, but rather orbit inan ordered fashion in the same plane as the outer disk. This is verydifferent than elliptical galaxies.

Subsequent studies (using the Hubble Space Telescope) show thatbulges of many galaxies are not devoid of dust, but rather show avaried and complex structure. This structure often looks similar toa spiral galaxy, but is much smaller. Giant spiral galaxies aretypically 2–100 times the size of those spirals that exist in bulges.When they exist these central spirals dominate the light of thebulge in which they reside. Many bulges also have young stars andongoing star formation, this is not a feature commonly found inelliptical galaxies. Typically the rate at which new stars are formedin pseudobulges is similar to the rates at which stars form in diskgalaxies. Sometimes bulges contain nuclear rings that are formingstars at much higher rates than (per area) is typically found inouter disks, as shown in NGC 4314 (right).

These properties (such as spiral structure and young stars) suggestthat some bulges did not form through the same process that madeelliptical galaxies and classical bulges. Yet the theories for theformation of pseudobulges is less certain than those of classicalbulges. Pseudobulges may be the result of extremely gas-richmergers than happened more recently than those mergers thatformed classical bulges (within the last 5 billion years). However,it is difficult for disks to survive the merging process, castingdoubt on this scenario.Many astronomers suggest that bulges that appear similar to disksform internally out of the disk, and are not the product of the merging process. When left alone disks galaxies canrearrange their stars and gas (as a response to instabilities). The products of this process (called secular evolution) areoften observed in disk galaxies; both spiral disks and galactic bars are can result from secular evolution of galaxydisks. Secular evolution is also expected to send gas and stars to the center of a galaxy. If this happens that wouldincrease the density at the center of a galaxy, and thus make a bulge that has properties similar to disk galaxies.

If secular evolution is responsible for the formation of a significant number of bulges, then that many galaxies havenot experienced a merger since the formation of their disk. This would then mean that current theories of galaxyformation and evolution greatly over-predict the number of mergers in the past few billion years.Most bulges are thought to host a supermassive black hole at their center. Such black holes by definition can not be observed (light cannot escape them), but various pieces of evidence strongly suggest their existence, both in the bulges of spiral galaxies and in the centers of ellipticals. The masses of the black holes correlate tightly with bulge properties; the tightest such correlation, the M-sigma relation, is between black hole mass and the velocity dispersion of stars in the bulge.[5] Until recently it was thought that one could not have a supermassive black hole without a bulge around it, but galaxies hosting supermassive black holes without accompanying bulges have now been

Galactic bulge 41

observed.[6]

See also• Galaxy formation and evolution• Galactic coordinate system• Disc galaxy

• Spiral arm• Galactic halo• Galactic spheroid• Galactic corona• M-sigma relation

References[1] Sandage, Allan "The Hubble Atlas of Galaxies" Washington: Carnegie Institution, 1961[2] The formation of galactic bulges edited by C.M. Carollo, H.C. Ferguson, R.F.G. Wyse. Cambridge, U.K. ; New York : Cambridge University

Press, 1999. (Cambridge contemporary astrophysics)[3] Kormendy, J. & Kennicutt, R.C. Annual Review of Astronomy and Astrophysics, vol. 42, Issue 1, pp.603-683[4] Athanassoula, E. (2005) MNRAS 358 p1477[5] Ferrarese, F. and Merritt, D. (2000), A Fundamental Relation between Supermassive Black Holes and Their Host Galaxies (http:/ / adsabs.

harvard. edu/ abs/ 2000ApJ. . . 539L. . . 9F)[6] SPACE.com - Even Thin Galaxies Pack Hefty Black Holes (http:/ / www. space. com/ scienceastronomy/ 080110-aas-fat-black-holes. html)

The Galactic Bulge: A Review (http:/ / arxiv. org/ abs/ 0710. 3104)

Galactic coronaThe terms galactic corona and gaseous corona have been used in the first decade of the twenty-first century todescribe a hot, ionised, gaseous component in the Galactic halo of the Milky Way. A similar body of very hot andtenuous gas in the halo of any spiral galaxy may also be described by these terms.This coronal gas may be sustained by the galactic fountain, in which superbubbles of ionised gas from supernovaremnants expand vertically through galactic chimneys into the halo. As the gas cools, it is pulled back into thegalactic disc of the galaxy by gravitational forces.

See also• Galaxy formation and evolution• Galactic coordinate system• Galactic bulge• Disc galaxy

• Spiral arm• Galactic halo• Galactic spheroid

Galactic corona 42

External links• THE GALACTIC CORONA [1], Jerry Bonnell, 1995• Absorption Line Studies in the Halo [2], Philipp Richter, 2003• Multi-phase High-Velocity Clouds toward HE 0226-4110 and PG 0953+414 [3], Andrew J. Fox et al., 2005• Galactic Corona or Local Group Intergalactic Medium? [4], Rik J. Williams, Smita Mathur, & Fabrizio Nicastro,

2005• NGC 5746: Detection of Hot Halo Gets Theory Out of Hot Water [5]

References[1] http:/ / antwrp. gsfc. nasa. gov/ diamond_jubilee/ papers/ lamb/ node4. html[2] http:/ / arxiv. org/ abs/ astro-ph/ 0309693[3] http:/ / arxiv. org/ abs/ astro-ph/ 0505299[4] http:/ / arxiv. org/ abs/ astro-ph/ 0511621[5] http:/ / chandra. harvard. edu/ photo/ 2006/ n5746/

Galactic discA disc is a component of disc galaxies, such as spiral galaxies, or lenticular galaxies.The galactic disc is the plane in which the spirals, bars and discs of disc galaxies exist. Galaxy discs tend to havemore gas and dust, and younger stars than galactic bulges, or galactic haloes.The galactic disc is mainly composed of gas, dust and stars. The gas and dust component of the galactic disk is calledthe gaseous disk. The star component of the galactic disk is called the stellar disk.

Inconsistent orbital velocities of starsIt has been noted that the orbital velocity of stars in the disc of most disc galaxies is inconsistent with the amount ofluminous matter calculated for the galaxy. A possible explanation for this problem is the non-luminous dark matter.

See also• Galactic spheroid• Galactic corona

Galactic halo 43

Galactic haloThe term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extendsbeyond the main, visible component. It can refer to any of several distinct components which share these properties:• the galactic spheroid (stars)• the galactic corona (hot gas, i.e. a plasma)• the dark matter haloThe distinction between the halo and the main body of the galaxy is clearest in spiral galaxies, where the sphericalshape contrasts with the flat disc. In an elliptical galaxy, there is no sharp transition between the body of the galaxyand the halo.

See also• Galaxy formation and evolution• Galactic coordinate system• Galactic bulge• Disc galaxy

• Spiral arm• Galactic corona

Ionization coneIonisation cones are cones of material extending out from spiral galaxies. They are visible because of theiremissions which are believed to be from re-emission of photons produced by nuclear activity within the galaxyitself.[1] [2]

There is not yet a scientific consensus on the mechanics of such cones.

References[1] Paper by Wilson on the properties of such cones (http:/ / www. springerlink. com/ content/ j4276u12119nr290/ )[2] Paper proposing a model for Ionisation cone operation (http:/ / www. springerlink. com/ content/ j4276u12119nr290/ )

Low-ionization nuclear emission-line region 44

Low-ionization nuclear emission-line region

The Sombrero Galaxy (M104) as observed by the Hubble Space Telescope (HST).The Sombrero Galaxy is an example of a LINER galaxy.[1] Credit:

HST/NASA/ESA.

A low-ionization nuclear emission-lineregion (LINER) is a type of galactic nucleusthat is defined by its spectral line emission.The spectra typically include line emissionfrom weakly ionized or neutral atoms, suchas O, O+, N+, and S+. Conversely, thespectral line emission from strongly ionizedatoms, such as O++, Ne++, and He+, isrelatively weak.[2] The class of galacticnuclei was first identified by TimothyHeckman in the third of a series of paperson the spectra of galactic nuclei that werepublished in 1980.[2]

Demographics of LINERgalaxies

Galaxies that contain LINERs are often referred to as LINER galaxies. LINER galaxies are very common;approximately one-third of all nearby galaxies (galaxies within approximately 20-40 Mpc) may be classified asLINER galaxies.[2] [3] Approximately 75% of LINER galaxies are either elliptical galaxies, lenticular galaxies, orS0/a-Sab galaxies (spiral galaxies with large bulges and tightly-wound spiral arms). LINERs are found lessfrequently in Sb-Scd galaxies (spiral galaxies with small bulges and loosely-wound spiral arms), and they are veryrare in nearby irregular galaxies.[3] LINERs also may be commonly found in luminous infrared galaxies (LIRGs), aclass of galaxies defined by their infrared luminosities that are frequently formed when two galaxies collide witheach other. Approximately one-quarter of LIRGs may contain LINERs.[4]

Scientific debates: energy sources and ionization mechanismsLINERs have been at the center of two major debates. First, astronomers have debated the source of energy thatexcites the ionized gas in the centers of these galaxies. Some astronomers have proposed that active galactic nuclei(AGN) with supermassive black holes are responsible for the LINER spectral emission.[2] [5] Other astronomers haveasserted that the emission is powered by star formation regions.[6] [7] The other major issue is related to how the ionsare excited. Some astronomers have suggested that shock waves propagating through the gas may ionize the gas,[2]

while others have suggested that photoionization (ionization by ultraviolet light) may be responsible.[6] [7] [5]

These debates are complicated by the fact that LINERs are found in a wide variety of objects with differentbrightnesses and morphologies. Moreover, the debate over the energy sources for LINERs is entangled with a similardebate over whether the light from star formation regions or the light from AGN produce the high infraredluminosities seen in LIRGs.[4]

Although both the energy sources and the excitation mechanisms for LINER emission are still being studied, manyLINERs are frequently referred to as AGN.[1]

Low-ionization nuclear emission-line region 45

Star formation in LINERsA number of surveys have been performed to explore the connection between star formation and LINER activity. If aconnection can be found between star formation activity and LINER activity, then this strengthens the possibilitythat LINERs are powered by the hot stars found in star formation regions. However, if star formation cannot befound in LINERs, then this definitively excludes star formation as powering LINER emission.

Star formation in LIRGs with LINERsRecent observations with the Spitzer Space Telescope show a clear connection between LINER emission inluminous infrared galaxies (LIRGs) and star formation activity. The mid-infrared spectra of LIRGs with LINERshave been shown to look similar to the mid-infrared spectra of starburst galaxies, which suggest that infrared-brightLINERs are powered by star formation activity. However, some mid-infrared spectral line emission from AGN havealso been detected in these galaxies, indicating that star formation may not be the only energy sources in thesegalaxies.[8]

Star formation in normal galaxies with LINERsNormal nearby galaxies with LINERs, however, appear to be different. A few near-infrared spectroscopic surveyshave identified some LINERs in normal galaxies that may be powered by star formation.[9] However, most LINERsin nearby galaxies have low levels of star formation activity.[9] [10] [11] Moreover, the stellar populations of manyLINERs appear to be very old,[12] [13] [11] and the mid-infrared spectra, as observed by the Spitzer Space Telescope,do not appear similar to the spectra expected from star formation.[8] These results demonstrate that most LINER innearby normal galaxies may not be powered by star formation, although a few exceptions clearly exist.

Notable LINER galaxies• Messier 94[1]

• NGC 5005[1]

• NGC 5195[1]

• Sombrero Galaxy[1]

See also• Seyfert galaxy - Another class of galaxies that contain AGN

References[1] L. C. Ho, A. V. Filippenko, W. L. W. Sargent (1997). "A Search for "Dwarf" Seyfert Nuclei. III. Spectroscopic Parameters and Properties of

the Host Galaxies" (http:/ / adsabs. harvard. edu/ abs/ 1997ApJS. . 112. . 315H). Astrophysical Journal Supplement 112: 315–390.doi:10.1086/313041. .

[2] T. M. Heckman (1980). "An optical and radio survey of the nuclei of bright galaxies - Activity in normal galactic nuclei" (http:/ / adsabs.harvard. edu/ abs/ 1980A& A. . . . 87. . 152H). Astronomy and Astrophysics 87: 152–164. .

[3] L. C. Ho, A. V. Filippenko, W. L. W. Sargent (1997). "A Search for "Dwarf" Seyfert Nuclei. V. Demographics of Nuclear Activity in NearbyGalaxies" (http:/ / adsabs. harvard. edu/ abs/ 1997ApJ. . . 487. . 568H). Astrophysical Journal 487: 568–578. doi:10.1086/304638. .

[4] S. Veilleux, D.-C. Kim, D. B. Sanders, J. M. Mazzarella, B. T. Soifer (1995). "Optical Spectroscopy of Luminous Infrared Galaxies. II.Analysis of the Nuclear and Long-Slit Data" (http:/ / adsabs. harvard. edu/ abs/ 1995ApJS. . . 98. . 171V). Astrophysical Journal SupplementSeries 98: 171–217. doi:10.1086/192158. .

[5] L. C. Ho, A. V. Filippenko, W. L. W. Sargent (1993). "A Reevaluation of the Excitation Mechanism of LINERs" (http:/ / adsabs. harvard.edu/ abs/ 1993ApJ. . . 417. . . 63H). Astrophysical Journal 417: 63–81. doi:10.1086/173291. .

[6] R. Terlevich, J. Melnick (1985). "Warmers - The missing link between Starburst and Seyfert galaxies" (http:/ / adsabs. harvard. edu/ abs/1985MNRAS. 213. . 841T). Monthly Notices of the Royal Astronomical Society 213: 841–856. .

[7] J. C. Shields (1992). "Normal O stars in dense media generate LINERs" (http:/ / adsabs. harvard. edu/ abs/ 1992ApJ. . . 399L. . 27S).Astrophysical Journal 399: L27–L30. doi:10.1086/186598. .

Low-ionization nuclear emission-line region 46

[8] E. Sturm, D. Rupke, A. Contursi, D.-C. Kim, D. Lutz, H. Netzer, S. Veilleux, R. Genzel, M. Lehnert, L. J. Tacconi, D. Maoz, J. Mazzarella,S. Lord, D. Sanders, A. Sternberg (2006). "Mid-Infrared Diagnostics of LINERS" (http:/ / adsabs. harvard. edu/ abs/ 2006ApJ. . . 653L. . 13S).Astrophysical Journal 653: L13–L16. doi:10.1086/510381. .

[9] J. E. Larkin, L. Armus, R. A. Knop, B. T. Soifer, K. Matthews (1998). "A Near-Infrared Spectroscopic Survey of LINER Galaxies" (http:/ /adsabs. harvard. edu/ abs/ 1998ApJS. . 114. . . 59L). Astrophysical Journal Supplement Series 114: 59–72. doi:10.1086/313063. .

[10] G. J. Bendo, R. D. Joseph, M. Wells, P. Gallais, M. Haas, A. M. Heras, U. Klaas, R. J. Laureijs, K. Leech, D. Lemke, L. Metcalfe, M.Rowan-Robinson, B. Schulz, C. Telesco (2002). "Star Formation in the Infrared Space Observatory Atlas of Bright Spiral Galaxies" (http:/ /adsabs. harvard. edu/ abs/ 2002AJ. . . . 124. 1380B). Astronomical Journal 124: 1380–1392. doi:10.1086/342283. .

[11] G. J. Bendo, R. D. Joseph (2004). "Nuclear Stellar Populations in the Infrared Space Observatory Atlas of Bright Spiral Galaxies" (http:/ /adsabs. harvard. edu/ abs/ 2004AJ. . . . 127. 3338B). Astronomical Journal 127: 3338–3360. doi:10.1086/420712. .

[12] R. Cid Fernandes, R. M. González Delgado, H. Schmitt, T. Storchi-Bergmann, Thaisa, L. P. Martins, E. Pérez, T. Heckman, C. Leitherer, D.Schaerer (2004). "The Stellar Populations of Low-Luminosity Active Galactic Nuclei. I. Ground-based Observations" (http:/ / adsabs. harvard.edu/ abs/ 2004ApJ. . . 605. . 105C). Astrophysical Journal 605: 105–126. doi:10.1086/382217. .

[13] R. M. González Delgado, R. Cid Fernandes, E. Pérez, L. P. Martins, T. Storchi-Bergmann, H. Schmitt, T. Heckman, C. Leitherer (2004)."The Stellar Populations of Low-Luminosity Active Galactic Nuclei. II. Space Telescope Imaging Spectrograph Observations" (http:/ / adsabs.harvard. edu/ abs/ 2004ApJ. . . 605. . 127G). Astrophysical Journal 605: 127–143. doi:10.1086/382216. .

Relativistic jetThe lower-energy non-relativistic version of this phenomenon is described at polar jet.

Relativistic jet. The environment around the AGN where the relativisticplasma is collimated into jets which escape along the pole of the

supermassive black hole

Relativistic jets are extremely powerful jets ofplasma which emerge from presumed massiveobjects at the centers of some active galaxies,notably radio galaxies and quasars. Their lengthscan reach several thousand[1] or even hundreds ofthousands of light years.[2] The hypothesis is thatthe twisting of magnetic fields in the accretion diskcollimates the outflow along the rotation axis ofthe central object, so that when conditions aresuitable, a jet will emerge from each face of theaccretion disk. If the jet is oriented along the lineof sight to Earth, relativistic beaming will changeits apparent brightness. The mechanics behind boththe creation of the jets[3] [4] and the composition ofthe jets[5] are still a matter of much debate in thescientific community; it is hypothesized that thejets are composed of an electrically neutral mixtureof electrons, positrons, and protons in someproportion.

Relativistic jet 47

Elliptical Galaxy M87 emitting a relativistic jet,as seen by Hubble Space Telescope's WFPC2 in

the visible spectrum.

Similar jets, though on a much smaller scale, can develop around theaccretion disks of neutron stars and stellar black holes. These systemsare often called microquasars. A famous example is SS433, whosewell-observed jet has a velocity of 0.23c, although other microquasarsappear to have much higher (but less well measured) jet velocities.Even weaker and less-relativistic jets may be associated with manybinary systems; the acceleration mechanism for these jets may besimilar to the magnetic reconnection processes observed in the Earth'smagnetosphere and the solar wind.

The general hypothesis among astrophysicists is that the formation ofrelativistic jets is the key to explaining the production of gamma-raybursts. These jets have Lorentz factors of ~100 (that is, speeds ofroughly 0.99995c), making them one of the swiftest celestial objectscurrently known.

Rotating black hole as energy source

Because of the enormous amount of energy needed to launch a relativistic jet, some jets are thought to be poweredby spinning black holes. There are two competing theories for how the energy is transferred from the black hole tothe jet.

• Blandford-Znajek process.[6] This is the most popular theory for the extraction of energy from the central blackhole. The magnetic fields around the accretion disk are dragged by the spin of the black hole. The relativisticmaterial is possibly launched by the tightening of the field lines.

• Penrose mechanism.[7] This extracts energy from a rotating black hole by frame dragging. This theory was laterproven to be able to extract relativistic particle energy and momentum,[8] and subsequently shown to be a possiblemechanism for the formation of jets.[9]

Other images

Centaurus A in x-rays showing therelativistic jet

The M87 jet seen by the Very Large Array inradio frequency (the viewing field is larger

and rotated with respect to the above image).

Hubble LegacyArchive Near-UV

image of therelativistic jet

coming out of 3C66B.

Relativistic jet 48

See also• Bipolar outflow• Polar jet• Blandford-Znajek process

Further reading• Melia, Fulvio, The Edge of Infinity. Supermassive Black Holes in the Universe 2003, Cambridge University Press,

ISBN 978-0-521-81405-8 (Cloth)

References[1] Biretta, J. (1999, January 6). Hubble Detects Faster-Than-Light Motion in Galaxy M87 (http:/ / www. stsci. edu/ ftp/ science/ m87/ m87.

html)[2] Yale University - Office of Public Affairs (2006, June 20). Evidence for Ultra-Energetic Particles in Jet from Black Hole (http:/ / web.

archive. org/ web/ 20080513034113/ http:/ / www. yale. edu/ opa/ newsr/ 06-06-20-01. all. html)[3] Meier, L. M. (2003). The Theory and Simulation of Relativistic Jet Formation: Towards a Unified Model For Micro- and Macroquasars,

2003, New Astron. Rev. , 47, 667. (http:/ / arxiv. org/ abs/ astro-ph/ 0312048)[4] Semenov, V.S., Dyadechkin, S.A. and Punsly (2004, August 13). Simulations of Jets Driven by Black Hole Rotation. Science, 305, 978-980.

(http:/ / www. sciencemag. org/ cgi/ content/ abstract/ sci;305/ 5686/ 978?maxtoshow=& HITS=10& hits=10& RESULTFORMAT=&fulltext=relativistic+ jet& searchid=1& FIRSTINDEX=10& resourcetype=HWCIT)

[5] Georganopoulos, M.; Kazanas, D.; Perlman, E.; Stecker, F. (2005) Bulk Comptonization of the Cosmic Microwave Background byExtragalactic Jets as a Probe of their Matter Content, The Astrophysical Journal , 625, 656. (http:/ / arxiv. org/ abs/ astro-ph/ 0502201)

[6] Blandford, R. D., Znajek, R. L. (1977), Monthly Notices of the Royal Astronomical Society, 179, 433[7] Penrose, R. (1969). Gravitational collapse: The role of general relativity. Nuovo Cimento Rivista, Numero Speciale 1, 252-276.[8] Williams, R. K. (1995, May 15). Extracting x rays, Ύ rays, and relativistic e-e+ pairs from supermassive Kerr black holes using the Penrose

mechanism. Physical Review, 51(10), 5387-5427.[9] Williams, R. K. (2004, August 20). Collimated escaping vortical polar e-e+ jets intrinsically produced by rotating black holes and Penrose

processes. The Astrophysical Journal, 611, 952-963. (http:/ / arxiv. org/ abs/ astro-ph/ 0404135)

Supermassive black hole 49

Supermassive black hole

Top: artist's conception of a supermassive blackhole tearing apart a star. Bottom: images believed

to show a supermassive black hole devouring astar in galaxy RXJ 1242-11. Left: X-ray image,

Right: optical image.[1]

A supermassive black hole is the largest type of black hole in agalaxy, on the order of hundreds of thousands to billions of solarmasses. Most, and possibly all galaxies, including the Milky Way,[2]

are believed to contain supermassive black holes at their centers.[3] [4]

Supermassive black holes have properties which distinguish them fromlower-mass classifications:• The average density of a supermassive black hole (defined as the

mass of the black hole divided by the volume within itsSchwarzschild radius) can be as low as the density of water for verylarge mass black holes. This is because the Schwarzschild radius isdirectly proportional to mass, while density is inversely proportionalto the volume. Since the volume of a spherical object (such as theevent horizon of a non-rotating black hole) is directly proportionalto the cube of the radius, and mass merely increases linearly, thevolume increases by a much greater factor than the mass as a blackhole grows. Thus, average density decreases for increasingly largerradii of black holes (due to volume increasing much faster thanmass).

• The tidal forces in the vicinity of the event horizon are significantly weaker. Since the central singularity is so faraway from the horizon, a hypothetical astronaut traveling towards the black hole center would not experiencesignificant tidal force until very deep into the black hole.

Formation

An artist's conception of a supermassive blackhole & accretion disk.

There are many models for the formation of black holes of this size.The most obvious is by slow accretion of matter starting from a blackhole of stellar size. Another model[5] of supermassive black holeformation involves a large gas cloud collapsing into a relativistic starof perhaps a hundred thousand solar masses or larger. The star wouldthen become unstable to radial perturbations due to electron-positronpair production in its core, and may collapse directly into a black holewithout a supernova explosion, which would eject most of its mass andprevent it from leaving a supermassive black hole as a remnant. Yetanother model[6] involves a dense stellar cluster undergoingcore-collapse as the negative heat capacity of the system drives thevelocity dispersion in the core to relativistic speeds. Finally, primordialblack holes may have been produced directly from external pressure in the first instants after the Big Bang.

The difficulty in forming a supermassive black hole resides in the need for enough matter to be in a small enoughvolume. This matter needs to have very little angular momentum in order for this to happen. Normally the process ofaccretion involves transporting a large initial endowment of angular momentum outwards, and this appears to be thelimiting factor in black hole growth, and explains the formation of accretion disks.Currently, there appears to be a gap in the observed mass distribution of black holes. There are stellar-mass black holes, generated from collapsing stars, which range up to perhaps 33 solar masses. The minimal supermassive black

Supermassive black hole 50

hole is in the range of a hundred thousand solar masses. Between these regimes there appears to be a dearth ofintermediate-mass black holes. Such a gap would suggest qualitatively different formation processes. However,some models[7] suggest that ultraluminous X-ray sources (ULXs) may be black holes from this missing group.

Doppler measurementsDirect Doppler measures of water masers surrounding the nucleus of nearby galaxies have revealed a very fastkeplerian motion, only possible with a high concentration of matter in the center. Currently, the only known objectsthat can pack enough matter in such a small space are black holes, or things that will evolve into black holes withinastrophysically short timescales. For active galaxies farther away, the width of broad spectral lines can be used toprobe the gas orbiting near the event horizon. The technique of reverberation mapping uses variability of these linesto measure the mass and perhaps the spin of the black hole that powers the active galaxy's "engine".Such supermassive black holes in the center of many galaxies are thought to be the "engine" of active objects such asSeyfert galaxies and quasars.

Supermassive black hole hypothesisAstronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, in a regioncalled Sagittarius A*[8] because:

• The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 lighthours from the center of the central object.[9]

• From the motion of star S2, we estimate the object's mass as 4.1 million solar masses.[10]

• We also know that the radius of the central object is significantly less than 17 light hours, because otherwise,S2 would either collide with it or be ripped apart by tidal forces. In fact, recent observations[11] indicate thatthe radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit.

• The only known object which can pack 4.1 million solar masses into a volume that small is a black hole.The Max Planck Institute for Extraterrestrial Physics and UCLA Galactic Center Group[12] have provided thestrongest evidence to date that Sagittarius A* is the site of a supermassive black hole,[8] based on data from theESO[13] and the Keck telescope.[14] Our galactic central black hole is calculated to have a mass of approximately 4.1million solar masses,[15] or about 8.2 × 1036 kg.

Supermassive black holes outside the Milky WayIt is now widely accepted that the center of nearly every galaxy contains a supermassive black hole.[16] [17] The closeobservational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, knownas the M-sigma relation[18] , strongly suggests a connection between the formation of the black hole and the galaxyitself.[16]

The explanation for this correlation remains an unsolved problem in astrophysics. It is believed that black holes andtheir host galaxies coevolved between 300-800 million years after the Big Bang, passing through a quasar phase anddeveloping correlated characteristics, but models differ on the causality of whether black holes triggered galaxyformation or vice versa, and sequential formation cannot be excluded. The unknown nature of dark matter is acrucial variable in these models.[19] [20]

At least one galaxy, Galaxy 0402+379, appears to have two supermassive black holes at its center, forming a binarysystem. If they collide, the event would create strong gravitational waves. Binary supermassive black holes arebelieved to be a common consequence of galactic mergers.[21] As of November 2008, another binary pair, in OJ 287,contains the most massive black hole known, with a mass estimated at 18 billion solar masses.[22]

Supermassive black hole 51

See also• Active galactic nucleus• Black hole• Fuzzball (string theory)• Galaxy• Galactic center• Hypercompact stellar system• Neutron star• Quasar• M-sigma relation• Sagittarius A*• Spin-flip

References[1] Chandra :: Photo Album :: RX J1242-11 :: 18 Feb 04 (http:/ / chandra. harvard. edu/ photo/ 2004/ rxj1242/ )[2] Schödel, R.; et al. (2002). "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way". Nature 419

(6908): 694–696. doi:10.1038/nature01121. PMID 12384690.[3] Antonucci, R. (1993). "Unified Models for Active Galactic Nuclei and Quasars". Annual Reviews in Astronomy and Astrophysics 31 (1):

473–521. doi:10.1146/annurev.aa.31.090193.002353.[4] Urry, C.; Paolo Padovani (1995). "Unified Schemes for Radio-Loud Active Galactic Nuclei". Publications of the Astronomical Society of the

Pacific 107: 803–845. doi:10.1086/133630.[5] Begelman, M. C.; et al. (Jun 2006). "Formation of supermassive black holes by direct collapse in pre-galactic haloes". Monthly Notices of the

Royal Astronomical Society 370 (1): 289–298. doi:10.1111/j.1365-2966.2006.10467.x.[6] Spitzer, L. (1987). Dynamical Evolution of Globular Clusters. Princeton University Press. ISBN 0691083096.[7] Winter, L.M.; et al. (Oct 2006). "XMM-Newton Archival Study of the ULX Population in Nearby Galaxies". Astrophysical Journal 649:

730–752. doi:10.1086/506579.[8] Henderson, Mark (December 9, 2008). "Astronomers confirm black hole at the heart of the Milky Way" (http:/ / www. timesonline. co. uk/

tol/ news/ uk/ science/ article5316001. ece). Times Online. . Retrieved 2009-05-17.[9] Schödel, R.; et. al. (17 October 2002). "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way" (http:/ /

www. nature. com/ nature/ journal/ v419/ n6908/ abs/ nature01121. html). Nature 419 (419): 694–696. doi:10.1038/nature01121.arXiv:astro-ph/0210426. PMID 12384690. . Retrieved 2009-07-27.

[10] Ghez, A. M.; et al. (December 2008). "Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with StellarOrbits" (http:/ / adsabs. harvard. edu/ abs/ 2008ApJ. . . 689. 1044G). Astrophysical Journal 689: 1044–1062. doi:10.1086/592738.arXiv:astro-ph/0808.2870. .

[11] Ghez, A. M.; Salim, S.; Hornstein, S. D.; Tanner, A.; Lu, J. R.; Morris, M.; Becklin, E. E.; Duchêne, G. (May 2005). "Stellar Orbits aroundthe Galactic Center Black Hole" (http:/ / www. journals. uchicago. edu/ doi/ abs/ 10. 1086/ 427175). The Astrophysical Journal 620 (2):744–757. doi:10.1086/427175. arXiv:astro-ph/0306130v2. . Retrieved 2008-05-10.

[12] UCLA Galactic Center Group (http:/ / www. astro. ucla. edu/ ~ghezgroup/ gc/ )[13] ESO - 2002 (http:/ / www. eso. org/ outreach/ press-rel/ pr-2002/ pr-17-02. html)[14] http:/ / www. keckobservatory. org/ news/ old_pages/ andreaghez. html[15] http:/ / www. skyandtelescope. com/ news/ 27621359. html[16] King, Andrew (2003-09-15). "Black Holes, Galaxy Formation, and the MBH-σ Relation" (http:/ / www. iop. org/ EJ/ article/ 1538-4357/

596/ 1/ L27/ 17559. text. html). The Astrophysical Journal (The American Astronomical Society.): 596:L27–L29. .[17] Richstone, D. et al. (January 13, 1997). "Massive Black Holes Dwell in Most Galaxies, According to Hubble Census" (http:/ / hubblesite.

org/ newscenter/ archive/ releases/ 1997/ 01/ text/ ). 189th Meeting of the American Astronomical Society. . Retrieved 2009-05-17.[18] Merritt, D.; Ferrarese, Laura (2001-01-15). "The MBH-σ Relation for Supermassive Black Holes" (http:/ / adsabs. harvard. edu/ abs/

2001ApJ. . . 547. . 140M). The Astrophysical Journal (The American Astronomical Society.) 547: 547:140–145. doi:10.1086/318372. .[19] Robert Roy Britt (2003-07-29). "The New History of Black Holes: 'Co-evolution' Dramatically Alters Dark Reputation" (http:/ / www.

space. com/ scienceastronomy/ blackhole_history_030128-1. html). .[20] "Astronomers crack cosmic chicken-or-egg dilemma" (http:/ / www. astronomy. com/ asy/ default. aspx?c=a& id=2165). 2003-07-22. .[21] D. Merritt and M. Milosavljevic (2005). "Massive Black Hole Binary Evolution." http:/ / relativity. livingreviews. org/ Articles/ lrr-2005-8/[22] Shiga, David (10 January 2008). "Biggest black hole in the cosmos discovered" (http:/ / space. newscientist. com/ article/

dn13166-biggest-black-hole-in-the-cosmos-discovered. html). NewScientist.com news service. .

Supermassive black hole 52

Further reading• Fulvio Melia (2003). The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge University

Press. ISBN 978-0-521-81405-8.• Laura Ferrarese and David Merritt (2002). "Supermassive Black Holes" (http:/ / adsabs. harvard. edu/ abs/

2002astro. ph. . 6222F). Physics World 15 (1): 41–46.• Fulvio Melia (2007). The Galactic Supermassive Black Hole. Princeton University Press.

ISBN 978-0-691-13129-0.• Julian Krolik (1999). Active Galactic Nuclei. Princeton University Press. ISBN 0-691-01151-6.

External links• Black Holes: Gravity's Relentless Pull (http:/ / www. hubblesite. org/ go/ blackholes) Award-winning interactive

multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute• Images of supermassive black holes (http:/ / chandra. harvard. edu/ photo/ 2002/ 0157/ 0157_composite. jpg)• NASA images of supermassive black holes (http:/ / antwrp. gsfc. nasa. gov/ apod/ image/ 0210/

mwcentre_eso_big. jpg)• The black hole at the heart of the Milky Way (http:/ / www. einstein-online. info/ en/ spotlights/ milkyway_bh/

index. html)• ESO video clip of orbiting star (http:/ / www. eso. org/ outreach/ press-rel/ pr-2002/ video/ vid-02-02. mpg) (533

KB MPEG Video)• Star Orbiting Massive Milky Way Centre Approaches to within 17 Light-Hours (http:/ / www. eso. org/ outreach/

press-rel/ pr-2002/ pr-17-02. html) ESO, October 21, 2002• Images, Animations, and New Results from the UCLA Galactic Center Group (http:/ / www. astro. ucla. edu/

research/ galcenter/ )• Washington Post article on Supermassive black holes (http:/ / www. washingtonpost. com/ wp-dyn/ content/

article/ 2007/ 10/ 30/ AR2007103002073. html?nav=most_emailed)• A simulation of the stars orbiting the Milky Way's central massive black hole (http:/ / www. orbitsimulator. com/

gravity/ articles/ mwblackhole. html)

53

Large-scale structure

Galaxy groups and clusters

The galaxies of HCG 87, about four hundred millionlight-years distant. The large edge-on spiral, the fuzzyelliptical galaxy immediately to its right, and the spiral

near the top of the image are members of the group,while the small spiral galaxy exactly in the middle is amore distant background galaxy. Credit: NASA/ESA.

Galaxy groups and clusters are the largest known gravitationallybound objects to have arisen thus far in the process of cosmicstructure formation.[1] They form the densest part of the large scalestructure of the universe. In models for the gravitational formationof structure with cold dark matter, the smallest structures collapsefirst and eventually build the largest structures, clusters ofgalaxies. Clusters are then formed relatively recently between 10billion years ago and now. Groups and clusters may contain fromten to thousands of galaxies. The clusters themselves are oftenassociated with larger groups called superclusters.

Groups of galaxies

Groups of galaxies are the smallest aggregates of galaxies. Theytypically contain fewer than 50 galaxies in a diameter of 1 to 2megaparsecs (Mpc) (see 1022 m for distance comparisons). Theirmass is approximately 1013 solar masses. The spread of velocitiesfor the individual galaxies is about 150 km/s. However, thisdefinition should be used as a guide only, as larger and moremassive galaxy systems are sometimes classified as galaxy groups.

Our own galaxy, the Milky Way, is contained in the Local Group of galaxies, which contains more than 40galaxies.[2]

Clusters of galaxies

Galaxy cluster ACO 3341.

Clusters are larger than groups, although there is no sharp dividing linebetween the two. When observed visually, clusters appear to becollections of galaxies held together by mutual gravitational attraction.However, their velocities are too large for them to remaingravitationally bound by their mutual attractions, implying thepresence of either an additional invisible mass component, or anadditional attractive force besides gravity. X-ray studies have revealedthe presence of large amounts of intergalactic gas known as theintracluster medium. This gas is very hot, between 107K and 108K, andhence emits X-rays in the form of bremsstrahlung and atomic lineemission. The total mass of the gas is greater than that of the galaxiesby roughly a factor of two. However this is still not enough mass tokeep the galaxies in the cluster. Since this gas is in approximate hydrostatic equilibrium with the overall cluster

Galaxy groups and clusters 54

gravitational field, the total mass distribution can be determined. It turns out the total mass deduced from thismeasurement is approximately six times larger than the mass of the galaxies or the hot gas. The missing componentis known as dark matter and its nature is unknown. In a typical cluster perhaps only 5% of the total mass is in theform of galaxies, maybe 10% in the form of hot X-ray emitting gas and the remainder is dark matter. Brownstein andMoffat[3] use a theory of modified gravity to explain X-ray cluster masses without dark matter. Observations of theBullet Cluster, however, are considered to be some of the strongest evidence for the existence of dark matter.Clusters typically have the following properties.• They contain 50 to 1,000 galaxies, hot X-ray emitting gas and large amounts of dark matter• The distribution of these three components is approximately the same in the cluster.• They have total masses of 1014 to 1015 solar masses.• They typically have a diameter from 2 to 10 Mpc (see 1023 m for distance comparisons).• The spread of velocities for the individual galaxies is about 800–1000 km/s.Notable galaxy clusters in the relatively nearby universe include the Virgo cluster, Fornax Cluster, Hercules Cluster,and the Coma Cluster. A very large aggregation of galaxies known as the Great Attractor, dominated by the Normacluster, is massive enough to affect the local expansion of the universe (Hubble flow).In the last few decades, they are also found to be relevant sites of particle acceleration, a feature which has beendiscovered by the observing non-thermal diffuse radio emissions as radio halos and radio relics.Note: clusters of galaxies should not be confused with star clusters such as galactic clusters and open clusters, whichare structures within galaxies, as well as globular clusters, which typically orbit galaxies.

Observational methodsClusters of galaxies have been found in surveys by a number of observational techniques and have been studied indetail using many methods:• Optical or infrared: The individual galaxies of clusters can be studied through optical or infrared imaging and

spectroscopy. Galaxy clusters are found by optical or infrared telescopes by searching for overdensities, and thenconfirmed by finding several galaxies at a similar redshift. Infrared searches are more useful for finding moredistant (higher redshift) clusters.

• X-ray: The hot plasma emits X-rays which can be detected by X-ray telescopes. The cluster gas can be studiedusing both X-ray imaging and X-ray spectroscopy. Clusters are quite prominent in X-ray surveys and along withAGN are the brightest X-ray emitting extragalactic objects.

• Radio: A number of diffuse structures emitting at radio frequencies have been found in clusters. Groups of radiosources (which may include diffuse structures or AGN have been used as tracers of cluster location. At highredshift imaging around individual radio sources (in this case AGN) has been used to detect proto-clusters(clusters in the process of forming).

• Sunyaev-Zel'dovich effect: The hot electrons in the intracluster medium scatter radiation from the cosmicmicrowave background through inverse Compton scattering. This produces a "shadow" in the observed cosmicmicrowave background at some radio frequencies.

• Gravitational lensing: Clusters of galaxies contain enough matter to distort the observed orientations of galaxiesbehind them. The observed distortions can be used to model the distribution of dark matter in the cluster.

Galaxy groups and clusters 55

Temperature and densityClusters of galaxies are the most recent and most massive objects to have arisen in the hierarchical structureformation of the universe and the study of clusters tells one about the way galaxies form and evolve. Clusters havetwo important properties: their masses are large enough to retain any energetic gas ejected from member galaxiesand the thermal energy of the gas within the cluster is observable within the X-Ray bandpass. The observed state ofgas within a cluster is determined by a combination of shock heating during accretion, radiative cooling, and thermalfeedback triggered by that cooling. The density, temperature, and substructure of the intracluster X-Ray gas thereforerepresents the entire thermal history of cluster formation. To better understand this thermal history one needs tostudy the entropy of the gas because entropy is the quantity most directly changed by increasing or decreasing thethermal energy of intracluster gas.

See also• Fossil group• Galactic orientation• List of galaxy clusters• Large-scale structure of the cosmos• Supercluster• Timeline of galaxies, clusters of galaxies, and large-scale structure• Intracluster medium• Entropy

References[1] Voit, G.M.; "Tracing cosmic evolution with clusters of galaxies"; Reviews of Modern Physics, vol. 77, Issue 1, pp. 207-258[2] Mike Irwin. "The Local Group" (http:/ / www. ast. cam. ac. uk/ ~mike/ local_more. html). . Retrieved 2009-11-07.[3] Galaxy Cluster Masses Without Non-Baryonic Dark Matter "Galaxy Cluster Masses Without Non-Baryonic Dark Matter" (http:/ / arxiv. org/

abs/ astro-ph/ 0507222). Mon.Not.Roy.Astron.Soc. 367 (2006) 527-540. July 8, 2005. Galaxy Cluster Masses Without Non-Baryonic DarkMatter. Retrieved 2008-12-20.

Galaxy supercluster 56

Galaxy supercluster

A map of the nearest (to Virgo) Superclusters

Superclusters are large groups of smaller galaxygroups and clusters and are among the largest structuresof the cosmos. They are so large that they are notgravitationally bound and, consequently, partake in theHubble expansion.

Existence

The existence of superclusters indicates that thegalaxies in our Universe are not uniformly distributed;most of them are drawn together in groups and clusters,with groups containing up to 50 galaxies and clustersup to several thousand. Those groups and clusters andadditional isolated galaxies in turn form even largerstructures called superclusters.

Once thought to be the largest structures in nature,superclusters are now understood to be subordinate to enormous walls or sheets, usually called "filaments",sometimes called "supercluster complexes", "walls" or "sheets", that can span a billion light-years in length, morethan 5% of the observable universe. Superclusters themselves can span several hundred million light-years. Thetypical speed of a galaxy is about 1000 km/s. Hubble's law implies that typical galaxies would only move about 30million light-years at that speed in a Hubble time of 1/H, which is approximately the age of the universe. While thisis a huge distance in human terms, it is much smaller than the size of superclusters. In an expanding universe, sayingthat the distance d an object has moved equals its present velocity v times the elapsed time t underestimates d when tis not small compared to 1/H. The calculation above still gives some idea of how long it would take the normalmovements of galaxies to form or obliterate these structures, and thus indicates their great age. When we observesuperclusters and larger structures today, we learn about the condition of the universe when these superclusters werecreated. The directions of the rotational axes of galaxies within superclusters also gives us insight into the formationprocess of galaxies early in the history of the Universe.[1]

According to some astronomers, no clusters of superclusters (“hyperclusters”) are known; the existence of structureslarger than superclusters is debated (see Galaxy filament). Interspersed among superclusters are large voids of spacein which few galaxies exist. Even though superclusters are the largest structures confirmed, the total number ofsuperclusters leaves possibilities for structural distribution.Superclusters are frequently subdivided into groups of clusters called galaxy clouds.

List of superclusters

Nearby superclusters

Galaxy supercluster 57

Galaxysupercluster

Data Notes

Local Supercluster • z=0.000 (0 lightyears away)

• Length = 33 Mpc(110 million lightyears)

It contains the Local Group with our galaxy, the Milky Way. It also contains the Virgo cluster nearits center, and is sometimes called the Virgo Supercluster.

Hydra-CentaurusSupercluster

It is composed of two lobes, sometimes also referred to as superclusters, or sometimes the entiresupercluster is referred to by these other two names

• Hydra Supercluster• Centaurus Supercluster

Perseus-PiscesSupercluster

Pavo-IndusSupercluster

Coma Supercluster Forms most of the CfA Homunculus, the center of the CfA2 Great Wall galaxy filament

PhoenixSupercluster

SculptorSuperclusters

SCl 9

HerculesSuperclusters

SCl 160

Leo Supercluster SCl 93

OphiuchusSupercluster

• 17h 10m -22°• cz=8500-9000 km/s

(centre)• 18 Mpc x 26 Mpc

Forming the far wall of the Ophiuchus Void, it may be connected in a filament, with thePavo-Indus-Telescopium Supercluster and the Hercules Supercluster. This supercluster is centeredon the cD cluster Ophiuchus Cluster, and has at least two more galaxy clusters, four more galaxygroups, several field galaxies, as members.[2]

ShapleySupercluster

The second supercluster found, after the Local Supercluster.

Distant superclusters

Galaxy supercluster Data Notes

Pisces-Cetus Supercluster

Bootes Supercluster SCl 138

Horologium Supercluster z=0.063 (700 Mly)

Length = 550 Mly

The entire supercluster is referred to as the Horologium-Reticulum Supercluster

Corona Borealis Supercluster

Columba Supercluster

Aquarius Supercluster

Aquarius B Supercluster

Aquarius-Capricornus Supercluster

Aquarius-Cetus Supercluster

Bootes A Supercluster

Caelum Supercluster SCl 59

Galaxy supercluster 58

Draco Supercluster

Draco-Ursa Major Supercluster

Fornax-Eridanus Supercluster

Grus Supercluster

Leo A Supercluster

Leo-Sextans Supercluster

Leo-Virgo Supercluster SCl 107

Microscopium Supercluster SCl 174

Pegasus-Pisces Supercluster SCl 3

Pisces Supercluster SCl 24

Pisces-Aries Supercluster

Ursa Major Supercluster

Virgo-Coma Supercluster SCl 111

Far distant superclusters

Galaxy supercluster Data Notes

Lynx Supercluster z=1.27 Discovered in 1999[3] (as ClG J0848+4453, a name now used to describe the western cluster, with ClGJ0849+4452 being the eastern one),[4] it contains at least two clusters RXJ 0848.9+4452 (z=1.26) andRXJ 0848.6+4453 (z=1.27) . At the time of discovery, it became the most distant known supecluster.[5]

Additionally, seven smaller groups of galaxies are associated with the supercluster.[6]

SCL @ 1338+27 atz=1.1

z=1.1

Length=70MpcA rich supercluster with several galaxy clusters was discovered around an unusual concentration of 23QSOs at z=1.1 in 2001. The size of the complex of clusters may indicate a wall of galaxies exists there,instead of a single supercluster. The size discovered approaches the size of the CfA2 Great Wallfilament. At the time of the discovery, it was the largest and most distant supercluster beyond z=0.5 [7][8]

SCL @ 1604+43 atz=0.9

z=0.91 This supercluster at the time of its discovery was the largest supercluster found so deep into space, in2000. It consisted of two known rich clusters and one newly discovered cluster as a result of the studythat discovered it. The then known clusters were Cl 1604+4304 (z=0.897) and Cl 1604+4321 (z=0.924),which then known to have 21 and 42 known galaxies respectively. The then newly discovered clusterwas located at 16h 04m 25.7s, +43° 14′ 44.7″ [9]

SCL @ 0018+16 atz=0.54 in SA26

z=0.54 This supercluster lies around radio galaxy 54W084C (z=0.544) and is composed of at least three largeclusters, CL 0016+16 (z=0.5455), RX J0018.3+1618 (z=0.5506), RX J0018.8+1602 .[10]

MS 0302+17 z=0.42

Length=6MpcThis supercluster has at least three member clusters, the eastern cluster CL 0303+1706, southern clusterMS 0302+1659 and northern cluster MS 0302+1717.[11]

SPT-CL_J0546-5345 z=1.07 The most massive supercluster yet found in the early universe at 7 billion years away. It has 800 trillionsuns packed into hundreds of galaxies, but is likely to be much larger by now.[12]

Galaxy supercluster 59

References[1] Hu, F. X.; Wu, G. X.; Song, G. X.; Yuan, Q. R.; Okamura, S. (2006). "Orientation of Galaxies in the Local Supercluster: A Review" (http:/ /

adsabs. harvard. edu/ cgi-bin/ nph-bib_query?bibcode=2006Ap& SS. 302. . . 43H). Astrophysics and Space Science 302 (1-4): 43–59.doi:10.1007/s10509-005-9006-7. .

[2] Hasegawa, Takashi; Wakamatsu, Ken-ichi; Malkan, Matthew; Sekiguchi, Kazuhiro; Menzies, John W.; Parker, Quentin A.; Jugaku, Jun;Karoji, Hiroshi; Okamura, Sadanori (2000) "Large-scale structure of galaxies in the Ophiuchus region" (http:/ / cdsads. u-strasbg. fr/ cgi-bin/nph-iarticle_query?2000MNRAS. 316. . 326H& amp;data_type=PDF_HIGH& amp;whole_paper=YES& amp;type=PRINTER&filetype=. pdf) (PDF) Monthly Notices of the Royal Astronomical Society, Volume 316, Issue 2, pp. 326-344Bibcode: 2000MNRAS.316..326H doi:10.1046/j.1365-8711.2000.03531.x

[3] Piero Rosati et al (1999) "An X-Ray-Selected Galaxy Cluster at z = 1.26" (http:/ / www. iop. org/ EJ/ abstract/ -link=10009729/ 1538-3881/118/ 1/ 76) The Astronomical Journal 118 76-85

[4] SIMBAD, "Lynx Supercluster" (http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=NAME LYNX SUPERCLUSTER)[5] Fumiaki Nakata, Tadayuki Kodama, Kazuhiro Shimasaku, Mamoru Doi, Hisanori Furusawa, Masaru Hamabe, Masahiko Kimura, Yutaka

Komiyama, Satoshi Miyazaki, Sadanori Okamura, Masami Ouchi, Maki Sekiguchi, Masafumi Yagi and Naoki Yasuda (2004) "Discovery of alarge-scale clumpy structure of the Lynx supercluster at z∼1.27" (http:/ / journals. cambridge. org/ download. php?file=/ IAU/IAU2004_IAUC195/ S1743921304000080a. pdf& code=d459918c34d5d3cd559d97c60a9e146d), (PDF) Proceedings IAU Colloquium No.195, doi:10.1017/S1743921304000080

[6] Kouji Ohta, Masayuki Akiyama, Yoshihiro Ueda, Toru Yamada, Kouichiro Nakanishi, Gavin B. Dalton, Yasushi Ogasaka, Tsuneo Kii,Kiyoshi Hayashida (2003) "Optical Identification of the ASCA Lynx Deep Survey: An Association of Quasi-Stellar Objects and aSupercluster at z = 1.3?" (http:/ / www. iop. org/ EJ/ article/ 0004-637X/ 598/ 1/ 210/ 58425. text. html) The Astrophysical Journal,598:210-215

[7] Ichi Tanaka (2004) "Subaru Observation of a Supercluster of Galaxies and QSOS at Z = 1.1" (http:/ / adsabs. harvard. edu/ abs/ 2004sgyu.conf. . . 61T) Studies of Galaxies in the Young Universe with New Generation Telescope, Proceedings of Japan-German Seminar, held inSendai, Japan, July 24-28, 2001 Bibcode: 2004sgyu.conf...61T

[8] Ichi Tanaka, Toru Yamada, Edwin L. Turner, Yasushi Suto (2000) "Superclustering of Faint Galaxies in the Field of a QSO Concentration atz ~ 1.1" (http:/ / www. iop. org/ EJ/ article/ 0004-637X/ 547/ 2/ 521/ 52628. text. html) The Astrophysical Journal, 547:521-530

[9] Lori M. Lubin et al (2000) "A Definitive Optical Detection of a Supercluster at z~0.91" (http:/ / www. iop. org/ EJ/ article/ -link=10009726/1538-4357/ 531/ 1/ L5/ 995833. web. pdf?request-doi=f88e19d1-e5ad-494f-82fa-209b71deb313) (PDF) The Astrophysical Journal,531:L5–L8 10.1086/312518

[10] A. J. Connolly et al (1996) "Superclustering at Redshift z = 0.54" (http:/ / www. iop. org/ EJ/ article/ -link=10009724/ 1538-4357/ 473/ 2/L67/ 5314. pdf?request-doi=57cce217-e8d5-46db-9f16-6aefdc96e347) (PDF) The Astrophysical Journal, 473:L67–L70 10.1086/310395

[11] University of Hawaii, "The MS0302+17 Supercluster" (http:/ / www. ifa. hawaii. edu/ ~kaiser/ pictures/ ms0302/ caption. html), NickKaiser. Retrieved 15 September 2009.

[12] Space.com, "The SPT-CL_J0546-5345 Supercluster" (http:/ / www. space. com/ scienceastronomy/most-massive-galaxy-cluster-distant-universe-101014. html), Mark Brodwin. Retrieved 16 October 2010.

External links• Overview of local superclusters (http:/ / www. atlasoftheuniverse. com/ superc. html)• The Nearest Superclusters (http:/ / www. atlasoftheuniverse. com/ nearsc. html)• Universe family tree: Supercluster (http:/ / www. astro. uu. nl/ ~strous/ AA/ en/ boom/ supercluster. html)

See also• Large-scale structure of the cosmos• Galaxy groups and clusters• Galaxy filament• Galaxy cloud• Galaxy

Galaxy filament 60

Galaxy filament

The present day dark matter distribution in a slice cut through a simulation of a flatuniverse with cosmological constant, using the overall pattern of structure in ourlocal neighbourhood as a simulation constraint[1] . The distribution reveals fine,

filamentary structures. The slice has a side length of 520 million light years, and athickness of 100 million light years. It contains the so-called "supergalactic plane".

The major nearby clusters, like Coma, Virgo, Perseus cluster, are labelled.[2]

In physical cosmology, filaments are thelargest known structures in the universe,thread-like structures with a typical lengthof 50 to 80 megaparsecs h-1 that form theboundaries between large voids in theuniverse.[3] Filaments consist ofgravitationally-bound galaxies; parts wherea large number of galaxies are very close toeach other are called superclusters.

Discoveries about "hyperclusters" (clustersof superclusters) started in the 1980s. In1987 astronomer R. Brent Tully of theUniversity of Hawaii’s Institute ofAstronomy identified what he called thePisces-Cetus Supercluster Complex.[4] [5] In1989 the CfA2 Great Wall was discovered,[6] followed by the Sloan Great Wall in2003.[7]

In 2006, scientists announced the discoveryof three filaments aligned to form the largeststructure known to humanity, composed ofdensely-packed galaxies and enormousblobs of gas known as Lyman alpha blobs.[8]

List

Filaments

Filament subtype of filaments have roughly similar major and minor axes in cross-section, along the lengthwise axis.

Filaments of Galaxies

Filament Date

Mean Distance Dimension

Notes

Coma Filament The Coma Supercluster lies within the Coma Filament. [9] It formspart of the CfA2 Great Wall.[10]

Perseus-Pegasus Filament 1985 Connected to the Pisces-Cetus Supercluster, with thePerseus-Pisces Supercluster being a member of the filament. [11]

Ursa Major Filament Connected to the CfA Homunculus, a portion of the filament formsa portion of the "leg" of the Homunculus. [12]

Lynx-Ursa Major Filament(LUM Filament)

1999 from 2000km/s to8000km/s in redshiftspace

Connected to and separate from the Lynx-Ursa Major Supercluster.[12]

Galaxy filament 61

z=2.38 filament aroundprotocluster ClG J2143-4423

2004 z=2.38 110Mpc A filament the length of the Great Wall was discovered in 2004. Asof 2008, it was still the largest structure beyond redshift 2. [13] [14][15] [16]

Galaxy wallsThe Galaxy wall subtype of filaments have a significantly greater major axis than minor axis in cross-section, alongthe lengthwise axis.

Walls of Galaxies

Wall Date

MeanDistance

Dimension Notes

CfA2 Great Wall (ComaWall, Great Wall, NorthernGreat Wall, Great NorthernWall, CfA Great Wall)

1989 z=0.03058 251Mpc long

750 Mlylong250 Mlywide20 Mlythick

This was the first super-large large-scale structure or pseudo-structure in theuniverse to be discovered. It is also the second largest. The CfA Homunculuslies at the heart of the Great Wall, and the Coma Supercluster forms most ofthe homunculus structure. The Coma Cluster lies at the core. [17] [18]

Sloan Great Wall (SDSSGreat Wall)

2005 z=0.07804 433Mpc long This is the largest known structure or pseudo-structure in the universediscovered so far. [17]

Sculptor Wall (SouthernGreat Wall, Great SouthernWall, Southern Wall)

8000km/s long5000km/s wide1000km/s deep(in redshiftspacedimensions)

The Sculptor Wall is "parallel" to the Fornax Wall and "perpendicular" to theGrus Wall. [19] [20]

Grus Wall The Grus Wall is "perpendicular" to the Fornax and Sculptor Walls. [20]

Fornax Wall The Fornax Cluster is part of this wall. The wall is "parallel" to the SculptorWall and "perpendicular" to the Grus Wall. [19] [20]

• A Centaurus Wall or Centaurus Great Wall has been proposed, and would have the Fornax Wall as a portion of it,visually created by the Zone of Avoidance. It would also include the Centaurus Supercluster and the LocalSupercluster, (this would then be the Local Wall or Local Great Wall) [19] [20]

• A wall has been proposed to be the physical embodiment of the Great Attractor, with the Norma Cluster as part ofthis wall. This wall is also referred to as the Great Attractor Wall or Norma Wall. [21]

• A wall has been proposed, in 2000, to lie at z=1.47 in the vicinity of radio galaxy B3 0003+387. [22]

• A wall has been proposed, in 2000, to lie at z=0.559 in the northern Hubble Deep Field (HDF North). [23] [24]

Galaxy filament 62

Map of nearest galaxy walls

The Universe within 500 million Light Years, showing the nearest galaxy walls

Maps of large scale distribution

The universewithin 1 billionlight-years (307Mpc) of Earth,showing localsuperclusters

forming filamentsand voids.

Map of nearestwalls, voids and

superclusters.

2dF survey map,containing the SDSS

Great Wall

2MASS XSC infrared sky map

Galaxy filament 63

See also• Galaxy• Galaxy clusters• Galaxy superclusters• List of galaxy clusters• List of galaxies• List of galaxy superclusters• Large-scale structure of the universe• Void (astronomy)

References[1] "Simulating the Local Galaxy Population" (http:/ / www. mpa-garching. mpg. de/ HIGHLIGHT/ 2001/ highlight0107_e. html).

Max-Planck-Institut für Astrophysik. .[2] http:/ / www. mpa-garching. mpg. de/ galform/ data_vis/[3] Bharadwaj, Somnath; Bhavsar, Suketu; Sheth, Jatush V. The Size of the Longest Filaments in the Universe (http:/ / www. journals. uchicago.

edu/ doi/ full/ 10. 1086/ 382140). Astrophys.J. 606 (2004) 25-31[4] Massive Clusters of Galaxies Defy Concepts of the Universe N.Y. Times Tue. November 10, 1987: (http:/ / www. nytimes. com/ 1987/ 11/

10/ science/ massive-clusters-of-galaxies-defy-concepts-of-the-universe. html?pagewanted=all)[5] Map of the Pisces-Cetus Supercluster Complex: (http:/ / plasmascience. net/ tpu/ LargeScale. html)[6] M. J. Geller & J. P. Huchra, Science 246, 897 (1989). (http:/ / www. sciencemag. org/ cgi/ content/ abstract/ 246/ 4932/ 897)[7] Sky and Telescope, "Refining the Cosmic Recipe" (http:/ / www. skyandtelescope. com/ news/ 3308301. html?page=1& c=y), 14 November

2003[8] Than, Ker (2006-07-28). "Scientists: Cosmic blob biggest thing in universe" (http:/ / edition. cnn. com/ 2006/ TECH/ space/ 07/ 28/ universe.

blob/ index. html). SPACE.com. . Retrieved 2007-03-11.[9] 'Astronomy and Astrophysics' (ISSN 0004-6361), vol. 138, no. 1, Sept. 1984, p. 85-92. Research supported by Cornell University "The

Coma/A 1367 filament of galaxies" 09/1984 Bibcode: 1984A&A...138...85F[10] THE ASTRONOMICAL JOURNAL, 115:1745-1777, 1998 May ; THE STAR FORMATION PROPERTIES OF DISK GALAXIES: Hα

IMAGING OF GALAXIES IN THE COMA SUPERCLUSTER (http:/ / www. iop. org/ EJ/ article/ 1538-3881/ 115/ 5/ 1745/ 970234. text.html)

[11] 'Astrophysical Journal', Part 1 (ISSN 0004-637X), vol. 299, Dec. 1, 1985, p. 5-14. "A possible 300 megaparsec filament of clusters ofgalaxies in Perseus-Pegasus" 12/1985 Bibcode: 1985ApJ...299....5B

[12] 'The Astrophysical Journal Supplement Series', Volume 121, Issue 2, pp. 445-472. "Photometric Properties of Kiso Ultraviolet-ExcessGalaxies in the Lynx-Ursa Major Region" 04/1999 Bibcode: 1999ApJS..121..445T

[13] NASA, GIANT GALAXY STRING DEFIES MODELS OF HOW UNIVERSE EVOLVED (http:/ / www. nasa. gov/ centers/ goddard/news/ topstory/ 2004/ 0107filament. html), January 7, 2004

[14] 'The Astrophysical Journal', Volume 602, Issue 2, pp. 545-554. The Distribution of Lyα-Emitting Galaxies at z=2.38 02/2004Bibcode: 2004ApJ...602..545P doi:10.1086/381145

[15] 'The Astrophysical Journal', Volume 614, Issue 1, pp. 75-83. The Distribution of Lyα-emitting Galaxies at z=2.38. II. Spectroscopy10/2004 Bibcode: 2004ApJ...614...75F doi:10.1086/423417

[16] 'Relativistic Astrophysics Legacy and Cosmology - Einstein's, ESO Astrophysics Symposia', Volume . ISBN 978-3-540-74712-3.Springer-Verlag Berlin Heidelberg, 2008, p. 358 Ultraviolet-Bright, High-Redshift ULIRGS 00/2008 Bibcode: 2008ralc.conf..358W

[17] Chin. J. Astron. Astrophys. Vol. 6 (2006), No. 1, 35–42 Super-Large-Scale Structures in the Sloan Digital Sky Survey (http:/ / www. iop.org/ EJ/ article/ 1009-9271/ 6/ 1/ 004/ chjaa_6_1_004. pdf)PDF

[18] 'Scientific American', Vol. 280, No. 6, p. 30 - 37 Mapping the Universe (http:/ / cosmos. phy. tufts. edu/ ~zirbel/ ast21/ sciam/MappingUniverse. pdf)PDF (1.43 MB) 06/1999 Bibcode: 1999SciAm.280f..30L

[19] Unveiling large-scale structures behind the Milky Way. Astronomical Society of the Pacific Conference Series, Vol. 67; Proceedings of aworkshop at the Observatoire de Paris-Meudon; 18-21 January 1994; San Francisco: Astronomical Society of the Pacific (ASP); c1994; editedby Chantal Balkowski and R. C. Kraan-Korteweg, p.21 ; Visualization of Nearby Large-Scale Structures (http:/ / adsabs. harvard. edu/ full/1994ASPC. . . 67. . . 21F) ; Fairall, A. P., Paverd, W. R., & Ashley, R. P. ; 1994ASPC...67...21F

[20] 'Astrophysics and Space Science', Volume 230, Issue 1-2, pp. 225-235 Large-Scale Structures in the Distribution of Galaxies 08/1995Bibcode: 1995Ap&SS.230..225F

[21] World Science, Wall of galaxies tugs on ours, astronomers find (http:/ / www. world-science. net/ exclusives/ exclusives-nfrm/060419_attractor. htm) April 19, 2006

[22] 'The Astronomical Journal', Volume 120, Issue 5, pp. 2331-2337. B3 0003+387: AGN-Marked Large-Scale Structure at Redshift 1.47?11/2000 Bibcode: 2000AJ....120.2331T doi:10.1086/316827

Galaxy filament 64

[23] FermiLab, Astronomers Find Wall of Galaxies Traversing the Hubble Deep Field (http:/ / www. spaceref. ca/ news/ viewpr. html?pid=634),DARPA, Monday, January 24, 2000

[24] 'The Astronomical Journal', Volume 119, Issue 6, pp. 2571-2582 ; QSOS and Absorption-Line Systems surrounding the Hubble Deep Field(http:/ / www. iop. org/ EJ/ article/ 1538-3881/ 119/ 6/ 2571/ 990560. text. html) ; 06/2000 ; doi:10.1086/301404 ;Bibcode: 2000AJ....119.2571V ;

Further reading• arXiv, Pulling out Threads from the Cosmic Tapestry:Defining Filaments of Galaxies (http:/ / www. publish.

csiro. au/ ?act=view_file& file_id=AS05006. pdf)PDF, Kevin A. Pimbblet, 14 March 2005

External links• Pictures of the filamentary network (http:/ / pil. phys. uniroma1. it/ twiki/ bin/ view/ Pil/ GalaxyStructures)• Astronomical Institute / Utrecht University - Astronomy Answers - Universe Family Tree: Filament (http:/ /

www. astro. uu. nl/ ~strous/ AA/ en/ boom/ filament. html#filament) (Dr Louis Strous)• Astronomical Institute / Utrecht University - Astronomy Answers - From the Astronomical Dictionary - filament

(http:/ / www. astro. uu. nl/ ~strous/ cgi-bin/ glossary. cgi?l=en& o=filaments) (Dr Louis Strous)

65

Types of galaxies

Active galaxyAn active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normalluminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission hasbeen observed in the radio, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting anAGN is called an active galaxy. The radiation from AGN is believed to be a result of accretion of mass by thesupermassive black hole at the centre of the host galaxy. AGN are the most luminous persistent sources ofelectromagnetic radiation in the universe, and as such can be used as a means of discovering distant objects; theirevolution as a function of cosmic time also provides constraints on models of the cosmos.

Hubble Space Telescope image of a 5000 light-year long jet being ejected from theactive nucleus of the active galaxy M87, a radio galaxy. The blue synchrotron

radiation of the jet contrasts with the yellow starlight from the host galaxy.

Discovery

The issue of the Activity of Nuclei ofGalaxies (AGN) was first raised bysoviet-armenian physicist Prof. VictorAmbartsumian in the early 50s. Althoughthe idea concerning the activity of galacticnuclei for the first time was accepted veryskeptically, after many years, as a result ofthe pressure of observations (the discoveryof quasars, radio outbursts of galaxies,consequences of explosions in nuclei,ejection from nuclei, etc.) did it gainrecognition. The concept of AGN now iswidely accepted.[1]

Models of the active nucleus

For a long time it has been argued[2] thatAGN must be powered by accretion ontomassive black holes (with masses between106 and 1010 times that of the Sun). AGNare both compact and persistently extremelyluminous; accretion can potentially give very efficient conversion of potential and kinetic energy to radiation, and amassive black hole has a high Eddington luminosity, so that it can provide the observed high persistent luminosity.Central supermassive black holes are now believed to exist in the centers of most or all massive galaxies: the mass ofthe black hole correlates well with the velocity dispersion of the galaxy bulge (the M-sigma relation) or with bulgeluminosity (e.g.[3] ). Thus AGN-like characteristics are expected whenever a supply of material for accretion comeswithin the sphere of influence of the central black hole.

Active galaxy 66

Accretion diskIn the standard model of AGN, cold material close to the central black hole forms an accretion disc. Dissipativeprocesses in the accretion disc transport matter inwards and angular momentum outwards, while causing theaccretion disc to heat up. The expected spectrum of an accretion disc around a supermassive black hole peaks in theoptical-ultraviolet waveband; in addition, a corona of hot material forms above the accretion disc and caninverse-Compton scatter photons up to X-ray energies. The radiation from the accretion disc excites cold atomicmaterial close to the black hole and this radiates via emission lines. A large fraction of the AGN's primary outputmay be obscured by interstellar gas and dust close to the accretion disc, but (in a steady-state situation) this will bere-radiated at some other waveband, most likely the infrared.

Relativistic jetsAt least some accretion discs produce jets, twin highly collimated and fast outflows that emerge in oppositedirections from close to the disc (the direction of the jet ejection must be determined either by the angularmomentum axis of the disc or the spin axis of the black hole). The jet production mechanism and indeed the jetcomposition on very small scales are not known at present, as observations cannot distinguish between the varioustheoretical models that exist. The jets have the most obvious observational effects in the radio waveband, whereVery Long Baseline Interferometry can be used to study the synchrotron radiation they emit down to sub-parsecscales. However, they radiate in all wavebands from the radio through to the gamma-ray via the synchrotron andinverse-Compton process, and so AGN with jets have a second potential source of any observed continuumradiation.

Radiatively inefficient AGNThere exists a class of 'radiatively inefficient' solutions to the equations that govern accretion. The most widelyknown of these is the Advection Dominated Accretion Flow (ADAF),[4] but others exist. In this type of accretion,which is important for accretion rates well below the Eddington limit, the accreting matter does not form a thin discand consequently does not radiate away the energy that it has acquired in moving close to the black hole. Radiativelyinefficient accretion has been used to explain the lack of strong AGN-type radiation from massive black holes in thecentres of elliptical galaxies in clusters, where otherwise we might expect high accretion rates and correspondinghigh luminosities[5] . Radiatively inefficient AGN would be expected to lack many of the characteristic features ofstandard AGN with an accretion disc.

Observational characteristicsThere is no single observational signature of an AGN. The list below covers some of the historically importantfeatures that have allowed systems to be identified as AGN.• Nuclear optical continuum emission. This is visible whenever we have a direct view of the accretion disc. Jets can

also contribute to this component of the AGN emission. The optical emission has a roughly power-lawdependence on wavelength.

• Nuclear infra-red emission. This is visible whenever the accretion disc and its environment are obscured by gasand dust close to the nucleus and then re-emitted ('reprocessing'). As it is thermal emission, it can be distinguishedfrom any jet or disc-related component.

• Broad optical emission lines. These come from cold material close to the central black hole. The lines are broadbecause the emitting material is revolving around the black hole with high speeds, emitting photons at varyingDoppler shifts.

• Narrow optical emission lines. These come from more distant cold material, and so are narrower than the broadlines.

Active galaxy 67

• Radio continuum emission. This is always due to a jet. It shows a spectrum characteristic of synchrotronradiation.

• X-ray continuum emission. This can arise both from a jet and from the hot corona of the accretion disc viascattering processes: in both cases it shows a power-law spectrum. In some radio-quiet AGN there is a `softexcess' in the X-ray emission in addition to the power-law component. The origin of the soft excess is not clear atpresent.

• X-ray line emission. This is a result of illumination of cold heavy elements by the X-ray continuum. Fluorescencegives rise to various emission lines, the best-known of which is the iron feature around 6.4 keV. This line may benarrow or broad: relativistically broadened iron lines can be used to study the dynamics of the accretion disc veryclose to the nucleus and therefore the nature of the central black hole.

Types of active galaxyIt is convenient to divide AGN into two classes, conventionally called radio-quiet and radio-loud. In the radio-loudobjects a contribution from the jet(s) and the lobes they inflate dominates the luminosity of the AGN, at least at radiowavelengths but possibly at some or all others. Radio-quiet objects are simpler since jet and jet-related emission canbe neglected.AGN terminology is often confusing, since the distinctions between different types of AGN sometimes reflecthistorical differences in how objects were discovered or initially classified, rather than real physical differences.

Radio-quiet AGN• Low-ionization nuclear emission-line regions (LINERs). As the name suggests, these systems show only weak

nuclear emission-line regions, and no other signatures of AGN emission. It is debatable whether all such systemsare true AGN (powered by accretion on to a supermassive black hole). If they are, they constitute thelowest-luminosity class of radio-quiet AGN. Some may be radio-quiet analogues of the low-excitation radiogalaxies (see below).

• Seyfert galaxies. Seyferts were the earliest distinct class of AGN to be identified. They show optical nuclearcontinuum emission, narrow and (sometimes) broad emission lines, (sometimes) strong nuclear X-ray emissionand sometimes a weak small-scale radio jet. Originally they were divided into two types known as Seyfert 1 and2: Seyfert 1s show strong broad emission lines while Seyfert 2s do not, and Seyfert 1s are more likely to showstrong low-energy X-ray emission. Various forms of elaboration on this scheme exist: for example, Seyfert 1swith relatively narrow broad lines are sometimes referred to as narrow-line Seyfert 1s. The host galaxies ofSeyferts are usually spiral or irregular galaxies.

• Radio-quiet quasars/QSOs. These are essentially more luminous versions of Seyfert 1s: the distinction is arbitraryand is usually expressed in terms of a limiting optical magnitude. Quasars were originally 'quasi-stellar' in opticalimages, and so had optical luminosities that were greater than that of their host galaxy. They always show strongoptical continuum emission, X-ray continuum emission, and broad and narrow optical emission lines. Someastronomers use the term QSO (Quasi-Stellar Object) for this class of AGN, reserving 'quasar' for radio-loudobjects, while others talk about radio-quiet and radio-loud quasars. The host galaxies of quasars can be spirals,irregulars or ellipticals: there is a correlation between the quasar's luminosity and the mass of its host galaxy, sothat the most luminous quasars inhabit the most massive galaxies (ellipticals).

• 'Quasar 2s'. By analogy with Seyfert 2s, these are objects with quasar-like luminosities but without strong opticalnuclear continuum emission or broad line emission. They are hard to find in surveys, though a number of possiblecandidate quasar 2s have been identified.

Active galaxy 68

Radio-loud AGNSee main article radio galaxies for discussion of the large-scale behaviour of the jets. Here only the active nuclei arediscussed.• Radio-loud quasars. These behave exactly like radio-quiet quasars with the addition of emission from a jet. Thus

they show strong optical continuum emission, broad and narrow emission lines, and strong X-ray emission,together with nuclear and often extended radio emission.

• 'Blazars' (BL Lac objects and OVV quasars). These classes are distinguished by rapidly variable, polarizedoptical, radio and X-ray emission. BL Lac objects show no optical emission lines, broad or narrow, so that theirredshifts can only be determined from features in the spectra of their host galaxies. The emission-line featuresmay be intrinsically absent or simply swamped by the additional variable component: in the latter case, emissionlines may become visible when the variable component is at a low level.[6] OVV quasars behave more likestandard radio-loud quasars with the addition of a rapidly variable component. In both classes of source, thevariable emission is believed to originate in a relativistic jet oriented close to the line of sight. Relativistic effectsamplify both the luminosity of the jet and the amplitude of variability.

• Radio galaxies. These objects show nuclear and extended radio emission. Their other AGN properties areheterogeneous. They can broadly be divided into low-excitation and high-excitation classes.[7] [8] Low-excitationobjects show no strong narrow or broad emission lines, and the emission lines they do have may be excited by adifferent mechanism.[9] Their optical and X-ray nuclear emission is consistent with originating purely in a jet.[10]

[11] They may be the best current candidates for AGN with radiatively inefficient accretion. By contrast,high-excitation objects (narrow-line radio galaxies) have emission-line spectra similar to those of Seyfert 2s. Thesmall class of broad-line radio galaxies, which show relatively strong nuclear optical continuum emission[12]

probably includes some objects that are simply low-luminosity radio-loud quasars. The host galaxies of radiogalaxies, whatever their emission-line type, are essentially always ellipticals.

SummaryThese galaxies can be broadly summarised by the following table:

Differences between active galaxy types and normal galaxies.

Galaxy Type Active Nuclei Emission Lines X-rays Excess of Strong Radio Jets Variable Radio loud

Narrow Broad UV Far-IR

Normal no weak none weak none none none none no no

Starburst no yes no some no yes some no no no

Seyfert I yes yes yes some some yes no no yes no

Seyfert II yes yes no some some yes no yes yes no

Quasar yes yes yes some yes yes some some yes 10%

Blazar yes no some yes yes no yes yes yes yes

BL Lac yes no none/faint yes yes no yes yes yes yes

OVV yes no stronger than BL Lac yes yes no yes yes yes yes

Radio galaxy yes some some some some yes yes yes yes yes

Active galaxy 69

Unification

Unification by viewing angle. From bottom totop: down the jet - Blazar, at an angle to thejet - Quasar/Seyfert 1 Galaxy, at 90 degrees

from the jet - Radio galaxy / Seyfert 2Galaxy[13]

Unified models of AGN unite two or more classes of objects, based on thetraditional observational classifications, by proposing that they are reallya single type of physical object observed under different conditions. Thecurrently favoured unified models are 'orientation-based unified models'meaning that they propose that the apparent differences between differenttypes of objects arise simply because of their different orientations to theobserver. For an overview of these see[14] and [15] , though some details inthe discussion below have emerged since these reviews were written.

Radio-quiet unification

At low luminosities, the objects to be unified are Seyfert galaxies. Theunified models propose that in Seyfert 1s the observer has a direct view ofthe active nucleus. In Seyfert 2s it is observed through an obscuringstructure which prevents a direct view of the optical continuum,broad-line region or (soft) X-ray emission. The key insight oforientation-dependent accretion models is that the two types of object canbe the same if only certain angles to the line of sight are observed. Thestandard picture is of a torus of obscuring material surrounding theaccretion disc. It must be large enough to obscure the broad-line regionbut not large enough to obscure the narrow-line region, which is seen inboth classes of object. Seyfert 2s are seen through the torus. Outside thetorus there is material that can scatter some of the nuclear emission intoour line of sight, allowing us to see some optical and X-ray continuumand, in some cases, broad emission lines—which are strongly polarized,showing that they have been scattered and proving that some Seyfert 2sreally do contain hidden Seyfert 1s. Infrared observations of the nuclei ofSeyfert 2s also support this picture.

At higher luminosities, quasars take the place of Seyfert 1s, but, as already mentioned, the corresponding 'quasar 2s'are elusive at present. If they do not have the scattering component of Seyfert 2s they would be hard to detect exceptthrough their luminous narrow-line and hard X-ray emission.

Radio-loud unification

Historically work on radio-loud unification has concentrated on high-luminosity radio-loud quasars. These can beunified with narrow-line radio galaxies in a manner directly analoguous to the Seyfert 1/2 unification (but withoutthe complication of much in the way of a reflection component: narrow-line radio galaxies show no nuclear opticalcontinuum or reflected X-ray component, although they do occasionally show polarized broad-line emission). Thelarge-scale radio structures of these objects provide compelling evidence that the orientation-based unified modelsreally are true.[16] [17] [18] X-ray evidence, where available, supports the unified picture: radio galaxies showevidence of obscuration from a torus, while quasars do not, although care must be taken since radio-loud objects alsohave a soft unabsorbed jet-related component, and high resolution is necessary to separate out thermal emission fromthe sources' large-scale hot-gas environment.[19] At very small angles to the line of sight, relativistic beamingdominates, and we see a blazar of some variety.

Active galaxy 70

However, the population of radio galaxies is completely dominated by low-luminosity, low-excitation objects. Thesedo not show strong nuclear emission lines — broad or narrow — they have optical continua which appear to beentirely jet-related,[10] and their X-ray emission is also consistent with coming purely from a jet, with no heavilyabsorbed nuclear component in general.[11] These objects cannot be unified with quasars, even though they includesome high-luminosity objects when looking at radio emission, since the torus can never hide the narrow-line regionto the required extent, and since infrared studies show that they have no hidden nuclear component:[20] in fact thereis no evidence for a torus in these objects at all. Most likely, they form a separate class in which only jet-relatedemission is important. At small angles to the line of sight, they will appear as BL Lac objects.[21]

Cosmological uses and evolutionFor a long time, active galaxies held all the records for the highest-redshift objects known, because of their highluminosity (either in the optical or the radio): they still have a role to play in studies of the early universe, but it isnow recognised that by its nature an AGN gives a highly biased picture of the 'typical' high-redshift galaxy.More interesting is the study of the evolution of the AGN population. Most luminous classes of AGN (radio-loudand radio-quiet) seem to have been much more numerous in the early universe. This suggests (1) that massive blackholes formed early on and (2) that the conditions for the formation of luminous AGN were more readily available inthe early universe — for example, that there was a much higher availability of cold gas near the centre of galaxiesthan there is now. It also implies, of course, that many objects that were once luminous quasars are now much lessluminous, or entirely quiescent. The evolution of the low-luminosity AGN population is much less well constrainedbecause of the difficulty of detecting and observing these objects at high redshifts.

See also• Radio galaxy• Quasar• Supermassive black hole• M-sigma relation• Relativistic jet

References[1] http:/ / www. astroscu. unam. mx/ massive_stars/ news/ news24. pdf[2] Lynden-Bell, D. (1969). "Galactic Nuclei as Collapsed Old Quasars". Nature 223 (5207): 690–694. doi:10.1038/223690a0.[3] Marconi, A.; L. K. Hunt (2003). "The Relation between Black Hole Mass, Bulge Mass, and Near-Infrared Luminosity". The Astrophysical

Journal 589 (1): L21–L24. doi:10.1086/375804.[4] Narayan, R.; I. Yi (1994). "Advection-Dominated Accretion: A Self-Similar Solution". Journal reference: Astrophys. J 428: L13.[5] Fabian, A. C.; M. J. Rees (1995). "The accretion luminosity of a massive black hole in an elliptical galaxy". Monthly Notices of the Royal

Astronomical Society 277 (2): L55–L58.[6] Vermeulen, R. C.; P. M. Ogle, H. D. Tran, I. W. A. Browne, M. H. Cohen, A. C. S. Readhead, G. B. Taylor, R. W. Goodrich (1995). "When

Is BL Lac Not a BL Lac?". The Astrophysical Journal Letters 452 (1): 5–8.[7] HINE, RG; MS LONGAIR (1979). "Optical spectra of 3 CR radio galaxies". Royal Astronomical Society, Monthly Notices 188: 111–130.[8] Laing, R. A.; C. R. Jenkins, J. V. Wall, S. W. Unger (1994). "Spectrophotometry of a Complete Sample of 3CR Radio Sources: Implications

for Unified Models". The First Stromlo Symposium: the Physics of Active Galaxies. ASP Conference Series, 54.[9] Baum, S. A.; E. L. Zirbel, C. P. O'Dea (1995). "Toward Understanding the Fanaroff-Riley Dichotomy in Radio Source Morphology and

Power". The Astrophysical Journal 451: 88. doi:10.1086/176202.[10] Chiaberge, M.; A. Capetti, A. Celotti (2002). "Understanding the nature of FRII optical nuclei: a new diagnostic plane for radio galaxies".

Journal reference: Astron. Astrophys 394: 791–800. doi:10.1051/0004-6361:20021204.[11] Hardcastle, M. J.; D. A. Evans, J. H. Croston (2006). "The X-ray nuclei of intermediate-redshift radio sources". Monthly Notices of the

Royal Astronomical Society 370 (4): 1893–1904. doi:10.1111/j.1365-2966.2006.10615.x.[12] Grandi, S. A.; D. E. Osterbrock (1978). "Optical spectra of radio galaxies". Astrophysical Journal 220 (Part 1).[13] http:/ / www. whatsnextnetwork. com/ technology/ media/ active_galactic_nuclei. jpg

Active galaxy 71

[14] Antonucci, R. (1993). "Unified Models for Active Galactic Nuclei and Quasars". Annual Reviews in Astronomy and Astrophysics 31 (1):473–521. doi:10.1146/annurev.aa.31.090193.002353.

[15] Urry, P.; Paolo Padovani (1995). "Unified schemes for radio�loud AGN". Publications of the Astronomical Society of the Pacific 107:803–845. doi:10.1086/133630.

[16] Laing, R. A. (1988). "The sidedness of jets and depolarization in powerful extragalactic radio sources". Nature 331 (6152): 149–151.doi:10.1038/331149a0.

[17] Garrington, S. T.; J. P. Leahy, R. G. Conway, RA LAING (1988). "A systematic asymmetry in the polarization properties of double radiosources with one jet". Nature 331 (6152): 147–149. doi:10.1038/331147a0.

[18] Barthel, P. D. (1989). "Is every quasar beamed?". Astrophysical Journal 336: 606–611. doi:10.1086/167038.[19] Belsole, E.; D. M. Worrall, M. J. Hardcastle (2006). "High-redshift Faranoff-Riley type II radio galaxies: X-ray properties of the cores".

Monthly Notices of the Royal Astronomical Society 366 (1): 339–352. doi:10.1111/j.1365-2966.2005.09882.x.[20] Ogle, P.; D. Whysong, R. Antonucci (2006). "Spitzer Reveals Hidden Quasar Nuclei in Some Powerful FR II Radio Galaxies". The

Astrophysical Journal 647 (1): 161–171. doi:10.1086/505337.[21] Browne, I. W. A. (1983). "Is it possible to turn an elliptical radio galaxy into a BL Lac object?". Royal Astronomical Society, Monthly

Notices (ISSN 0035-8711), 204: 23P–27P.

External links• Media related to Active galactic nuclei at Wikimedia Commons

Barred lenticular galaxy

NGC 2787 is an example of a barred lenticular galaxy

A barred lenticular galaxy is a lenticularversion of a barred spiral galaxy. They havethe Hubble type of SB0

See also

• Unbarred lenticular galaxy

Barred irregular galaxy 72

Barred irregular galaxyA barred irregular galaxy is an irregular version of a barred spiral galaxy. Examples include the Large MagellanicCloud[1] and NGC 6822.[2] Some barred irregular galaxies (like the Large Magellanic Cloud) may actually be dwarfspiral galaxies, which have been distorted into an irregular shape by tidal interactions with a more massive neighbor.

References[1] Sidney van den Bergh, The Local Group of Galaxies, National Research Council of Canada (http:/ / arxiv. org/ pdf/ astro-ph/ 9908050)[2] Norbert Przybilla, Quantitative Spectroscopy of Supergiants, Munich, 2002 (http:/ / edoc. ub. uni-muenchen. de/ archive/ 00000082/ 01/

Przybilla_Norbert. pdf)

Barred spiral galaxy

NGC 1300, viewed nearly face-on; Hubble Space Telescope image.

A barred spiral galaxy is a spiralgalaxy with a central bar-shapedstructure composed of stars. Bars arefound in approximately two-thirds ofall spiral galaxies.[1] Bars generallyaffect both the motions of stars andinterstellar gas within spiral galaxiesand can affect spiral arms as well.[1]

Edwin Hubble classified these types ofspiral galaxies as "SB" (Spiral, Barred)in his Hubble sequence, and arrangedthem into three sub-categories basedon how open the arms of the spiral are.SBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SBbtype galaxies lie in between. A fourth type, SBm, was subsequently created to describe somewhat irregular barredspirals, such as the Magellanic Cloud galaxies, which were once classified as irregular galaxies, but have since beenfound to contain barred spiral structures. Among other types in Hubble's classifications for the galaxies are: spiralgalaxy, elliptical galaxy and irregular galaxy.

In 2005, observations by the Spitzer Space Telescope backed up previously collected evidence that suggested the

Barred spiral galaxy 73

The Sculptor Galaxy, a barred spiral starburst galaxy, (2MASS).

Milky Way is a barred spiral galaxy.Observations by radio telescopes hadfor years suggested our galaxy to bebarred, but Spitzer's vision in theinfrared region of the spectrum hasprovided a more definite calculation.

The bars

Barred spiral galaxies are relativelycommon, with surveys showing that upto two-thirds of all spiral galaxiescontain a bar.[2] The current hypothesisis that the bar structure acts as a type ofstellar nursery, fueling star birth attheir centers. The bar is thought to actas a mechanism that channels gasinwards from the spiral arms throughorbital resonance, in effect funnelingthe flow to create new stars.[3] This process is also thought to explain why many barred spiral galaxies have activegalactic nuclei, such as that seen in the Southern Pinwheel Galaxy.

The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxywhose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, whichcreates a self-perpetuating bar structure.[4] Another possible cause of bar creation is gravitational disruptionsbetween galaxies or a collision of two galaxies.Bars are thought to be a temporary phenomenon in the life of spiral galaxies, the bar structure decaying over time,transforming the galaxy from a barred spiral to a "regular" spiral pattern. Past a certain size the accumulated mass ofthe bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated intheir center tend to have short, stubby bars.[5] Since so many spiral galaxies have a bar structure, it is likely that it is arecurring phenomenon in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barredspiral galaxy is thought to take on the average about two billion years.[6]

Recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the "formative years"end. A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology in Pasadenadiscovered that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70percent of their modern counterparts.[7]

The bulgesStudying the core of the Milky Way, scientists found out that the Milky Way's bulge was peanut-shaped. This led tothe conclusion that all barred spiral galaxies have a peanut shaped bulge. When observing a distant spiral galaxy witha rotational axis perpendicular to the line of sight, or one that appears "edge-on" to the observer, the shape of thebulge can be easily observed, and therefore quickly classified as either a barred spiral or a regular spiral. GalaxyNGC 4565 has been tentatively classified as a barred spiral galaxy using this method.[8]

Barred spiral galaxy 74

Grades

Under the de Vaucouleurs classification system, SB-galaxies are one of threetypes of spiral galaxy

Example Type Image Information Notes

SB0- SB0- is a type of lenticular galaxy

SB0 SB0 is a type of lenticular galaxy

SB0+ SB0+ is a type of lenticular galaxy

SB0/a SB0/a can also be considered a type of barred lenticular galaxy

NGC 4314 SBa This is actually an "SB(rs)a"

NGC 4921 SBab This is actually an "SB(rs)ab"

Messier 95 SBb This is actually an "SB(r)b"

NGC 3953 SBbc This is actually an "SB(r)bc"

Barred spiral galaxy 75

NGC 1073 SBc This is actually an "SB(rs)c"

Messier 108 SBcd This is actually an "SB(s)cd"

NGC 2903 SBd This is actually an "SB(s)d"

NGC 5398 SBdm SBdm can also be considered a type of barred Magellanic spiral This is actually an "SB(rs)dm"

NGC 55 SBm SBm is a type of Magellanic spiral (Sm) This is actually an "SB(s)m"

Examples

Name Type Constellation

M58 SBc Virgo

M91 SBb Coma Berenices

M95 SBb Leo

M109 SBb Ursa Major

NGC 1300 SBbc Eridanus

NGC 1365 SBc Fornax

Magellanic Clouds SBm Dorado, Tucana

See also• Galaxy morphological classification• Galaxy formation and evolution• Lenticular galaxy• Spiral galaxy• Firehose instability

Barred spiral galaxy 76

External links• Britt, Robert Roy. "Milky Way’s Central Structure Seen with Fresh Clarity." [9] SPACE.com [10] 16 August 2005.

• An article about the Spitzer Space Telescope's Milky Way discovery• Devitt, Terry. "Galactic survey reveals a new look for the Milky Way." [11] 16 August 2005.

• The original press release regarding the article above, from the Univ. of Wisconsin• SPACE.com staff writers. "'Barred' Spiral Galaxy Pic Highlights Stellar Birth." [12] SPACE.com [10] 2 March

2001.• Hastings, George and Jane Hastings. Classifying Galaxies: Barred Spirals [13], 1995.• Buta, Ronald, D. A. Crocker, and G. G. Byrd. "Astronomers Find Multiple Generations of Star Formation in

Central Starburst Ring of a Barred Spiral Galaxy." [14] January 15, 2000.• A press release concerning NGC 1326

• Barred spirals come and go [15] Sky & Telescope April 2002.• "ESO Provides An Infrared Portrait of the Barred Spiral Galaxy Messier 83." [16] November 29, 2001.

• A press release from the European Southern Observatory.• 04/03/07: Hubble: Barred Spiral Galaxy NGC 1672 [17]

References[1] D. Mihalas (1968). Galactic Astronomy. W. H. Freeman. ISBN 9780716703266.[2] P. B. Eskridge, J. A. Frogel (1999). "What is the True Fraction of Barred Spiral Galaxies?" (http:/ / adsabs. harvard. edu/ abs/ 1999Ap& SS.

269. . 427E). Astrophysics and Space Science 269/270: 427–430. doi:10.1023/A:1017025820201. .[3] J. H. Knapen, D. Pérez-Ramírez, S. Laine (2002). "Circumnuclear regions in barred spiral galaxies - II. Relations to host galaxies" (http:/ /

adsabs. harvard. edu/ abs/ 2002MNRAS. 337. . 808K). Monthly Notice of the Royal Astronomical Society 337 (3): 808–828.doi:10.1046/j.1365-8711.2002.05840.x. .

[4] F. Bournaud, F. Combes (2002). "Gas accretion on spiral galaxies: Bar formation and renewal" (http:/ / adsabs. harvard. edu/ abs/ 2002A& A.. . 392. . . 83B). Astronomy and Astrophysics 392: 83–102. doi:10.1051/0004-6361:20020920. .

[5] Barred Spirals Come and Go (http:/ / web. archive. org/ web/ 20020512044348/ http:/ / www. govertschilling. nl/ artikelen/ archief/ 2002/0204/ 020401_st. htm), Sky and Telescope, April 2002

[6] Ripples in a Galactic Pond (http:/ / www. sciamdigital. com/ index. cfm?fa=Products. ViewIssuePreview&ARTICLEID_CHAR=3BC08F0C-2B35-221B-67A9F2AE04AFC79A), Scientific American, October 2005

[7] Barred Spiral Galaxies are Latecomers to the Universe (http:/ / newswise. com/ articles/ view/ 542997/ ) Newswise, Retrieved on July 29,2008.

[8] INTERMEDIATE-BAND SURFACE PHOTOMETRY OF THE EDGE-ON GALAXY NGC 4565 at http:/ / www. iop. org/ EJ/ article/1538-3881/ 123/ 3/ 1364/ 201272. text. html

[9] http:/ / www. space. com/ scienceastronomy/ 050816_milky_way. html[10] http:/ / www. space. com/[11] http:/ / www. news. wisc. edu/ 11405. html[12] http:/ / www. space. com/ scienceastronomy/ astronomy/ hubble_bar_010302. html[13] http:/ / www. smv. org/ hastings/ bsmain. htm[14] http:/ / bama. ua. edu/ ~rbuta/ press-release. html[15] http:/ / www. govertschilling. nl/ artikelen/ archief/ 2002/ 0204/ 020401_st. htm[16] http:/ / www. spaceref. com/ news/ viewpr. html?pid=6736[17] http:/ / www. exploration-space. com/ 03-apr-2007-esa-2. html

Blazar 77

BlazarA blazar (blazing quasi-stellar object) is a very compact quasar (quasi-stellar object) associated with a presumedsupermassive black hole at the center of an active, giant elliptical galaxy. Blazars are among the most violentphenomena in the universe and are an important topic in extragalactic astronomy.Blazars are members of a larger group of active galaxies, also termed active galactic nuclei (AGN). A few rareobjects may be "intermediate blazars" that appear to have a mixture of properties from both OVV quasars and BLLac objects. The name "blazar" was originally coined in 1978 by astronomer Edward Spiegel to denote thecombination of these two classes.Blazars are AGN with a relativistic jet that is pointing in the general direction of the Earth. We observe "down" thejet, or nearly so, and this accounts for the rapid variability and compact features of both types of blazars. Manyblazars have apparent superluminal features within the first few parsecs of their jets, probably due to relativisticshock fronts.[1]

The generally accepted picture is that OVV quasars are intrinsically powerful radio galaxies while BL Lac objectsare intrinsically weak radio galaxies. In both cases the host galaxies are giant ellipticals.Alternative models, for example, gravitational microlensing, may account for a few observations of some blazarswhich are not consistent with the general properties.

Structure

Blazars, like all AGN, are thought to be ultimately powered bymaterial falling onto a supermassive black hole at the center of the hostgalaxy. Gas, dust and the occasional star are captured and spiral intothis central black hole creating a hot accretion disk which generatesenormous amounts of energy in the form of photons, electrons,positrons and other elementary particles. This region is quite small, approximately 10−3 parsecs in size.

There is also a larger opaque toroid extending several parsecs from the central black hole, containing a hot gas withembedded regions of higher density. These "clouds" can absorb and then re-emit energy from regions closer to theblack hole. On Earth the clouds are detected as emission lines in the blazar spectrum.Perpendicular to the accretion disk, a pair of relativistic jets carries a highly energetic plasma away from the AGN.The jet is collimated by a combination of intense magnetic fields and powerful winds from the accretion disk andtoroid. Inside the jet, high energy photons and particles interact with each other and the strong magnetic field. Theserelativistic jets can extend as far as many tens of kiloparsecs from the central black hole.All of these regions can produce a variety of observed energy, mostly in the form of a nonthermal spectrum rangingfrom very low frequency radio to extremely energetic gamma rays, with a high polarization (typically a few percent)at some frequencies. The nonthermal spectrum consists of synchrotron radiation in the radio to X-ray range, andinverse Compton emission in the X-ray to gamma-ray region. A thermal spectrum peaking in the ultraviolet regionand faint optical emission lines are also present in OVV quasars, but faint or non-existent in BL Lac objects.

Blazar 78

Relativistic Beaming

Viewing angle - 1. at 90 degrees to the jet::Radio galaxy / Seyfert 2 Galaxy; 2, 3. at anangle to the jet: Quasar/Seyfert 1 Galaxy; 4.

down the jet: Blazar.[2]

The observed emission from a Blazar is greatly enhanced by relativisticeffects in the jet, a process termed relativistic beaming.The bulk speed ofthe plasma that constitutes the jet can be in the range of 95%–99% of thespeed of light. (This bulk velocity is not the speed of a typical electron orproton in the jet. The individual particles move in many directions withthe result being that the net speed for the plasma is in the rangementioned.)

The relationship between the luminosity emitted in the rest frame of thejet and the luminosity observed from Earth depends on the characteristicsof the jet. These include whether the luminosity arises from a shock frontor a series of brighter blobs in the jet, as well as details of the magneticfields within the jet and their interaction with the moving particles.A simple model of beaming however, illustrates the basic relativisticeffects connecting the luminosity emitted in the rest frame of the jet, Seand the luminosity observed on Earth, So. These are connected by a termreferred to in astrophysics as the doppler factor, D, where So isproportional to Se × D2.

When looked at in much more detail than shown here, three relativisticeffects are at involved:• Relativistic Aberration contributes a factor of D2. Aberration is a

consequence of special relativity where directions which appearisotropic in the rest frame (in this case, the jet) appear pushed towardsthe direction of motion in the observer's frame (in this case, the Earth).

• Time Dilation contributed a factor of D+1. This effect speeds up theapparent release of energy. If the jet emits a burst of energy everyminute in its own rest frame this may be observed on Earth as being amuch faster release, perhaps one burst every ten seconds.

• Windowing can contribute a factor of D−1 and then works to decrease the amount of boosting. This happens for asteady flow, because there are then D fewer elements of fluid within the observed window, as each element hasbeen expanded by factor D. However, for a freely propagating blob of material, the radiation is boosted by the fullD+3.

An ExampleConsider a jet with an angle to the lines of sight θ = 5 degrees and a speed of 99.9% of the speed of light. On Earththe observed luminiosity is 70 times that of the emitted luminosity. However if θ is at the minimum value of 0degrees the jet will appear 600 times brighter from Earth.

Beaming AwayRelativistic beaming also has another critical consequence. The jet which is not approaching Earth will appeardimmer because of the same relativistic effects. Therefore two intrinsically identical jets will appear significantlyasymmetric. Indeed, in the example given above any jet where θ < 35 degrees will be observed on Earth as lessluminous than it would be from the rest frame of the jet.

Blazar 79

A further consequence is that a population of intrinsically identical AGN scattered in space with random jetorientations will look like a very inhomogeneous population on Earth. The few objects where θ is small will haveone very bright jet, while the rest will apparently have considerably weaker jets. Those where θ varies from 90° willappear to have asymmetric jets.This is the essence behind the connection between blazars and radio galaxies. AGN which have jets oriented close tothe line of sight with Earth can appear extremely different from other AGN even if they are intrinsically identical.

DiscoveryMany of the brighter blazars were first identified, not as powerful distant galaxies, but as irregular variable stars inour own galaxy. These blazars, like genuine irregular variable stars, changed in brightness on periods of days oryears, but with no pattern.The early development of radio astronomy had shown that there are numerous bright radio sources in the sky. By theend of the 1950s the resolution of radio telescopes was sufficient to be able to identify specific radio sources withoptical counterparts, leading to the discovery of quasars. Blazars were highly represented among these early quasars,and indeed the first redshift was found for 3C 273 — a highly variable quasar which is also a blazar.In 1968 a similar connection between the "variable star" BL Lacertae and a powerful radio source VRO 42.22.01[3]

was made. BL Lacertae shows many of the characteristics of quasars, but the optical spectrum was devoid of thespectral lines used to determine redshift. Faint indications of an underlying galaxy — proof that BL Lacertae was nota star — were found in 1974.The extragalactic nature of BL Lacertae was not a surprise. In 1972 a few variable optical and radio sources weregrouped together and proposed as a new class of galaxy: BL Lacertae-type objects. This terminology was soonshortened to "BL Lacertae object," "BL Lac object," or simply "BL Lac." (Note that the latter term can also mean theoriginal blazar and not the entire class.)As of 2003, a few hundred BL Lac objects are known.

Current visionBlazars are thought to be active galaxy nuclei, with relativistic jets oriented close to the line of sight with theobserver.The special jet orientation explains the general peculiar characteristics: high observed luminosity, very rapidvariation, high polarization (when compared with non-blazar quasars), and the apparent superluminal motionsdetected along the first few parsecs of the jets in most blazars.A Unified Scheme or Unified Model has become generally accepted where highly variable quasars are related tointrinsically powerful radio galaxies, and BL Lac objects are related to intrinsically weak radio galaxies. Thedistinction between these two connected populations explains the difference in emission line properties in blazars.Alternate explanations for the relativistic jet/unified scheme approach which have been proposed includegravitational microlensing and coherent emission from the relativistic jet. Neither of these explain the overallproperties of blazars. For example microlensing is achromatic, that is all parts of a spectrum will rise and falltogether. This is very clearly not observed in blazars. However it is possible that these processes, as well as morecomplex plasma physics can account for specific observations or some details.Some examples of blazars include 3C 454.3, 3C 273, BL Lacertae, PKS 2155-304, Markarian 421, and Markarian501. The latter two are also called "TeV Blazars" for their high energy (Tera electron volt range) gamma-rayemission.

Blazar 80

See also• Astrophysics

Notes[1] Biretta, John (1999-01-06). "HUBBLE DETECTS FASTER-THAN-LIGHT MOTION IN GALAXY M87" (http:/ / www. stsci. edu/ ftp/

science/ m87/ press. txt). Baltimore, Maryland: Space Telecsope Science Institute. .[2] http:/ / www. whatsnextnetwork. com/ technology/ media/ active_galactic_nuclei. jpg[3] Schmitt J. L. (1968): "BL Lac identified as radio source", Nature 218, 663

External links• AAVSO High Energy Network (http:/ / www. aavso. org/ observing/ programs/ hen/ blazar. shtml)• Expanding Gallery of Hires Blazar Images (http:/ / www. perseus. gr/ Astro-DSO-Quasars-Blazars. htm)• News service (April 2008). "Michigan telescope helps give astronomers insight into blazars" (http:/ / www. ns.

umich. edu/ htdocs/ releases/ story. php?id=6499). University of Michigan. Retrieved 2008-06-04. (withanimation)

Blue compact dwarf galaxy

NGC 1705, a nearby example of a blue compact dwarfgalaxy. Image from the Hubble Space Telescope.

In astronomy, a blue compact dwarf galaxy (BCD galaxy) is asmall galaxy which contains large clusters of young, hot, massivestars. These stars cause the galaxy to appear blue in color.[1]

Nearby examples include NGC 1705, NGC 2915 and NGC3353.[1] [2]

See also

• Pea galaxy

References[1] blue compact dwarf galaxy (BCD) (http:/ / www. daviddarling. info/

encyclopedia/ B/ blue_compact_dwarf_galaxy. html), David Darling, entry inThe Internet Encyclopedia of Science. Accessed on line October 14, 2007.

[2] Optical observations of NGC 2915: A nearby blue compact dwarf galaxy(http:/ / adsabs. harvard. edu/ abs/ 1994AJ. . . . 107. 2021M), G. R. Meurer, G.Mackie and C. Carignan, The Astronomical Journal 107, #6 (June 1994), pp.2021–2035.

Dark galaxy 81

Dark galaxyA dark galaxy is a hypothetical galaxy composed of dark matter[1] . Dark galaxies receive their name because theyhave no stars and are theoretically invisible. An influential community of scientist conjecture the existence of darkgalaxies to support theories based on other well studied celestial bodies[2] . There is experimental evidence to supportthe existence of dark galaxies, although scientists have no conclusive evidence and continue their research[3] .

Observational EvidenceAstronomers first suspected that there was an invisible galaxy, dark galaxy, out there when they spied galaxy NGC4254. This unusual-looking galaxy appears to be one partner in a cosmic collision. The only evidence is thefollowing: gas is being siphoned away into a tenuous stream, and one of its spiral arms is being stretched out. Theother partner in this collision is nowhere to be seen. The researchers calculated that an object with 100 billion solarmasses theoretically careened past NGC 4254 within the last 100 million years creating the gas stream and tearing atone of its arms. This was the clue that an invisible dark matter galaxy might be lurking nearby[4] .

Nature of Dark Galaxy

OriginIn 2000 astronomers found a gas cloud VIRGOHI21 and tried to find a theory of what it was and or why it couldcause such a gravitational pull from NGC 4254 galaxy. After years of running out of other explanations some haveconcluded that VIRGOHI21 is a dark galaxy, due to the massive effect it had on NGC 4254[1] .

SizeThe actual size of a dark galaxy is unknown, because they cannot be spotted with a normal telescope. There havebeen various estimations that dark galaxies. Two potential sizes could be either double the size of the Milky Way[4]

or the size of a small quasar.

StructureDark galaxies are composed of dark matter. Furthermore, dark galaxies are theoretically composed of hydrogen anddust[1] . Some scientists support the idea that dark galaxies may contain stars[3] . Yet the exact composition of darkgalaxies is unknown because there is no conclusive way to spot them so far.

Methodology to Observer Dark BodiesDark galaxies contain no stars, and are not visible through conventional methods, i.e. telescopes. Arecibo GalaxyEnvironment Survey (AGES) a current study using the Arecibo radio telescope to search for dark galaxies. TheArecibo radio telescope is useful where others are not because of its ability to detect neutral-hydrogen wavelengths[5]

.

Dark galaxy 82

Alternative TheoriesScientists do not have much explanation for some astronomic events, so some use the idea of a dark galaxy toexplain these events. Little is known about dark galaxies, and some scientists believe dark galaxy is actually a newlyforming galaxy. One such candidate is in the Virgo cluster. This candidate contains very few stars. Scientist classifythis galaxy as a newly forming galaxy, rather than a dark galaxy. [6] .

Potential Dark Galaxies

HE0450-2958HE0450-2958 is an unusual quasar (a star like object that may send out radio waves and other forms of energy). Thisone in particular has many large red shifts [7] . HE0450-2958 has no visible host galaxy (a galaxy surrounding thequasar) detected around it. It has been suggested that this may be a dark galaxy in which a quasar has become active.However subsequent observations revealed that a normal host galaxy is probably present [8] .

HVC 127-41-330HVC 127-41-330 is a cloud at high speed between the Andromeda and the Triangulum Galaxy. Astronomer JoshSimon considers this cloud to be a dark galaxy because of the speed of its rotation and its predicted mass[9] .

VIRGOHI21The discovery of VIRGOHI21 was announced in February 2005, and it was the first good candidate to be a true darkgalaxy [3] [2] [10] [11] . It was found when AGES was looking for the 21cm-wavelength radio waves emitted byhydrogen (H). Its dynamics are apparently inconsistent with the predictions of the Modified Newtonian Dynamics(MOND) theory [12] . Some researchers have since discounted the possibility of VIRGOHI21 being a dark galaxyand believe it is more likely a "tidal tail"[13] . of nearby galaxy NGC 4254, which is experiencing gravitationalperturbations as it enters the Virgo cluster

See also• Low surface brightness galaxy• Dwarf spheroidal galaxy• Dark matter halo• Dark matter

References[1] http:/ / www. universetoday. com/ 1888/ no-stars-shine-in-this-dark-galaxy/[2] Clark, Stuart (2005-02-23). "Astronomers claim first 'dark galaxy' find" (http:/ / www. newscientist. com/ article. ns?id=dn7056).

NewScientist.com news service. . Retrieved 2006-10-26.[3] Stuart Clark. "Dark galaxy' continues to puzzle astronomers" (http:/ / space. newscientist. com/ article/

dn12100-dark-galaxy-continues-to-puzzle-astronomers. html). New Scientist. . Retrieved 2008-02-26.[4] http:/ / www. spacedaily. com/ reports/ Arecibo_Survey_Produces_Dark_Galaxy_Candidate. html[5] http:/ / www. dailygalaxy. com/ my_weblog/ 2009/ 12/ darkmatter-galaxy-ten-billion-xs-the-mass-of-the-sun. html[6] http:/ / newsblaze. com/ story/ 20091130170354drex. nb/ topstory. html[7] Magain, P. et al. (2005), Discovery of a bright quasar without a massive host galaxy (http:/ / adsabs. harvard. edu/ abs/ 2005Natur. 437. .

381M), Nature, 437, 381[8] Merritt, D. et al. (2005), The nature of the HE0450-2958 System (http:/ / arxiv. org/ abs/ astro-ph/ 0511315), arXiv:astro-ph/0511315[9] Josh Simon (2005). Dark Matter in Dwarf Galaxies: Observational Tests of the Cold Dark Matter Paradigm on Small Scales (http:/ / www.

astro. caltech. edu/ ~jsimon/ thesis/ jdsthesis. pdf). .[10] Shiga, David (2005-02-26). "Ghostly Galaxy: Massive, dark cloud intrigues scientists" (http:/ / www. sciencenews. org/ view/ generic/ id/

5929/ title/ Ghostly_Galaxy_Massive,_dark_cloud_intrigues_scientists). Science News Online (Society for Science &#38) 167 (9): 131.

Dark galaxy 83

doi:10.2307/4015891. . Retrieved 2008-09-14.[11] Britt, Roy (2005-02-23). "First Invisible Galaxy Discovered in Cosmology Breakthrough" (http:/ / www. space. com/ scienceastronomy/

050223_dark_galaxy. html). Space.com. .[12] Funkhouser, Scott (2005). "Testing MOND with VirgoHI21" (http:/ / arxiv. org/ abs/ astro-ph/ 0503104). Monthly Notices of the Royal

Astronomical Society 364: 237. doi:10.1111/j.1365-2966.2005.09565.x. . Retrieved 2006-10-26.[13] Haynes, Martha P.; Giovanelli, Riccardo; Kent, Brian R. (2007). "NGC 4254: An Act of Harassment Uncovered by the Arecibo Legacy Fast

ALFA Survey". Astrophysical Journal 665 (1): L19–22. doi:10.1086/521188.

• Battersby, Stephen (2003-10-20). "Astronomers find first 'dark galaxy'" (http:/ / www. newscientist. com/ article.ns?id=dn4272). New Scientist.

External links• Universe Today, Some Galaxies Are Made Almost Entirely of Dark Matter (http:/ / www. universetoday. com/

2007/ 02/ 26/ some-galaxies-are-made-almost-entirely-of-dark-matter/ )

Disc galaxy

The Sculptor Galaxy (NGC 253)

Disc galaxies are galaxies which have discs, a flattened circularvolume of stars. These galaxies may, or may not include a centralnon-disc-like region (central bulge).

Disc galaxy types include• spiral galaxies

• barless spiral galaxies (type S, SA)• barred spiral galaxies (type SB)• intermediate barred spiral galaxies (type SAB)

• lenticular galaxies (type E8, S0, SA0, SB0, SAB0)

Dwarf elliptical galaxy 84

Dwarf elliptical galaxy

The dwarf elliptical galaxy Messier 110 (also known as NGC205)Credit: John Lanoue.

Dwarf elliptical galaxies, or dEs, are elliptical galaxiesthat are much smaller than others. They are classifiedas dE, and are quite common in galaxy groups andclusters, and are usually companions to other galaxies.

Examples

One of the most nearby Dwarf ellipticals (dEs) isMessier 110 (also known as NGC205), a satellite of theAndromeda galaxy. It was discovered by the Frenchcomet hunter Charles Messier in 1773. It remained theonly known dwarf elliptical galaxy until, in 1944,Walter Baade confirmed NGC147 and NGC185 asmembers of the Local Group by resolving them into individual stars. Resolving stars in NGC147 and NGC185 wasonly possible because these dEs are very nearby galaxies. In the 1950s, dEs were also discovered in the nearbyFornax and Virgo clusters.[1]

Comparison with giant ellipticalsDwarf elliptical galaxies have blue absolute magnitudes within the range -18 mag < M < -14 mag, fainter than giantelliptical galaxies. While the surface brightness profiles of giant elliptical galaxies are well described by deVaucouleur's law, dEs have exponentially declining surface brightness profiles. However, both types can be well fitby the same more general law, Sersic's law, and there is a continuity of Sersic index as a function of luminosity,[2]

suggesting that dwarf and giant elliptical galaxies belong to a single sequence. Still fainter elliptical-like galaxies,called dwarf spheroidal galaxies, appear to be genuinely distinct.

Two hypotheses for originsDwarf ellipticals may be primordial objects. Within the currently favoured cosmological Lambda-CDM model, smallobjects (consisting of dark matter and gas) are the first to form. Because of their mutual gravitational attraction,some of these will coalesce and merge, forming more massive objects. Further mergers lead to ever more massiveobjects.The process of coalescence is thought to lead to the present-day galaxies, and has been called "hierarchical merging".If this hypothesis is correct, dwarf galaxies may be the building blocks of today's giant galaxies.An alternative suggestion[3] is that dEs could be the remnants of low-mass spiral galaxies that obtained a roundershape through the action of repeated gravitational interactions with giant galaxies within a cluster. This process ofchanging a galaxy's morphology by interactions has been called "galaxy harassment". Evidence for this latterhypothesis has been found in the form taken by stellar disks and spiral arms of spiral galaxies. Under this alternativehypothesis the disks and arms are modified version of the original stellar disk of the transformed spiral galaxy, andsimilarly, small remnants of disks and arms are embedded within "harassed" dEs.

Dwarf elliptical galaxy 85

See also• Dwarf galaxy• Dwarf spheroidal galaxy• Elliptical galaxy• Galaxy morphological classification• Irregular galaxy

References[1] G. Reaves (1956), Dwarf galaxies in the Virgo cluster (http:/ / adsabs. harvard. edu/ abs/ 1956AJ. . . . . 61. . . 69R)[2] A. Graham and R. Guzman (2003), HST Photometry of Dwarf Elliptical Galaxies in Coma, and an Explanation for the Alleged Structural

Dichotomy between Dwarf and Bright Elliptical Galaxies (http:/ / adsabs. harvard. edu/ abs/ 2003AJ. . . . 125. 2936G)[3] Moore, B. et al. (1996), Galaxy harassment and the evolution of clusters of galaxies (http:/ / adsabs. harvard. edu/ abs/ 1996Natur. 379. .

613M)

Dwarf galaxyA dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our ownMilky Way's 200-400 billion stars. The Large Magellanic Cloud, containing over 30 billion stars, is sometimesclassified as a dwarf galaxy while others consider it a full-fledged galaxy going around the Milky Way galaxy.

Creation of dwarf galaxiesCurrent theory states that most galaxies, including dwarf galaxies, form in association with dark matter or out of gascontaining metals. However, NASA's Galaxy Evolution Explorer space probe identified new dwarf galaxies formingout of gases lacking metals. These galaxies were located in the Leo Ring, a cloud of hydrogen and helium aroundtwo massive galaxies in the constellation Leo.[1]

Local dwarfsThere are many dwarf galaxies in the Local Group: these small galaxies frequently orbit around larger galaxies, suchas the Milky Way, the Andromeda Galaxy and the Triangulum Galaxy. A 2007 paper[2] has suggested that manydwarf galaxies were created by tidal forces during the early evolution of the Milky Way and Andromeda. Tidaldwarf galaxies are produced when galaxies collide and their gravitational masses interact. Streams of galacticmaterial are pulled out away from the parent galaxies and the halos of dark matter that surround them.[3]

The Milky Way has 14 known dwarf galaxies orbiting it, and recent observations[4] have also led astronomers tobelieve the largest globular cluster in the Milky Way, Omega Centauri, is in fact the core of a dwarf galaxy with ablack hole in its center, which was at some time absorbed by the Milky Way.

Dwarf galaxy 86

Dwarf galaxiesDwarf galaxies come in many different morphologies:• Elliptical galaxy: dwarf elliptical galaxy (dE) and its subtype dwarf spheroidal galaxy (dSph)• Irregular galaxy: dwarf irregular galaxy (dI)• Spiral galaxy: dwarf spiral galaxy

Hobbit galaxiesThe recently coined term, hobbit galaxy has been used to describe galaxies smaller and dimmer than dwarfgalaxies.[5] [6]

Ultra Compact DwarfsUltra Compact Dwarf galaxies (UCD) are a recently discovered class of very compact galaxies with very highstellar population counts. They are thought to be on the order of 200 light years across, with a hundred millionstars.[7] It is theorized that these are the cores of nucleated dwarf elliptical galaxies, that have been stripped of gasand outlying stars by tidal interactions, travelling through the hearts of rich clusters.[8] UCDs have been found in theVirgo Cluster, Fornax Cluster, Abell 1689, Coma Cluster, amongst other clusters.[9]

Partial list of dwarf galaxies• Aquarius Dwarf• Canis Major Dwarf Galaxy• I Zwicky 18• Irregular Galaxy IC 10• Large Magellanic Cloud• NGC 1569• NGC 1705• Pegasus Dwarf Irregular Galaxy• Phoenix Dwarf• Sagittarius Dwarf Elliptical Galaxy• Sagittarius Dwarf Irregular Galaxy• Sculptor Dwarf Galaxy• Sculptor Dwarf Irregular Galaxy• Sextans A• Sextans Dwarf• Small Magellanic Cloud• Tucana Dwarf• Ursa Minor Dwarf• Willman 1• Carina Dwarf• Draco Dwarf• Fornax Dwarf• Leo II (dwarf galaxy)

Dwarf galaxy 87

See also• Galaxy morphological classification• List of nearest galaxies• Pea galaxy

External links• Milky Way Satellite Galaxies [10]

• SPACE.com article on "hobbit galaxies" [11]

• Science article on "hobbit galaxies" [12]

References[1] UPI, "New Recipe For Dwarf Galaxies: Start With Leftover Gas", Science Daily, 19 Feb 2009 (http:/ / www. sciencedaily. com/ releases/

2009/ 02/ 090218132145. htm)[2] Metz, M (2007) Dwarf-spheroidal satellites: are they of tidal origin? http:/ / arxiv. org/ abs/ astro-ph/ 0701289[3] New Recipe for Dwarf Galaxies: Start with Leftover Gas (http:/ / newswise. com/ articles/ view/ 549307/ ) Newswise, Retrieved on February

20, 2009.[4] Noyola, E. and Gebhardt, K. and Bergmann, M. (apr 2008). "Gemini and Hubble Space Telescope Evidence for an Intermediate-Mass Black

Hole in ω Centauri" (http:/ / adsabs. harvard. edu/ abs/ 2008ApJ. . . 676. 1008N). The Astrophysical Journal 676: 1008–1015.doi:10.1086/529002. arXiv:0801.2782. .

[5] SPACE.com - New 'Hobbit' Galaxies Discovered Around Milky Way (http:/ / www. space. com/ scienceastronomy/070115_mm_hobbit_galaxies. html)

[6] http:/ / sciencenow. sciencemag. org/ cgi/ content/ full/ 2007/ 109/ 1[7] Anglo-Australian Observatory Astronomers discover dozens of mini-galaxies (http:/ / www. aao. gov. au/ press/ mini_galaxies. html) 0100

AEST Friday 2 April 2004[8] arXiv:astro-ph/0307362 Galaxies and Overmerging: What Does it Take to Destroy a Satellite Galaxy? Mon, 21 Jul 2003 10:33:02 GMT[9] arXiv:astro-ph/0406613 Ultra Compact Dwarf galaxies in Abell 1689: a photometric study with the ACS, Mon, 28 Jun 2004 08:34:37 GMT[10] http:/ / www. astro. uu. se/ ~ns/ mwsat. html[11] http:/ / www. space. com/ scienceastronomy/ 070115_mm_hobbit_galaxies. html[12] http:/ / sciencenow. sciencemag. org/ cgi/ content/ full/ 2007/ 109/ 1

Dwarf spheroidal galaxy 88

Dwarf spheroidal galaxy

NGC 147, a dwarf spheroidal galaxy of the Local Group

Dwarf spheroidal galaxy (dSph) is a termin astronomy applied to low luminositygalaxies that are companions to the MilkyWay and to the similar systems that arecompanions to the Andromeda Galaxy M31.While similar to dwarf elliptical galaxies inappearance and properties such as little tono gas or dust or recent star formation, theyare approximately spheroidal in shape,generally lower luminosity, and are onlyrecognized as satellite galaxies in the LocalGroup.[1]

While there were nine "classical" dSphgalaxies discovered up until 2005, the SloanDigital Sky Survey has resulted in thediscovery of 11 more dSph galaxies -- thishas radically changed the understanding of these galaxies by providing a much larger sample to study.[2]

Recently, as growing evidence has indicated that the vast majority of dwarf ellipticals have properties that are not atall similar to elliptical galaxies, but are closer to irregular and late-type spiral galaxies, this term has been used torefer to all of the galaxies that share the properties of those above. These sorts of galaxies may in fact be the mostcommon type of galaxies in the universe, but are much harder to see than other types of galaxies because they are sofaint.Because of the faintness of the lowest luminosity dwarf spheroidals and the nature of the stars contained withinthem, some astronomers suggest that dwarf spheroidals and globular clusters may not be clearly separate and distincttypes of objects.[3] Other recent studies, however, have found a distinction in that the total amount of mass inferredfrom the motions of stars in dwarf spheroidals is many times that which can be accounted for by the mass of the starsthemselves. In the current predominantly accepted Cold Dark Matter cosmology, this is seen as a sure sign ofdark matter, and the presence of dark matter is often cited as a reason to classify dwarf spheroidals as a differentclass of object from globular clusters (which show little to no signs of dark matter). Because of the extremely largeamounts of dark matter in these objects, they may deserve the title "most dark matter-dominated galaxies" [4]

See also• Galaxy• Dwarf galaxy• Dwarf elliptical galaxy• Galaxy morphological classification• Galaxy formation and evolution• Groups and clusters of galaxies• Irregular galaxy• Local group• List of nearest galaxies• Dark galaxy

Dwarf spheroidal galaxy 89

External links• A popular overview [5]

• Universe Today, Some Galaxies Are Made Almost Entirely of Dark Matter [6]

References[1] Mashchenko, Sergey; Sills, Alison; Couchman, H. M. (March 2006), "Constraining Global Properties of the Draco Dwarf Spheroidal Galaxy"

(http:/ / adsabs. harvard. edu/ abs/ 2006ApJ. . . 640. . 252M), The Astrophysical Journal 640 (1): 252–269, doi:10.1086/499940,[2] Simon, Josh; Geha, Marla (November 2007), "The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite

Problem" (http:/ / adsabs. harvard. edu/ abs/ 2007ApJ. . . 670. . 313S), The Astrophysical Journal 670: 313–331, doi:10.1086/521816,[3] van den Bergh, Sidney (November 2007), "Globular Clusters and Dwarf Spheroidal Galaxies" (http:/ / adsabs. harvard. edu/ abs/

2007arXiv0711. 4795V), MNRAS (Letters), in press 385: L20, doi:10.1111/j.1745-3933.2008.00424.x,[4] Strigari, Louie; Koushiappas, et al; Bullock, James S.; Kaplinghat, Manoj; Simon, Joshua D.; Geha, Marla; Willman, Beth (September 2007),

"The Most Dark Matter Dominated Galaxies: Predicted Gamma-ray Signals from the Faintest Milky Way Dwarfs" (http:/ / adsabs. harvard.edu/ abs/ 2007arXiv0709. 1510S), The Astrophysical Journal 678: 614, doi:10.1086/529488,

[5] http:/ / www. astro. uu. se/ ~ns/ review. html[6] http:/ / www. universetoday. com/ 2007/ 02/ 26/ some-galaxies-are-made-almost-entirely-of-dark-matter/

Dwarf spiral galaxyA dwarf spiral galaxy is the dwarf version of a spiral galaxy. Dwarf galaxies are characterized as having lowluminosities, small diameters (less than 5 kpc), low surface brightnesses, and low hydrogen masses.[1] The galaxiesmay be considered a subclass of low surface brightness galaxies.Dwarf spiral galaxies, particularly the dwarf counterparts of Sa-Sc type spiral galaxies, are quite rare. In contrast,dwarf elliptical galaxies, dwarf irregular galaxies, and the dwarf versions of Sm type galaxies (which may beconsidered transitory between spiral and irregular in terms of morphology) are very common.[1]

LocationMost identified dwarf spiral galaxies are located outside clusters. Strong gravitational interactions between galaxiesand interactions between galaxies and intracluster gas are expected to destroy the disks of most dwarf spiralgalaxies.[1] [2] Nonetheless, dwarf galaxies with spiral-like structure have been identified within the Virgo Clusterand Coma Cluster.[3] [4] [2] [5]

References[1] J. M. Schombert, R. A. Pildis, J. A. Eder, A. Oelmer, Jr. (1995). "Dwarf Spirals" (http:/ / adsabs. harvard. edu/ abs/ 1995AJ. . . . 110. 2067S).

Astronomical Journal 110: 2067–2074. doi:10.1086/117669. .[2] A. W. Graham, H. Jerjen, R. Guzmán (2003). "Hubble Space Telescope Detection of Spiral Structure in Two Coma Cluster Dwarf Galaxies"

(http:/ / adsabs. harvard. edu/ abs/ 2003AJ. . . . 126. 1787G). Astronomical Journal 126: 1787–1793. doi:10.1086/378166. .[3] H. Jerjen, A. Kalnajs, B. Binggeli (2000). "IC3328: A "dwarf elliptical galaxy" with spiral structure" (http:/ / adsabs. harvard. edu/ abs/

2000A& A. . . 358. . 845J). Astronomy and Astrophysics 358: 845–849. .[4] F. D. Barazza, B. Binggeli, H. Jerjen (2002). "More evidence for hidden spiral and bar features in bright early-type dwarf galaxies" (http:/ /

adsabs. harvard. edu/ abs/ 2002A& A. . . 391. . 823B). Astronomy and Astrophysics 391: 823–831. doi:10.1051/0004-6361:20020875. .[5] T. Lisker, E. K. Grebel, B. Binggeli (2006). "Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. I. On the Possible

Disk Nature of Bright Early-Type Dwarfs" (http:/ / adsabs. harvard. edu/ abs/ 2006AJ. . . . 132. . 497L). Astronomical Journal 132: 497–513.doi:10.1086/505045. .

Elliptical galaxy 90

Elliptical galaxy

The giant elliptical galaxy ESO 325-G004.

An elliptical galaxy is a galaxy having an approximately ellipsoidalshape and a smooth, nearly featureless brightness profile. They rangein shape from nearly spherical to highly flat and in size from hundredsof millions to over one trillion stars. They can be the result of twogalaxies colliding.

Elliptical galaxies are one of the three main classes of galaxy originallydescribed by American astronomer Edwin Hubble in his 1936 workThe Realm of the Nebulae,[1] along with spiral and lenticular galaxies.

Most elliptical galaxies are composed of older, low-mass stars, with asparse interstellar medium and minimal star formation activity. Theyare surrounded by large numbers of globular clusters. Ellipticalgalaxies are believed to make up approximately 10–15% of galaxies inthe local Universe[2] but are not the dominant type of galaxy in theuniverse overall. They are preferentially found close to the centers ofgalaxy clusters[3] and are less common in the early Universe.

General characteristicsElliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. Themotion of stars in elliptical galaxies is predominantly radial, unlike the disks of spiral galaxies, which are dominatedby rotation. Furthermore, there is very little interstellar matter (neither gas nor dust), which results in low rates ofstar formation, few open star clusters, and few young stars; rather elliptical galaxies are dominated by old stellarpopulations, giving them red colours. Large elliptical galaxies typically have an extensive system of globularclusters.[4]

The dynamical properties of elliptical galaxies and the bulges of disk galaxies are similar, [5] suggesting that they areformed by the same physical processes, although this remains controversial. The luminosity profiles of both ellipticalgalaxies and bulges are well fit by Sersic's law.Elliptical galaxies are preferentially found in galaxy clusters and in compact groups of galaxies.

Star formationThis traditional portrait of elliptical galaxies paints them as galaxies where star formation has finished after the initialburst, leaving them to shine with only their aging stars. Very little star formation is thought to occur, because of thelack of gas, dust, and space. In general, they appear yellow-red, which is in contrast to the distinct blue tinge of atypical spiral galaxy, a colour emanating largely from the young, hot stars in its spiral arms.

Sizes and shapesThere is a wide range in size and mass for elliptical galaxies: as small as a tenth of a kiloparsec to over 100kiloparsecs, and from 107 to nearly 1013 solar masses. This range is much broader for this galaxy type than for anyother. The smallest, the Dwarf elliptical galaxies, may be no larger than a typical globular cluster, but contain aconsiderable amount of dark matter not present in clusters. Most of these small galaxies may not be related to otherellipticals.

Elliptical galaxy 91

The Hubble classification of elliptical galaxies contains an integer that describes how elongated the galaxy image is.The classification is determined by the ratio of the major (a) to the minor (b) axes of the galaxy's isophotes:

Thus for a spherical galaxy with a equal to b, the number is 0, and the Hubble type is E0. The limit is about E7,which is believed to be due to a bending instability that causes flatter galaxies to puff up. The most common shape isclose to E3. Hubble recognized that his shape classification depends both on the intrinsic shape of the galaxy, as wellas the angle with which the galaxy is observed. Hence, some galaxies with Hubble type E0 are actually elongated.There are two physical types of ellipticals; the "boxy" giant ellipticals, whose shapes result from random motionwhich is greater in some directions than in others (anisotropic random motion), and the "disky" normal and lowluminosity ellipticals, which have nearly isotropic random velocities but are flattened due to rotation.Dwarf elliptical galaxies have properties that are intermediate between those of regular elliptical galaxies andglobular clusters. Dwarf spheroidal galaxies appear to be a distinct class: their properties are more similar to those ofirregulars and late spiral-type galaxies.At the large end of the elliptical spectrum, there is further division, beyond Hubble classification. Beyond gE giantellipticals, lies D-galaxies and cD-galaxies. These are similar to their smaller brethren, but more diffuse, with largerhaloes. Some even appear more akin to lenticular galaxies.

EvolutionCurrent thinking is that an elliptical galaxy may be the result of a long process where two galaxies of comparablemass, of any type, collide and merge.Such major galactic mergers are thought to have been common at early times, but may carry on more infrequentlytoday. Minor galactic mergers involve two galaxies of very different masses, and are not limited to giant ellipticals.For example, our own Milky Way galaxy is known to be "ingesting" a couple of small galaxies right now. The MilkyWay galaxy, is also, depending upon an unknown tangential component, on a collision course in 3-4 billion yearswith the Andromeda Galaxy. It has been theorized that an elliptical galaxy will result from a merger of the twospirals.Every bright elliptical galaxy is believed to contain a supermassive black hole at its center. The mass of the blackhole is tightly correlated with the mass of the galaxy, via the M-sigma relation. It is believed that black holes mayplay an important role in limiting the growth of elliptical galaxies in the early universe by inhibiting star formation.

Examples• M32• M49• M59• M60 (NGC 4649)• M87 (NGC 4486)• M89• M105 (NGC 3379)• Maffei 1, the closest giant elliptical galaxy.

Elliptical galaxy 92

See also• Firehose instability• Galaxy color-magnitude diagram• Galaxy morphological classification• Hubble sequence• Lenticular galaxy• M-sigma relation• Osipkov-Merritt model• Sersic profile

References[1] Hubble, E. P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. ISBN 36018182.[2] Loveday, J. (February 1996). "The APM Bright Galaxy Catalogue." (http:/ / articles. adsabs. harvard. edu/ full/ 1996MNRAS. 278. 1025L).

Monthly Notices of the Royal Astronomical Society 278 (4): 1025–1048. . Retrieved 2007-09-15.[3] Dressler, A. (March 1980). "Galaxy morphology in rich clusters - Implications for the formation and evolution of galaxies." (http:/ / articles.

adsabs. harvard. edu/ full/ 1980ApJ. . . 236. . 351D). The Astrophysical Journal 236: 351–365. doi:10.1086/157753. . Retrieved 2007-09-15.[4] Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 9780691025650. OCLC 39108765.[5] Merritt, D. (February 1999). "Elliptical galaxy dynamics" (http:/ / adsabs. harvard. edu/ abs/ 1999PASP. . 111. . 129M). The Astronomical

Journal 756: 129–168. doi:10.1086/316307. .

External links• Elliptical Galaxies (http:/ / www. seds. org/ messier/ elli. html), SEDS Messier pages• Elliptical Galaxies (http:/ / csep10. phys. utk. edu/ astr162/ lect/ galaxies/ elliptical. html)

Faint blue galaxyThe faint blue galaxy (FBG) problem in astrophysics first arose with observations starting in 1978 that there weremore galaxies with a bolometric magnitude > 22 than then-current theory predicted.[1] [2] [3] Galaxies can appearfaint because they are small or because they are far away. Neither explanation, nor any combination, initiallymatched the observations. The distribution of these galaxies has since been found to be consistent with CosmicInflation, measurements of the Cosmic Microwave Background, and a non-zero cosmological constant, that is, withthe existence of the now-accepted dark energy.[4] [5] It thus serves as a confirmation of supernova observationsrequiring dark energy.A second problem arose in 1988, with even deeper observations showing a much greater excess of faint galaxies.[6]

These are now interpreted as dwarf galaxies undergoing large bursts of star formation, resulting in blue light fromyoung, massive stars.[7] Thus FBGs are extremely bright for their size and distance.Most FBGs appear between redshift 0.5 and 2. It is believed that they disappear as separate objects by merger withother galaxies.[8] [9] However, the problem has not been definitively solved.

Faint blue galaxy 93

References[1] Kron R 1978 Ph.D. Thesis, University of California, Berkeley[2] Peterson, B.A.; Ellis, R.S.; Kibblewhite, E.J.; Bridgeland, M.T.; Hooley, T.; Horne, D. (Nov. 1, 1979), "Number magnitude counts of faint

galaxies" (http:/ / adsabs. harvard. edu/ abs/ 1979ApJ. . . 233L. 109P), Astrophysical Journal, Part 2 - Letters to the Editor 233: L109-L113,doi:10.1086/183087,

[3] Tyson, J.A.; Jarvis, J.F. (June 15, 1979), "Evolution of galaxies - Automated faint object counts to 24th magnitude" (http:/ / adsabs. harvard.edu/ abs/ 1979ApJ. . . 230L. 153T), Astrophysical Journal, Part - 2 Letters to the Editor 230: L153-L156, doi:10.1086/182982,

[4] Yoshii, Yuzuru; Takahara, Fumio (Nov. 1, 1989), "On the redshift-volume measurement of the cosmological density parameter" (http:/ /adsabs. harvard. edu/ abs/ 1989ApJ. . . 346. . . 28Y), Astrophysical Journal, Part 1 346: 28–33, doi:10.1086/167983,

[5] David C., Koo (June 21-23, 1989). "The evolution of field galaxies - Is Omega = 1?" (http:/ / adsabs. harvard. edu/ abs/ 1990ASPC. . . 10. .268K). . Berkeley, CA: Astronomical Society of the Pacific. pp. 268-285. .

[6] Broadhurst, T.J.; Ellis, R.S.; Shanks, T. (Dec. 1, 1988), "The Durham/Anglo-Australian Telescope faint galaxy redshift survey" (http:/ /adsabs. harvard. edu/ abs/ 1988MNRAS. 235. . 827B), Monthly Notices of the Royal Astronomical Society 235: 827–856,

[7] Colless, Matthew; Ellis, Richard S.; Broadhurst, T.J.; Taylor, Keith; Peterson, Bruce A. (03/1993), "Faint blue galaxies - High or lowredshift?" (http:/ / adsabs. harvard. edu/ abs/ 1993MNRAS. 261. . . 19C), Monthly Notices of the Royal Astronomical Society 261: 19–38,

[8] Carlberg, R.G. (11/1992), "Merging and fast galaxy evolution" (http:/ / adsabs. harvard. edu/ abs/ 1992ApJ. . . 399L. . 31C), AstrophysicalJournal, Part 2 - Letters 399 (1): L31-L34,

[9] Carlberg, R.G.; Charlot, Stephane (09/1992), "Faint galaxy evolution via interactions" (http:/ / adsabs. harvard. edu/ abs/ 1992ApJ. . . 397. . . .5C), Astrophysical Journal, Part 1 397 (1): 5–13, doi:10.1086/171759,

Field galaxyA field galaxy is a galaxy that does not belong to a larger cluster of galaxies, but is gravitationally alone. The vastmajority of galaxies exist outside of clusters.Most low surface brightness galaxies are field galaxies[1] .

References[1] An Introduction to Galaxies and Cosmology by David J. Adams and others

Flocculent spiral galaxy 94

Flocculent spiral galaxy

NGC 4414, a flocculent spiral

A flocculent spiral galaxy is a type of spiral galaxy, that is thefunctional opposite of the grand design spiral galaxy.[1] Unlike the welldefined spiral architecture of a grand design galaxy, flocculent galaxiesare patchy, with discontinuous spiral arms.[2] Approximately 30% ofspirals are flocculent, 10% are grand design, and the rest are"multi-armed".[3] The "multiple arm" type is sometimes folded into theflocculent-type.[4]

The prototypical flocculent spiral is NGC 2841.[5]

Examples

Example Class Image Constellation Notes

NGC 4414 SA(rs)c Coma Berenices [6]

NGC 2841 SA(r)b Ursa Major [5]

NGC 7793 SA(s)d Sculptor [5]

Pinwheel Galaxy SAB(rs)cd Ursa Major [7]

References[1] COSMOS - SAO Encyclopedia of Astronomy, "Grand Design Spiral" (http:/ / astronomy. swin. edu. au/ cms/ astro/ cosmos/ G/ Grand+

Design+ Spiral) (accessed 23 April 2010)[2] COSMOS - SAO Encyclopedia of Astronomy, "Flocculent Spiral" (http:/ / astronomy. swin. edu. au/ cms/ astro/ cosmos/ F/ Flocculent+

Spiral) (accessed 23 April 2010)[3] Case Western Reserve University, Chris Mihos, ASTR222 - Spring 2008, "Spiral Structure" (http:/ / burro. cwru. edu/ Academics/ Astr222/

Galaxies/ Spiral/ spiral. html) (accessed 23 April 2010)[4] University of Virginia, Mark Whittle, ASTR 553/554 : Extragalactic Astronomy (2007), "Lecture 5: Spiral Galaxies" (http:/ / www. astro.

virginia. edu/ class/ whittle/ astr553/ Topic05/ Lecture_5. html) (accessed 23 April 2010)[5] "A Near-Infrared Atlas of Spiral Galaxies", Debra Meloy Elmegreen, "CH3. Discussion" (http:/ / nedwww. ipac. caltech. edu/ level5/

Elmegreen/ Elm3. html) (accessed 23 April 2010)[6] APOD, "NGC 4414: A Flocculent Spiral Galaxy" (http:/ / apod. nasa. gov/ apod/ ap020403. html), 3 April 2002 (accessed 23 April 2010)[7] Spitzer/JPL/NASA, "The Pinwheel Galaxy, M101, in the Infrared" (http:/ / spitzer. caltech. edu/ images/

1945-ssc2008-14b-The-Pinwheel-Galaxy-M101-in-the-Infrared), 07.21.08 (accessed 23 April 2010)

• PDF (http:/ / nedwww. ipac. caltech. edu/ level5/ Elmegreen/ paper. pdf) "A Near-Infrared Atlas of SpiralGalaxies", Debra Meloy Elmegreen, 1981, doi:10.1086/190757, Bibcode: 1981ApJS...47..229E

Flocculent spiral galaxy 95

External links• COSMOS astronomy encyclopedia - Flocculent Spiral (http:/ / astronomy. swin. edu. au/ cms/ astro/ cosmos/ F/

Flocculent+ Spiral)

Grand design spiral galaxy

A Spitzer Space Telescope Image of Messier 81,a grand design spiral

A grand design spiral galaxy is a type of spiral galaxy withprominent and well-defined spiral arms, as opposed to multi-arm andflocculent spirals which have subtler structural features. The spiralarms of a grand design galaxy extend clearly around the galaxythrough many radians and can be observed over a large fraction of thegalaxy's radius. Approximately ten percent of spiral galaxies areclassified as grand design type spirals,[1] including M81, M51 andM74.

The origin of Grand Design structure

Density wave theory is the preferred explanation for the well-definedstructure of grand design spirals.[2] According to this theory, the spiral arms are created inside density waves thatturn around the galaxy at different speeds from the stars in the galaxy’s disk. Stars are clumped in these denseregions due to gravitational attraction towards the dense material, though their location in the spiral arm may not bepermanent. When they come close to the spiral arm, they are pulled towards the dense material by the force ofgravity; and as they travel through the arm, they are slowed from exiting by the same gravitational pull. This causesmaterial to clump in the dense regions.

References[1] Mihos, Chris (2002-01-11), Spiral Structure (http:/ / burro. cwru. edu/ Academics/ Astr222/ Galaxies/ Spiral/ spiral. html), , retrieved

2007-05-30[2] Masters, Karen (2002-09), What is the Origin of Spiral Structure in Galaxies (http:/ / curious. astro. cornell. edu/ question. php?number=199),

, retrieved 2007-05-30

Host galaxy 96

Host galaxyA host galaxy is one with an active galactic nucleus at its core. Most powerful quasars and all BL Lacertae objectsappear to be situated within giant elliptical galaxies.

Interacting galaxy

The Whirlpool Galaxy with its satellite NGC 5195

The Mice Galaxies

Interacting galaxies (Colliding galaxies) are galaxieswhose gravitational fields result in a disturbance of oneanother. An example of a minor interaction is a satellitegalaxy's disturbing the primary galaxy's spiral arms. Anexample of a major interaction is a galactic collision.

Satellite interaction

A giant galaxy interacting with its satellites is common.A satellite's gravity could attract one of the primary'sspiral arms. Or the satellite could dive in to the primary(e.g. Sagittarius Dwarf Elliptical Galaxy). This couldtrigger a small amount of star formation.

Galaxy collision

Colliding galaxies are common in galaxy evolution.Due to the extremely tenuous distribution of matter ingalaxies, these are not collisions in the normal sense ofthe word, but rather gravitational interaction. Collidingmay lead to merging. This occurs when two galaxiescollide and do not have enough momentum to continuetraveling after the collision. Instead, they fall back intoeach other and eventually merge after many passesthrough each other, forming one galaxy. If one of thecolliding galaxies is much larger than the other, it willremain largely intact after the merger; that is, the largergalaxy will look much the same while the smallergalaxy will be stripped apart and become part of thelarger galaxy. Through-passes are less disruptive ofgalaxy shapes than mergers in that both galaxies largelyretain their material and shape after the pass.

Galaxy collisions are now frequently simulated oncomputers, with all the realistic physics, including gravity forces, gas dissipation, star formation and feedback.Dynamical friction slows down galaxy pairs, which may or may not merge at some point, according to the initialrelative energy of the orbits. A simulated library of galaxy collisions can be found at the Paris Observatory website:GALMER [1]

Interacting galaxy 97

Galactic cannibalismGalactic cannibalism refers to the process by which a large galaxy, through tidal gravitational interactions with acompanion, merges with that companion, resulting in a larger, often irregular galaxy.The most common result of the gravitational merger of two or more galaxies is an irregular galaxy of one form oranother, although elliptical galaxies may also result.It has been suggested that galactic cannibalism is currently occurring between the Milky Way and the Large andSmall Magellanic Clouds. Streams of gravitationally-attracted hydrogen arcing from these dwarf galaxies to theMilky Way is taken as evidence for this theory.

Notable interacting galaxies

Name Type Distance(million ly)

Magnitude Notes

Whirlpool Galaxy (M51) SAc (SB0-a) 37 +8.4 Satellite interacting with its primary

NGC 2207 and IC 2163 SAc/SAbc 114 +11 galaxies going through the first phase in galactic collision

Mice Galaxies (IC 819/20) S0/SB(s)ab 300 +13.5 galaxies going through the second phase in galactic collision

NGC 1097 SB(s)bc (E6) 45 +9.5 Satellite interacting with its primary

Antennae Galaxies (NGC 4038/9) SAc/SBm 45 +10.3 galaxies going through the third phase in galactic collision

NGC 520 S 100 +11.3 galaxies going through the third phase in galactic collision

Future collision of the Milky Way with AndromedaAstronomers have estimated that our galaxy, the Milky Way galaxy, will collide with the Andromeda galaxy inabout 3 billion years. It is thought that the two spiral galaxies will merge to become an elliptical galaxy. [2] [3]

See also• Galactic tide• Galaxy merger• NGC 7318• Whirlpool Galaxy

References[1] GALMER 27 March 2010 (http:/ / galmer. obspm. fr)[2] Hazel Muir, " Galactic merger to 'evict' Sun and Earth," New Scientist 4 May 2007 (http:/ / space. newscientist. com/ article/

dn11852-galactic-merger-to-evict-sun-and-earth. html)[3] Astronomy, June 2008, page 28, by Abraham Loeb and T.J.Cox

External links• Galaxy Collisions (http:/ / www. public. iastate. edu/ ~curt/ cg/ homepage. html)• Galactic cannibalism (http:/ / www. iac. es/ gabinete/ noticias/ 2001/ mar08i. htm)• Galactic Collision Simulation (http:/ / burro. cwru. edu/ JavaLab/ GalCrashWeb/ )• GALMER: Galaxy Merger Simulations (http:/ / galmer. obspm. fr)

Intermediate spiral galaxy 98

Intermediate spiral galaxy

Messier 61; M61 is an intermediate spiral galaxy

An intermediate spiral galaxy is a galaxy that is in between theclassifications of a barred spiral galaxy and an unbarred spiralgalaxy.[1] [2] It is designated as SAB in the galaxy morphologicalclassification scheme.[1] [2]

Grades

Under the de Vaucouleurs classification system, SAB-galaxies are intermediatebetween SA-galaxies and SB-galaxies

Intermediate spiral galaxy 99

Example Type Image Information Notes

SAB0- SAB0- is a type of lenticular galaxy

SAB0 SAB0 is a type of lenticular galaxy

SAB0+ SAB0+ is a type of lenticular galaxy

SAB0/a SAB0/a can also be considered a type of intermediatelenticular galaxy

Messier 65 SABa M65 is actually an "SAB(rs)a"

NGC 4725 SABab NGC 4725 is actually an "SAB(r)abpec"

Messier 66 SABb M66 is actually an "SAB(s)b"

Messier 106 SABbc M106 is actually an "SAB(s)bc"

SculptorGalaxy

SABc Sculptor is actually an "SAB(s)c"

NGC 2403 SABcd NGC 2403 is actually an"SAB(s)cd"

SABd

SABdm SABdm can also be considered a type of intermediateMagellanic spiral

NGC 4625 SABm SABm is a type of Magellanic spiral (Sm) NGC 4625 is actually an "SAB(rs)mpec"

References[1] "Building Galaxies", Leslie Kipp Hunt, 15 October 2004[2] "Galaxy Formation", Malcolm S. Longair, 1998, Springer, ISSN 0941-7834

Tags

Irregular galaxy 100

Irregular galaxy

The Large and Small Magellanic Clouds areirregular dwarf galaxies.

NGC 1427A, an example of an irregular galaxyabout 52 Mly distant.

An irregular galaxy is a galaxy that does not have a regular shape,like a spiral or an elliptical galaxy.[1] The shape of an irregular galaxyis uncommon – they do not fall into any of the regular classes of theHubble sequence, and they are often chaotic in appearance, withneither a nuclear bulge nor any trace of spiral arm structure.[2]

Collectively they are thought to make up about a quarter of allgalaxies. Most irregular galaxies were once spiral or elliptical galaxiesbut were deformed by disorders in gravitational pull. Irregular galaxiesalso contain abundant amounts of gas and dust.

There are two major Hubble types of irregular galaxies:[3]

• An Irr-I galaxy (Irr I) is an irregular galaxy that features somestructure but not enough to place it cleanly into the Hubblesequence. De Vaucouleurs subtypes this into galaxies that havesome spiral structure Sm, and those that do not Im.

• An Irr-II galaxy (Irr II) is an irregular galaxy that does not appearto feature any structure that can place it into the Hubble sequence.

A third classification of irregular galaxies are the dwarf irregulars,labelled as dI or dIrrs.[4] This type of galaxy is now thought to beimportant to understand the overall evolution of galaxies, as they tendto have a low level of metallicity and relatively high levels of gas, andare thought to be similar to the earliest galaxies that populated theUniverse. They may represent a local (and therefore more recent)version of the faint blue galaxies known to exist in deep field galaxysurveys.

Some of the irregular galaxies are small spiral galaxies that are beingdistorted by the gravity of a larger neighbor.

The Magellanic Cloud galaxies were once classified as irregular galaxies, but have since been found to containbarred spiral structures, and have been since re-classified as "SBm", a fourth type of barred spiral galaxy, the barredMagellanic spiral type.

See also• Dwarf galaxy• Dwarf elliptical galaxy• Dwarf spheroidal galaxy

References[1] Butz, Stephen D. (2002). Science of Earth Systems. Cengage Learning. p. 107. ISBN 978-0766833913.[2] Morgan, W. W. & Mayall, N. U. (1957). "A Spectral Classification of Galaxies." Publications of the Astronomical Society of the Pacific. 69

(409): 291–303.[3] Gallagher, J. S. & Hunter, D. A. (1984). "Structure and Evolution of Irregular Galaxies." Annual Review of Astronomy and Astrophysics. 22:

37-74. doi:10.1146/annurev.aa.22.090184.000345[4] Grebal, Eva K. (2004). The evolutionary history of Local Group irregular galaxies. in McWilliam, Andrew; Rauch, Michael (eds) Origin and

evolution of the elements. Cambridge University Press. p. 234-254. ISBN 978-0521755788.

Lenticular galaxy 101

Lenticular galaxy

The Spindle Galaxy (NGC 5866), a lenticulargalaxy in the Draco constellation.

Credit:NASA/ESA

Dust ring near NGC 1553's nucleus by HST. 0.3′view

A lenticular galaxy is a type of galaxy which is intermediate betweenan elliptical galaxy and a spiral galaxy in galaxy morphologicalclassification schemes.[1] Lenticular galaxies are disc galaxies (likespiral galaxies) which have used up or lost most of their interstellarmatter and therefore have very little ongoing star formation.[2] As aresult, they consist mainly of aging stars (like elliptical galaxies). Thedust in most lenticular galaxies is generally found only near thenucleus and generally follows the light profile of the galaxies' bulges.Because of their ill-defined spiral arms, if they are inclined face-on it isoften difficult to distinguish between them and elliptical galaxies.Despite the morphological differences, lenticular and elliptical galaxiesshare common properties like spectral features, scaling relations andboth can be considered as early type galaxies which are passivelyevolving, at least in the local universe.

Morphological variations

Although lenticular galaxies do not vary in shape as much as spiralgalaxies, they may still be divided into a series of subclasses based ontheir appearance. Some of these subclasses are described below.

Bars

In the de Vaucouleurs classification system, lenticular galaxies may besplit into three subclasses based on the presence or absence of a centralbar structure. The SA0 designation is used for lenticular galaxies withno apparent bars. The SB0 designation is used for galaxies with astrong bar. The SAB0 designation is an intermediate class that may bereferred to as weakly barred.[1] It is also referred to as thequatridienticedent.

Inner ring and S-shaped subtypes

Some lenticular galaxies are also given inner ring (S0(r)) and S-shaped (S0(s)) designations as well as anintermediate designation (S0(rs)). However, these designations were defined primarily for spiral galaxies, andfinding objects that represent some of these classes is very difficult.[1]

Examples

• IC 1101, the largest known galaxy• NGC 2787, a barred lenticular galaxy

Lenticular galaxy 102

References[1] R. J. Buta, H. G. Corwin, Jr., S. C. Odewahn (2007s). The de Vaucouleurs Atlas of Galaxies. Cambridge: Cambridge University.

ISBN 0-521-82048-6.[2] DeGraaff, Regina Barber; Blakeslee, John P.; Meurer, Gerhardt R.; Putman, Mary E. (December 2007). "A Galaxy in Transition: Structure,

Globular Clusters, and Distance of the Star-Forming S0 Galaxy NGC 1533 in Dorado" (http:/ / adsabs. harvard. edu/ abs/ 2007ApJ. . . 671.1624D). The Astrophysical Journal 671 (2): 1624–1639. doi:10.1086/523640. .

Low surface brightness galaxyA low surface brightness galaxy, or LSB galaxy, is a diffuse galaxy with a surface brightness that, when viewedfrom Earth, is at least one magnitude lower than the ambient night sky.Most LSBs are dwarf galaxies, and most of their baryonic matter is in the form of neutral gaseous hydrogen, ratherthan stars. They appear to have over 95% of their mass as non-baryonic dark matter. There appears to be nosupernova activity in these galaxies.Rotation curve measurements indicate an extremely high mass-to-light ratio, meaning that stars and luminous gascontribute only very little to the overall mass balance of an LSB. The centers of LSBs show no large overdensities instars, unlike e.g. the bulges of normal spiral galaxies. Therefore they seem to be dark matter-dominated even in theircenters which makes them excellent laboratories for the study of dark matter.In comparison to the more well-studied high-surface brightness galaxies, LSBs are mainly isolated field galaxies,found in regions devoid of other galaxies. In their past, they had fewer tidal interactions or mergers with othergalaxies, which could have triggered enhanced star formation. This is an explanation for the small stellar content.

Examples

• Andromeda V• Pegasus Dwarf Spheroidal Galaxy

• IC 10

• Phoenix Dwarf Galaxy• Sagittarius Dwarf Irregular Galaxy (SagDIG)• Sextans A

• Sextans B• Wolf-Lundmark-Melotte Galaxy (WLM)

References• K. O'Neil, The HI Content and Extent of Low Surface Brightness Galaxies - Could LSB Galaxies be Responsible

for Damped Ly-alpha Absorption? [1]. For publication in Extragalactic Gas at Low Redshift, by Mulchaey, et al.,eds (2001).

• S.D. Rosenbaum and D.J.Bomans, The environment of Low Surface Brightness Galaxies [2]. Astronomy &Astrophysics Letters, 422, 5-8 (2004).

• A.J. Barth, A normal stellar disk in the galaxy Malin 1 [3]. Astronomical Journal 133, 1085-1091 (2007).

References[1] http:/ / aps. arxiv. org/ abs/ astro-ph/ 0107064[2] http:/ / adsabs. harvard. edu/ abs/ 2004A%26A. . . 422L. . . 5R[3] http:/ / lanl. arxiv. org/ abs/ astro-ph/ 0701018

Luminous infrared galaxy 103

Luminous infrared galaxy

Ultraluminous Infrared Galaxy IRAS 19297-0406

A luminous infrared galaxy (LIRG), is a galactic body whosedefining characteristic is in emitting more than 1011 solarluminosities in the far-infrared part of the electromagneticspectrum. A more luminous system, emitting more than 1012 solarluminosities in the far-infrared, is called ultraluminous infraredgalaxy (ULIRG). An even more luminous system, emitting morethan 1013 solar luminosities in the far-infrared, is calledhyperluminous infrared galaxy (HLIRG). Most LIRGs andULIRGs emit at least 90% of their light in the infrared.

Most LIRGs and all ULIRGs show signs of recent or continuinginteractions and disruptions. Many are starburst galaxies, and somealso contain an active galactic nucleus. On average, ULIRGsspawn about 100 new stars yearly, as compared to our own galaxywhich spawns one new star a year. ULIRGs are implicated in avariety of interesting astrophysical phenomena including theformation of quasars and elliptical galaxies. Local examples of ULIRGs are often used as analogs of galaxyformation at high redshift. ULIRGs seem to be embedded in dark matter halo with masses of around 10 trillion solarmasses.

External links• Nearby Extreme Galaxies Linked To Humble Roots [1] (SkyNightly) Jun 07, 2006• How To Bake A Galaxy [2] (SpaceDaily) Jun 19, 2006• The Great Observatory All-sky LIRG Survey [3]

See also• II Zw 96 - a luminous infrared galaxy containing young powerful starburst regions.• NGC 6240 - a luminous infrared galaxy with two nuclei.• Arp 220 - the closest known ultraluminous infrared galaxy.

References[1] http:/ / www. skynightly. com/ reports/ Nearby_Extreme_Galaxies_Linked_To_Humble_Roots. html[2] http:/ / www. spacedaily. com/ reports/ How_To_Bake_A_Galaxy. html[3] http:/ / goals. ipac. caltech. edu

Lyman-alpha emitter 104

Lyman-alpha emitter

A Lyman alpha emitter (left) and an artistsimpression of what one might look like if viewed

at a relatively close distance (right).

Lyman alpha emitters (LAEs) are a type of distant galaxy that emitsLyman-alpha radiation. They are extremely distant and because of thefinite travel time of light they are glimpses of the past history of theuniverse. They are thought to be the progenitors of most modern MilkyWay type galaxies. These galaxies can be found nowadays rather easilyin narrow-band searches by an excess of their narrow-band flux at awavelength which may be interpreted as their redshift:

where z is the redshift, is the observed wavelength, and 1215.67 Å is the wavelength of Lyman-alpha emission.The Lyman-alpha line is thought to be caused by an ongoing outburst of star-formation. Experimental observationsof the redshift of LAEs are important in cosmology[1] because they trace dark matter halos and subsequently theevolution of matter distribution in the universe.

See also• Lyman series• Lyman alpha blob• Lyman-break galaxy• Damped Lyman-alpha system• Lyman-alpha forest• Lyman limit

References[1] "The Lyman-alpha Emission Line as a Cosmological Tool" (http:/ / arxiv. org/ abs/ 0711. 2199). arXiv.org. . Retrieved 2008-12-01.

Lyman-break galaxy 105

Lyman-break galaxyLyman-break galaxies are star-forming galaxies at high redshift that are selected using the differing appearance ofthe galaxy in several imaging filters due to the position of the Lyman limit. The technique has primarily been used toselect galaxies at redshifts of z=3-4 using ultraviolet and optical filters, but progress in infrared astronomy hasallowed the use of this technique at higher redshifts using infrared filters.The Lyman-break galaxy selection technique relies on the fact that radiation at higher energies than the Lyman limitat 912 Å is almost completely absorbed by neutral gas around star-forming regions of galaxies. In the rest frame ofthe emitting galaxy, the emitted spectrum is bright at wavelengths longer than 912 Å, but very dim or imperceptibleat shorter wavelengths--this sharp drop (or "break") can be used to find the position of the Lyman limit. Light with awavelength shorter than 912 Å is in the far-ultraviolet range and is blocked by the Earth's atmosphere, but for verydistant galaxies the wavelengths of light are stretched considerably because of the expansion of the Universe. For agalaxy at redshift z=3, the Lyman break will appear to be at wavelengths of about 3600 Å, which is long enough tobe detected by ground- or space-based telescopes.Candidate galaxies around redshift z=3 can then be selected by looking for galaxies which appear in optical images(which are sensitive to wavelengths greater than 3600 Å), but do not appear in ultraviolet images (which aresensitive to light at wavelengths shorter than 3600 Å). The technique may be adapted to look for galaxies at otherredshifts by choosing different sets of filters--the method works as long as images may be taken through at least onefilter above and below the wavelength of the redshifted Lyman break. In order to confirm the redshift estimated bythe color selection, follow-up spectroscopy is performed. Although spectroscopic measurements are necessary toobtain a high-precision redshift, spectroscopy is typically much more time-consuming than imaging, so the selectionof candidate galaxies via the Lyman-break technique greatly improves the efficiency of high-redshift galaxysurveys.[1] [2]

See also• Lyman series• Lyman-alpha forest• Lyman alpha emitter• Lyman alpha blob• Damped Lyman-alpha system• Lyman limit

References[1] "Spectroscopic Confirmation of a Population of Normal Star-forming Galaxies at Redshifts z > 3" (http:/ / adsabs. harvard. edu/ abs/

1996ApJ. . . 462L. . 17S). http:/ / www. iop. org/ EJ/ journal/ apj. . Retrieved 2010-01-08.[2] "Lyman Break Galaxies at z~3 and Beyond" (http:/ / arxiv. org/ abs/ astro-ph/ 9812167). arXiv.org. . Retrieved 2010-01-08.

Magellanic spiral 106

Magellanic spiralMagellanic spiral galaxies are galaxies which are classified as the type Sm (and SAm , SBm , SABm). They aregalaxies with one spiral arm, and are named after their prototype, the Large Magellanic Cloud, an SBm galaxy.

Magellanic spiralsSAm galaxies are a type of unbarred spiral galaxy, while SBm are a type of barred spiral galaxy.[1] SABm are a typeof intermediate spiral galaxy.Type Sm and Im galaxies have also been categorized as irregular galaxies with some structure (type Irr-1). Smgalaxies are typically disrupted and asymmetric.[2] dSm galaxies are dwarf spiral galaxies or dwarf irregulargalaxies, depending on categorization scheme.The Magellanic spiral classification was introduced by Gerard de Vaucouleurs, along with Magellanic irregular (Im),when he revamped the Hubble classification of galaxies.

Grades

Magellanic Spiral Galaxies

Example Type Image Information Notes

SAdm

SAm

SABdm

SABm

SBdm

SBm

Sdm

Sm

Example Type Image Information Notes

dSAdm

dSAm

dSABdm

dSABm

dSBdm

dSBm

dSdm

dSm

List of Magellanic spiralsThis list is incomplete.

Barred (SBm)• Large Magellanic Cloud (LMC)• NGC 1311 [3]

• NGC 4618 [4]

References[1] Linda S. Sparke, John Sill Gallagher, "Galaxies in the Universe: An Introduction", 2ed., Cambridge University Press, '2007', ISBN

9780521855938[2] citeBase; Neutral Hydrogen in the Interacting Magellanic Spirals NGC 4618/4625 (http:/ / www. citebase. org/ abstract?id=oai:arXiv.

org:astro-ph/ 0409209); Stephanie J. Bush; Eric M. Wilcots; (accessed 1 March 2009)[3] Paul B. Eskridge; " Recent Star and Cluster Formation in the Nearby Magellanic Spiral NGC 1311 (http:/ / adsabs. harvard. edu/ abs/

2006AAS. . . 208. 1404E)"; 'American Astronomical Society Meeting' 208, #14.04; Bulletin of the American Astronomical Society, Vol.

Magellanic spiral 107

38, p.93; (accessed 1 March 2009)[4] University of Wisconsin, BARRED MAGELLANIC SPIRALS (http:/ / www. astro. wisc. edu/ ~ewilcots/ research/ sbm/ ) (accessed 1 March

2009)

See also• Galaxy classification

Pea galaxy

Galaxy Zoo Green Peas

Three HST pictures of Starburst Peas.

A Pea galaxy, also referred to as a Peaor Green Pea, is a rare class oflow-mass compact galaxy which isundergoing very high rates of starformation.[1] Pea Galaxies were firstdiscovered in 2007 by the volunteerusers within the forum section of theonline astronomy project Galaxy Zoo(GZ).[2] This project seeks to classifyup to one million galaxies and has beenonline since July 2007. Pea galaxiesare so-named because of their smallsize and greenish appearance in theimages taken by the Sloan Digital SkySurvey (SDSS) that are used as thedatabase for GZ. One of GZ's foundersKevin Schawinski said, "This is agenuine citizen science project. It's agreat example of how a new way ofdoing Science produced a result thatwouldn't have been possibleotherwise."[3]

History of discoveryAlongside its main pages that enable the user to classify galaxies, GZ has an online forum. On this forum, users get to ask questions and post interesting images, ideas or unusual objects. In July 2007, a few days after the start of GZ, a thread was started by Hanny Van Arkel called "Give peas a chance" in which various green objects were posted. This thread started humorously, but by December 2007, it had become clear that some of these unusual objects were a distinct group of galaxies. These "Pea galaxies" appear in the SDSS as unresolved green images. This is because the Peas have a very bright, or powerful, emission line in their spectra for highly-ionized Oxygen, which in SDSS color composites increases the luminosity, or brightness, of the "r" color band with respect to the two other color bands "g" and "i". The "r" color band shows as green in SDSS images.[1] [4] Enthusiasts, calling themselves the "Peas Corps", collected over a hundred of these Peas, which were eventually placed together into a dedicated thread started by Carolin Cardamone in July 2008. The collection, once refined, provided values that could be used in a systematic computer search of the GZ database of one million objects, which eventually resulted in a sample of 251 Green Peas. In July 2009, a paper titled "Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely Star-Forming

Pea galaxy 108

Galaxies" (Cardamone et al. 2009) was published by the MNRAS.[1] (e.g. [5] [6] [7] [8] ) Within the Galaxy Zoo GreenPeas paper, 10 GZ volunteers are acknowledged as having made a particularly significant contribution. They are:Elisabeth Baeten, Gemma Coughlin, Dan Goldstein, Brian Legg, Mark McCallum, Christian Manteuffel, RichardNowell, Richard Proctor, Alice Sheppard and Hanny Van Arkel. They are thanked for "giving Peas a chance." InApril 2010 in a paper which appears as a letter to The Astrophysical Journal, R. Amorin, E. Perez-Montero and J.Vilchez from the IAA-CISC explore issues concerning the metallicity of the 80 starburst Peas.[9] Ongoing research isalso taking place, including a forthcoming paper based on observations from the ESO at La Silla in Chile, as well asa survey by the GMRT in India.

DescriptionPea galaxies are essentially compact oxygen-rich emission line galaxies (ELG) that are at redshifts between z=0.112and 0.360. These low-mass galaxies have an upper size limit generally no bigger than 16300 light-years (5000 pc)[1]

across, and typically they reside in environments less than two-thirds the density of normal galaxy environments. Anaverage starburst Pea has a redshift of z=0.258, a mass of around 3,160 million solar masses, a star formation rate of13 solar masses a year, an [OIII] equivalent width of 69.4 nm and a low metallicity.[1] [9] They have a strongemission line at the [OIII] wavelength of 500.7 nm. [OIII], O++ or doubly ionized oxygen, is a forbidden line of thevisual spectrum and is only possible at very low densities. Pea galaxies are among the most active star-forminggalaxies ever found.[3]

Comparing a Pea galaxy to the Milky Way can be useful when trying to visualize these star-forming rates. TheMilky Way is a spiral galaxy and has a solar mass (M☉) of 580,000 million M☉.[10] [11] Research by the EuropeanSpace Agency and NASA has shown the Milky Way makes around 4 M☉/yr.[12]

An average starburst Pea galaxy has a mass of around 3,162 million M☉.[1] So, approximately, the Milky Way hasthe mass of 175 Peas. An average Pea makes around 13 M☉/yr, or 3.25 times as many M☉/yr as the Milky Way.[1] Ifthe mass of a Pea is made the same as the Milky Way, it can be seen that these Peas make solar masses 568 times asfast. If the Pea with the highest star-forming rate of 59 M☉/yr is compared, SDSS reference number587728906099687546 has a mass of 7,075 million M☉.[1] This Pea forms stars 14.75 times as fast as the Milky Way,which has the same mass as 80 of this Pea. If the mass of SDSS 587..546 is made the same as the Milky Way, it canbe seen that this Pea makes solar masses 1,180 times as fast.Pea galaxies exist at a time when the Universe was three-quarters of its present age and so are clues as to how galaxyformation took place in the earlier Universe.[13] [14] "These galaxies would have been normal in the early Universe,but we just don’t see such active galaxies today," said Schawinski. "Understanding the Green Peas may tell ussomething about how stars were formed in the early Universe and how galaxies evolve."[3]

Pea galaxy 109

Physics

Graph showing specific star formation rateplotted against galaxy mass, with the Peas (purple

diamonds) and the Galaxy Zoo Merger Sample(black points).

To date only five Pea galaxies have been imaged by the Hubble SpaceTelescope (HST). Three of these images, above right, reveal Peas to bemade up of bright clumps of star formation and low surface densityfeatures indicative of recent or ongoing galaxy mergers.[1] These threeHST images were imaged as part of a study of local ultraviolet(UV)-luminous galaxies in 2005.[15] Major mergers are frequently sitesof active star-formation and to the right a graph is shown that plotsspecific star formation rate (SFR / Galaxy Mass) against galaxymass.[16] In this graph, the Peas are compared to the 3003 mergersfrom the Galaxy Zoo Merger Sample.[6] It shows that the Peas havelow masses typical of dwarf galaxies and much higher star-formingrates compared to the GZ mergers. The black, dashed line shows aconstant SFR of 10 M☉/yr. Most Peas have SFR between 3 and 30M☉/yr, so follow this line closely.

Graph showing 103 Peas plotted as Starburstgalaxies (red stars), transition objects (green

crosses) or A.G.N. (blue diamonds).

Pea galaxies are rare. Of the one million objects that make up GZ'simage bank, only 251 Green Peas were found. After having to discard148 of these 251 because of atmospheric contamination of theirspectra, the 103 that were left, with the highest signal-to-noise ratio(SNR), were analyzed further and 80 were found to be starburstgalaxies.[1] The graph left, classifies 103 narrow-line Peas (all withSNR ≥ 3 in the emission lines) as 10 Active Galactic Nuclei (AGN)(blue diamonds), 13 transition objects (green crosses) and 80 starbursts(red stars). The solid line is: Kewley et al. (2001) maximal starburstcontribution (labelled Ke01).[17] [18] The dashed line is: Kauffmann etal. (2003) separating purely star-forming objects from AGN (labelledKa03).[19]

Histogram showing [OIII] Eq.Wth. of 10,000comparison galaxies (red); 215 UV-luminous

Galaxies (blue); Peas (green).

Pea galaxies have a strong emission line when compared to the rest oftheir spectral continuum.[20] On an SDSS spectrum, this shows up as alarge peak with [OIII] at the top.[21] The wavelength of [OIII](500.7 nm) was chosen to determine the luminosities of the Peas usingEquivalent Width (Eq.Wth.). The histogram on the right shows on thehorizontal scale the Eq.Wth. of a comparison of 10,000 normalgalaxies (marked red), UV-luminous Galaxies (marked blue) and Peas(marked green).[1] As can be seen from the histogram, the Eq.Wth. ofthe Peas is much larger than normal for even prolific starburst galaxiessuch as UV-luminous Galaxies.[22]

Within the GZ Green Peas paper, comparisons are made with othercompact galaxies, namely Blue Compact Dwarfs and UV-luminousGalaxies, at local and much higher distances.[23] The findings showthat Peas form a different class of galaxies than Ultra Blue CompactDwarfs, but may be similar to the most luminous members of the Blue Compact Dwarf Galaxy category.[24] The

Pea galaxy 110

Green Peas are also similar to UV luminous high redshift galaxies such as Lyman-break Galaxies and Lyman-alphaemitters.[25] [26] [27] It is concluded that if the underlying processes occurring in the Peas are similar to that found inthe UV-luminous high redshift galaxies, the Peas may be the last remnants of a mode of star formation common inthe early Universe.[1] [28] [29]

GANDALF spectrum for 587724241767825591

When compiling the paper, spectral classification was made using GasAnd Absorption Line Fitting (GANDALF).[1] This sophisticatedsoftware was programmed by Marc Sarzi, who helped analyze theSDSS spectra.[30] Also, a classic emission line diagnostic by Baldwin,Phillips and Terlevich was used to separate starbursts from AGN.[31]

On the left is an example spectral fit from GANDALF, showing thespectrum from a typical star-forming Pea. In black is the rest-frameobserved spectrum and in red the fit from GANDALF. The SDSS filterband passes are included as blue dotted lines, shifted into the

rest-frame of the Pea. Notice in the example, the [OIII] wavelength at 500.7 nm is redshifted inside the "r" colorband.

Histogram showing reddening values for Peas.

Pea galaxies have low interstellar reddening values, as shown in thehistogram on the right, with nearly all Peas having E(B-V) ≤ 0.25. Thedistribution shown indicates that the line-emitting regions ofstar-forming Peas are not highly reddened, particularly when comparedto more typical star-forming or starburst galaxies.[1] This lowreddening combined with very high UV luminosity is rare in galaxiesin the local Universe and is more typically found in galaxies at higherredshifts.[7]

Cardamone et al. describe Pea galaxies as having a low metallicity, butthat the oxygen present is highly ionized. It should be explained thatAstronomers label all elements other than hydrogen or helium as 'metals'. The average Pea has a metallicity oflog[O/H]+12~8.69, which is solar or sub-solar, depending on which set of standard values is used.[1] [32] [33] [34] [35]

Although the Peas are in general consistent with the mass-metallicity relation, they depart from it at the highest massend and thus do not follow the trend. Peas have a range of masses, but a more uniform metallicity than the samplecompared against.[36] These metallicities are common in low mass galaxies such as Peas.[1]

However, in April 2010, Amorin et al. dispute the metallicities calculated in the original Cardamone et al. GreenPeas paper, which are found in Table 4, Column 8, page 16.[1] [9] In a paper, which appears as a letter to TheAstrophysical Journal, R. Amorin, E. Perez-Montero and J. Vilchez from the IAA-CISC, use a differentmethodology from Cardamone et al. to produce metallicity values more than one fifth (20%) of the previous values(about 20% solar or one fifth solar). These mean values are log[O/H]+12~8.05, which shows a clear offset of0.65dex between the two papers' values. It should be noted that Amorin et al. use a smaller sample of 80 Peas, ofwhich all are starburst galaxies, rather than the sample of over 200 that were used by Cardamone et al. For these 80Peas, Amorin et al., using a direct method, rather than strong-line methods as used in Cardamone et al., calculatephysical properties, as well as oxygen and nitrogen ionic abundances.[37] These metals pollute hydrogen and helium,which make up the majority of the substances present in galaxies. As these metals are produced in Supernovae, theolder a galaxy is, the more metals it would have. As Peas are in the nearby, or older, Universe, they should havemore metals than galaxies at an earlier time.

Pea galaxy 111

Amorin's Pea Metallicity graph.

Amorin et al. find that the amount of metals, including the abundanceof nitrogen, are different from normal values and that Peas are notconsistent with the mass-metallicity relation, as concluded byCardamone et al.[1] [38] This analysis indicates that Peas can beconsidered as genuine metal-poor galaxies. They then argue that thisoxygen under-abundance is due to a recent interaction-induced inflowof gas, possibly coupled with a selective metal-rich gas loss driven bySupernovae winds and that this can explain their findings.[36] [39] Thisfurther suggests that Peas are likely very short-lived as the intense starformation in them would quickly enrich the gas.[9]

As well as the optical images from the SDSS, measurements from the GALEX survey were used to determine theultraviolet values.[40] This survey is well matched in depth and area, and 139 of the sampled 251 Green Peas arefound in GALEX Release 4 (G.R.4).[41] For the 56 of the 80 star-forming Peas with GALEX detections, the medianluminosity is 30,000 million .

Facts, figures and analysis of the Peas paper

The 80 starburst Peas.

These figures are from Table 4, pages 16–17 of "Galaxy ZooGreen Peas" showing the 80 starburst Peas that were analyzedin the Peas paper. The long 18-digit numbers are the SDSSreference numbers, which link to the appropriate entry at theSDSS Skyserver website.

Greatest Least Average Nearest to Average

Distance z=0.348(587732134315425958 [42])

z=0.141(587738947196944678 [43])

z=0.2583 z=0.261(587724240158589061 [44])

Mass 1010.48 M☉(588023240745943289 [45])

108.55 M☉(587741392649781464 [46])

109.48 M☉ 109.48 M☉(587724241767825591 [47])

Rate of star-forming 59 M☉/yr(587728906099687546 [48])

2 M☉/yr(588018090541842668 [49])

13.02 M☉/yr 13 M☉/yr(588011122502336742 [50])

Luminosity ([OIII] Eq.Wth.) 238.83 nm(587738410863493299 [51])

1.2 nm(587741391573287017 [52])

69.4 nm 67.4 nm(588018090541842668 [49])

Luminosity (UV) 36.1×1036 W(587733080270569500 [53])

1.9×1036 W(588848899919446344 [54])

12.36×1036 W 12.3×1036 W(588018055652769997 [55])

Pea galaxy 112

r-i vs g-r color-color diagram for 251 Peas (greencrosses), a sample of normal galaxies (red points)

and all quasars (purple points).

Color selection was by using the difference in the levels of three filters,in order to capture these color limits: u-r ≤ 2.5 (1), r-i ≤ -0.2 (2), r-z ≤0.5 (3), g-r ≥ r-i + 0.5 (4), u-r ≥ 2.5 (r-z) (5).[1] If the diagram on theright (one of two in the paper) is looked at, the effectiveness of thiscolor selection can be seen. The color-color diagram shows ~100Green Peas (green crosses), 10,000 comparison galaxies (red points)and 9,500 comparison quasars (purple stars) at similar redshifts toPeas. The black lines show how these figures directly above are on thediagram.

One of the original ways of recognizing Pea galaxies, before SQLprogramming was involved, was because of a discrepancy about howthe SDSS labels them within Skyserver.[56] Out of the 251 of theoriginal sample that were identified by the SDSS spectroscopicpipeline as having galaxy spectra, only 7 were targeted by the SDSSspectral fibre allocation as galaxies i.e. 244 were not.[1] [57] Indeed themajority of them are incorrectly classified as "stars". If the SDSS Skyserver pages for the following three (randomlychosen) Peas are looked at, then it can be read on the second line down that the SDSS incorrectly classifies these asstars, whereas further down the page under the picture they are correctly classified as galaxies:

1. 587726102030451047 [58]

2. 587724240158589061 [44]

3. 587742014876745993 [59]

Peas of varying colorsSpeculative examples of Peas at differing distances have been found throughout the search for green Peas. Thesehave a variety of colors, according to redshift. Some examples are given here: these are classified as 'stars' and thenas 'galaxies', thereby following the trend of green Peas. 587727179003723785, 587731521744928978,587727180597625000, 587739408378626262, 587735666917114003, 587736585507766663. None of the galaxieslisted directly above are able to have citations, but seem to be good candidates at this time.

References[1] Cardamone, C.; K. Schawinski, M. Sarzi, S. Bamford, N. Bennert, C. Urry, C. Lintott, W. Keel et al. (2009). "Galaxy Zoo Green Peas:

Discovery of A Class of Compact Extremely Star-Forming Galaxies". MNRAS. arXiv:0907.4155v1.[2] Jordan Raddick, M.; G. Bracey, P. Gay, C. Lintott, P. Murray, K. Schawinski, A. Szalay, J. Vandenberg (2009). "Exploring the motivations

of citizen science volunteers". MNRAS 389 (1179). arXiv:0909.2925v1.[3] "Galaxy Zoo Hunters Help Astronomers Discover Rare ‘Green Pea’ Galaxies" (http:/ / opa. yale. edu/ news/ article. aspx?id=6807). Yale

Bulletin. July 27, 2009. . Retrieved December 29, 2009.[4] "SDSS Color" (http:/ / cas. sdss. org/ dr6/ en/ proj/ basic/ color/ ). SDSS. . Retrieved 2010-01-23.[5] Raiter, A.; R. Fosbury, H. Teimoorinia (February 2010). "Ly-alpha emitters in the GOODS-S field: a powerful pure nebular SED with

Nitrogen IV emission at z=5.563". Astronomy & Astrophysics 510. doi:10.1051/0004-6361/200912429arXiv:0912.4305.[6] Darg, D.; S. Kaviraj, C. Lintott, K. Schawinski, M. Sarzi, S. Bamford, J. Silk, R. Proctor, et al. (2009). "Galaxy Zoo: The fraction of merging

galaxies in the SDSS and their morphologies". MNRAS 619. arXiv:0903.4937.[7] Masters, K.; R. Nichol, S. Bamford, M. Mosleh, C. Lintott et al. (2010). "Galaxy Zoo: Dust in Spirals". MNRAS. arXiv:1001.1744.[8] Overzier, R.; T. Heckman, D. Schiminovich, A. Basu-Zych, T. Goncalves, D. Martin, R. Rich (2009). "Morphologies of local Lyman break

galaxy analogs II: A Comparison with galaxies at z=2-4 in ACS and WFC3 images of the Hubble Ultra Deep Field". Astrophysical Journal.arXiv:0911.1279.

[9] Amorín, Ricardo O; Pérez-Montero, Enrique; Vílchez, Jose M (2010). "On the oxygen and nitrogen chemical abundances and the evolutionof the "green pea" galaxies.". The Astrophysical Journal Letters 715 (L128): 8. doi:10.1088/2041-8205/715/2/L128. arXiv:1004.4910.

[10] Karachentsev, Igor D; Kashibadze, Olga G (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field" (http:/ / adsabs. harvard. edu/ cgi-bin/ nph-bib_query?bibcode=2006Ap. . . . . 49. . . . 3K). The Journal of Astrophysics 49 (1):

Pea galaxy 113

3–18. doi:10.1007/s10511-006-0002-6. . Retrieved 2010-01-16.[11] Vayntrub, Alina (2000). "Mass of the Milky Way" (http:/ / hypertextbook. com/ facts/ 2000/ AlinaVayntrub. shtml). The Physics Factbook. .

Retrieved 2010-01-16.[12] "Milky Way Churns Out Seven New Stars Per Year, Scientists Say" (http:/ / www. physorg. com/ news9595. html/ ). PhysOrg. January 5,

2006. . Retrieved December 29, 2009.[13] "New Image of Infant Universe reveals era of first stars, age of cosmos, and more" (http:/ / www. nasa. gov/ centers/ goddard/ news/

topstory/ 2003/ 0206mapresults. html). NASA. 2003. . Retrieved 2010-01-16.[14] "Gamma-Ray Burst GRB 050724" (http:/ / www. eso. org/ public/ images/ eso0541a/ ). European Southern Observatory. 2005. . Retrieved

2010-01-16.[15] Heckman, T.; C. Hoopes, M. Seibert, D. Martin, S. Salim, R. Rich, G. Kauffmann et al. (2005). "The Properties of Ultraviolet-Luminous

Galaxies at the Current Epoch" (http:/ / adsabs. harvard. edu/ abs/ 2005ApJ. . . 619L. . 35H). Astrophysical journal 619. doi:10.1086/425979. .Retrieved 2010-01-14.

[16] Bauer, A.; N. Drory, G. Hill, G. Feulner (2005). "Specific Star Formation Rates to Redshift 1.5" (http:/ / www. iop. org/ EJ/ article/1538-4357/ 621/ 2/ L89/ 18787. web. pdf?request-id=f26140f5-4a2e-4eaa-a1fd-0a5113b195bc). The Astrophysical Journal. . Retrieved2010-01-14.

[17] Kewley, L.; M. Dopita, R. Sutherland, C. Heisler, J. Trevena (March 2001). "Theoretical Modeling of Starburst Galaxies" (http:/ / www.iop. org/ EJ/ article/ 0004-637X/ 556/ 1/ 121/ 53006. text. html). The Astrophysical Journal. . Retrieved 2010-01-14.

[18] Groves, B.; L. Kewley (2007). "Distinguishing Active Galactic Nuclei and Star Formation". ArXiv. arXiv:0707.0158v1.[19] Kauffmann, G.; T. Heckman, S. White, S. Charlot, C. Tremonti, J. Brinchman et al. (2003). "Stellar Masses and Star Formation Histories"

(http:/ / adsabs. harvard. edu/ abs/ 2003MNRAS. 341. . . 33K). MNRAS. doi:10.1046/j.1365-8711.2003.06291.x. . Retrieved 2010-01-14.[20] Strauss, M. (2003). "Measuring Spectra" (http:/ / www. sdss. org/ dr7/ products/ general/ edr_html/ node61. html). SDSS. . Retrieved

2010-01-16.[21] "SDSS_Spectra" (http:/ / cas. sdss. org/ astro/ en/ get/ specById. asp?id=219212317798170624). SDSS. . Retrieved 2010-01-17.[22] Hoopes, C.; T. Heckman, S. Salim, M. Seibert, C. Tremonti, D. Schiminovich et al. (2007). /441 "The Diverse Properties of the Most

Ultraviolet-Luminous Galaxies Discovered by GALEX" (http:/ / www. iop. org/ EJ/ abstract/ 0067-0049/ 173/ ). The Astrophysical Journal173. doi:10.1086/516644. /441. Retrieved 2010-01-16.

[23] Vaduvescu, O.; M. McCall, M. Richer (2007). "Chemical Properties of Star-Forming Dwarf Galaxies" (http:/ / www. iop. org/ EJ/ article/1538-3881/ 134/ 2/ 604/ 205645. text. html). The Astronomical Journal. . Retrieved 2010-01-14.

[24] Corbin, M.; W. Vacca, R. Cid Fernandes, J. Hibbard, R. Somerville, R. Windhorst (2006). "Ultracompact Blue Dwarf Galaxies: HSTImaging and Stellar Population Analysis" (http:/ / www. iop. org/ EJ/ article/ 0004-637X/ 651/ 2/ 861/ 65208. web. pdf). The AstrophysicalJournal. . Retrieved 2010-01-14.

[25] Bremer, M.; M. Lehnert, I. Waddington, M. Hardcastle, P. Boyce, S. Phillipps (2004). "The Properties of Galaxies at z~5". MNRAS.doi:10.1111/j.1365-2966.2004.07352.xarXiv:astro-ph/0306587.

[26] Gronwall, C.; R. Ciardullo, T. Hickey, E. Gawiser, J. Feldmeier, P. van Dokkum, C. Urry et al. (2007). "Lyα Emission-Line Galaxies at z =3.1 in the Extended Chandra Deep Field-South" (http:/ / www. iop. org/ EJ/ abstract/ 0004637X/ 667/ 1/ 79). The Astrophysical Journal.doi:10.1086/520324. . Retrieved 2010-01-14.

[27] L., Pentericci; A. Grazian, A. Fontana, M. Castellano, E. Giallongo, S. Salimbeni and P. Santini (February 1, 2009). "The physical propertiesof Ly$\alpha$ emitting galaxies: not just primeval galaxies?" (http:/ / www. aanda. org/ index. php?option=com_article& access=doi&doi=10. 1051/ 0004-6361:200810722& Itemid=129). Astronomy & Astrophysics 494 (2). doi: 10.1051/0004-6361:200810722. . Retrieved2010-06-15.

[28] Gawiser, E.; H. Francke, K. Lai, K. Schawinski, C. Gronwall, R. Ciardullo, R. Quadri, A. Orsi et al. (2007). "Lyα-Emitting Galaxies at z =3.1: Progenitors Experiencing Rapid Star Formation" (http:/ / adsabs. harvard. edu/ abs/ 2007ApJ. . . 671. . 278G). The Astrophysical Journal671. doi:10.1086/522955. . Retrieved 2010-01-14.

[29] Giavalisco, M.; M. Dickinson, H. Ferguson, S. Ravindranath, C. Kretchmer, L. Moustakas, P. Madau et al. (2004). "The Rest-FrameUltraviolet Luminosity Density of Star-forming Galaxies at Redshifts z > 3.51" (http:/ / www. iop. org/ EJ/ article/ 1538-4357/ 600/ 2/ L103/17427. text. html). The Astrophysical Journal 600. . Retrieved 2010-01-14.

[30] Sarzi, M.; J. Falcon-Barroso, R. Davies, R. Bacon, M. Bureau, M. Cappellari et al. (2006). "Integral-field emission-line kinematics of 48elliptical and lenticular galaxies" (http:/ / www. eso. org/ ~hkuntsch/ papers/ MNRAS_366_1151. pdf). MNRAS 366. . Retrieved 2010-01-14.

[31] Baldwin, J.; M. Phillips, R. Terlevich (1981). "Classification parameters for the emission-line spectra of extragalactic objects" (http:/ /adsabs. harvard. edu/ abs/ 1981PASP. . . 93. . . . 5B). The Astronomical Society of the Pacific 93. doi:10.1086/130766. . Retrieved 2010-01-15.

[32] Grevesse, N.; A. Sauval (1998). "Standard Solar Composition" (http:/ / adsabs. harvard. edu/ cgi-bin/ bib_query?1998SSRv. . . 85. . 161G).Space Science Reviews 85. doi:10.1023/A:1005161325181. . Retrieved 2010-01-16.

[33] Allende Prieto, Carlos; David L. Lambert and Martin Asplund (July 2001). "The Forbidden Abundance of Oxygen in the Sun" (http:/ /iopscience. iop. org/ 1538-4357/ 556/ 1/ L63). The Astrophysical Journal 556 (1). doi:10.1086/322874. . Retrieved 2010-05-06.

[34] Asplund, M.; Grevesse N., Sauval A.J. (2005). "Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis". TheAstronomical Society of the Pacific Conference Series 336. arXiv:astro-ph/0410214.

[35] Basu, Sarbani; Antia H.M. (2007). "Helioseismology and Solar Abundances". Physics Reports. doi: 10.1016/j.physrep.2007.12.002arXiv:0711.4590.

Pea galaxy 114

[36] Tremonti, C.; T. Heckman, G. Kauffmann, J. Brinchmann, S. Charlot et al. (2004). "The Origin of the Mass-Metallicity Relation: Insightsfrom 53,000 Star-forming Galaxies in the Sloan Digital Sky Survey" (http:/ / www. adsabs. harvard. edu/ abs/ 2004ApJ. . . 613. . 898T). TheAstrophysical Journal 613. doi:10.1086/423264. . Retrieved 2010-01-16.

[37] Perez-Montero, E.; Contini, T (2009). "The impact of the nitrogen-to-oxygen ratio on ionized nebulae diagnostics based on [NII]emissionlines". MNRAS 398 (2). doi:10.1111/j.1365-2966.2009.15145.xarXiv:0905.4621.

[38] Lequeux, J.; Peimbert, M., Rayo, J.F., Serrano, A., Torres-Peimbert, S. (1979). "Chemical composition and evolution of irregular and bluecompact galaxies" (http:/ / cdsads. u-strasbg. fr/ abs/ 1979A& A. . . . 80. . 155L). Astronomy and Astrophysics 80 (2): 155–166. . Retrieved2010-05-03.

[39] Finlator, Kristian; Davé, R. (2008). "The Origin of the Galaxy Mass-Metallicity Relation and Implications for Galactic Outflows". MNRAS385 (4). doi:10.1111/j.1365-2966.2008.12991.xarXiv:0704.3100.

[40] "GALEX Observes the Universe" (http:/ / www. nasa. gov/ missions/ deepspace/ galex_mission. html). NASA. 2003. . Retrieved2010-01-16.

[41] Morrissey, P.; T. Conrow, T. Barlow, T. Small, M. Seibert, T. Wyder et al. (2007). "The Calibration and Data Products of GALEX" (http:/ /adsabs. harvard. edu/ abs/ 2007ApJS. . 173. . 682M). The Astrophysical Journal Supplement 173. doi:10.1086/520512. . Retrieved2010-01-16.

[42] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=587732134315425958[43] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=587738947196944678[44] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=587724240158589061[45] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=588023240745943289[46] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=587741392649781464[47] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=587724241767825591[48] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=587728906099687546[49] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=588018090541842668[50] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=588011122502336742[51] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=587738410863493299[52] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=587741391573287017[53] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=587733080270569500[54] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=588848899919446344[55] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=588018055652769997[56] "SDSS Skyserver" (http:/ / cas. sdss. org/ dr7/ en/ ). SDSS. . Retrieved 2010-01-17.[57] Stoughton, C.; R. Lupton, M. Bernardi, M. Blanton, M. Burles, F. Castander et al. (2002). "Sloan Digital Sky Survey: Early Data Release"

(http:/ / www. adsabs. harvard. edu/ abs/ 2002AJ. . . . 123. . 485S). The Astronomical Journal 123. doi:10.1086/324741. . Retrieved2010-01-15.

[58] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=587726102030451047[59] http:/ / cas. sdss. org/ astro/ en/ tools/ explore/ obj. asp?id=587742014876745993

See also• Galaxy formation and evolution• Galaxy merger• Interacting galaxies• Starburst galaxy• Star formation• Dwarf galaxy• Blue compact dwarf galaxy• Astronomy• Ultraviolet astronomy• Sloan Digital Sky Survey

Peculiar galaxy 115

Peculiar galaxyA peculiar galaxy is a galaxy which is unusual in its size, shape, or composition. Peculiar galaxies come about as aresult of interactions between galaxies, and they may contain atypical amounts of dust or gas, may have higher orlower surface brightness than a typical galaxy, or may have features such as nuclear jets. They can be highlyirregular in shape due to the immense gravitational forces which act on them during encounters with other galaxies.Peculiar galaxies are designated by "p" or "pec" in some catalogs.

See also

• Atlas of Peculiar Galaxies• Irregular galaxy

• Ring galaxy• Starburst galaxy

External links• Peculiar galaxy [1]

References[1] http:/ / www. daviddarling. info/ encyclopedia/ P/ peculiar_galaxy. html

Polar-ring galaxy

NGC 4650A, an example of apolar-ring galaxy. Credit:Hubble

Space Telescope/NASA/ESA.

A polar-ring galaxy is a type of galaxy in which an outer ring of gas and starsrotates over the poles of the galaxy.[1] These polar rings are thought to formwhen two galaxies gravitationally interact with each other. One possibility isthat a material is tidally stripped from a passing galaxy to produce the polar ringseen in the polar-ring galaxy. The other possibility is that a smaller galaxycollides orthogonally with the plane of rotation of the larger galaxy, with thesmaller galaxy effectively forming the polar-ring structure.[2]

The best-known polar-ring galaxies are S0s (lenticular galaxies), but from thephysical point of view they are part of a wider category of galaxies, includingseveral ellipticals.

Polar-ring galaxy 116

NGC660 polar galaxy. 24-inch telescope on Mt. Lemmon, AZ. Courtesy Joseph D.Schulman

The first four S0 galaxies that wereidentified as polar-ring galaxies wereNGC 2685,[3] NGC 4650A,[4] [5] A0136 -0801,[2] and ESO 415 -G26.[5]

While these galaxies have beenextensively studied, many otherpolar-ring galaxies have since beenidentified.[6] Polar-ring S0 galaxiesmay be found around 0.5% of allnearby lenticular galaxies, and it ispossible that 5% of lenticular galaxiesmay have had polar rings at some pointduring their lifetimes.[6]

The first polar-ring elliptical galaxieswere identified in 1978. They wereNGC 5128, NGC 5363, NGC 1947 andCygnus A[7] , while the polar-ring S0galaxies NGC 2685 and NGC 4650were at that time indicated as resulting

from similar formation processes[7] . Only some years later, when the first observations of the stellar and gas motionof polar-ring elliptical and S0 galaxies were possible with a better spectroscopic technology, the external origin ofthe gaseous rings was clarified.[2] [5] [8] [9] In addition to the best-known example, NGC 5128 (Cen A), a very regularpolar ring elliptical, is NGC 5266[9]

References[1] James Binney, Michael Merrifield (1998). Galactic Astronomy. Princeton, New Jersey: Princeton University Press. ISBN 0-691-00402-1.[2] F. Schweizer, B. C. Whitmore, V. C. Rubin (1983). "Colliding and merging galaxies. II - S0 galaxies with polar rings" (http:/ / adsabs.

harvard. edu/ abs/ 1983AJ. . . . . 88. . 909S). Astronomical Journal 88: 909–925. doi:10.1086/113377. .[3] P. L. Schecter, J. E. Gunn (1978). "NGC 2685 - Spindle or pancake" (http:/ / adsabs. harvard. edu/ abs/ 1978AJ. . . . . 83. 1360S).

Astronomical Journal 83: 1360–1362. doi:10.1086/112324. .[4] J. L. Sérsic (1967). "Southern Peculiar Galaxies III" (http:/ / adsabs. harvard. edu/ abs/ 1967ZA. . . . . 67. . 306S). Zeitschrift für Astrophysik

67: 306–311. .[5] B. C. Whitmore, D. B. McElroy, F. Schweizer (1987). "The shape of the dark halo in polar-ring galaxies" (http:/ / adsabs. harvard. edu/ abs/

1987ApJ. . . 314. . 439W). Astrophysical Journal 314: 439–456. doi:10.1086/165077. .[6] B. C. Whitmore, R. A. Lucas, D. B. McElroy, T. Y. Steiman-Cameron, P. D. Sackett, R. P. Olling (1990). "New observations and a

photographic atlas of polar-ring galaxies" (http:/ / adsabs. harvard. edu/ abs/ 1990AJ. . . . 100. 1489W). Astronomical Journal 100:1489–1522, 1721–1755. doi:10.1086/115614. .

[7] Bertola, F. and Galletta, G. (1978). "A new type of galaxy with prolate structure" (http:/ / adsabs. harvard. edu/ abs/ 1978ApJ. . . 226L.115B). Astrophysical Journal 226: L115–L118. doi:10.1086/182844. .,

[8] Bertola, F., Galletta, G., Zeilinger, W.~W. (1985,). "Warped dust lanes in elliptical galaxies - Transient or stationary phenomena?" (http:/ /adsabs. harvard. edu/ abs/ 1985ApJ. . . 292L. . 51B). Astrophysical Journal 292,: L51–L55. doi:10.1086/184471. .

[9] Varnas, S.R. Bertola, F., Galletta, G., Freeman, K.C., Carter, D. (1987). "NGC 5266 - an elliptical galaxy with a dust ring" (http:/ / adsabs.harvard. edu/ abs/ 1987ApJ. . . 313. . . 69V). Astrophysical Journal 313: 69–88,. doi:10.1086/164949. .

Polar-ring galaxy 117

External links• Astronomy Picture of the Day

• Polar Ring Galaxy NGC 4650A (http:/ / antwrp. gsfc. nasa. gov/ apod/ ap990510. html) - May 10, 1999• Polar Ring Galaxy NGC 2685 (http:/ / apod. nasa. gov/ apod/ ap070216. html) - 2007 February 16• Polar Ring Galaxy NGC 660 (http:/ / apod. nasa. gov/ apod/ ap091203. html) - 2009 December 3

• Internet Voters Get Two Galaxies in One from Hubble (http:/ / hubblesite. org/ newscenter/ archive/ releases/1999/ 16/ )

• X marks the spot in dark matter web (http:/ / space. newscientist. com/ article/ mg19726455.500-x-marks-the-spot-in-dark-matter-web. html?feedId=online-news_rss20) - Polar ring galaxies offer first-handevidence of the existence of the cosmic web, New Scientist, 29 February 2008

See also• List of polar-ring galaxies• Ring galaxy

ProtogalaxyIn physical cosmology, a protogalaxy, which could also be called a "primeval galaxy", is a cloud of gas which isforming into a galaxy. It is believed that the rate of star formation, during this period of galactic evolution, willdetermine whether a galaxy is a spiral or elliptical galaxy; a slower star formation tends to produce a spiral galaxy.The smaller clumps of gas in a protogalaxy form into stars. The term protogalaxy was mainly used in the Big BangTheory.

See also• Dwarf galaxy• Globular cluster• Big Bang

External links• Rare Blob Unveiled: Evidence For Hydrogen Gas Falling Onto A Dark Matter Clump? [1] European Southern

Observatory (ScienceDaily) July 3, 2006

References[1] http:/ / www. sciencedaily. com/ releases/ 2006/ 07/ 060703163148. htm

Quasar 118

Quasar

An artist's impression of a growing quasar.

A quasi-stellar radio source ("quasar") is a veryenergetic and distant active galactic nucleus. They arethe most luminous objects in the universe. Quasarswere first identified as being high redshift sources ofelectromagnetic energy, including radio waves andvisible light, that were point-like, similar to stars, ratherthan extended sources similar to galaxies.

While there was initially some controversy over thenature of these objects—as recently as the early 1980s,there was no clear consensus as to their nature—there isnow a scientific consensus that a quasar is a compactregion in the center of a massive galaxy surrounding itscentral supermassive black hole. Its size is 10–10,000times the Schwarzschild radius of the black hole. Thequasar is powered by an accretion disc around the black hole.

OverviewQuasars show a very high redshift, which is an effect of the expansion of the universe between the quasar and theEarth.[1] They are the most luminous, powerful, and energetic objects known in the universe. They tend to inhabit thevery centers of active young galaxies and can emit up to a thousand times the energy output of the Milky Way.When combined with Hubble's law, the implication of the redshift is that the quasars are very distant—and thus, itfollows, objects from much earlier in the universe's history. The most luminous quasars radiate at a rate that canexceed the output of average galaxies, equivalent to one trillion (1012) suns. This radiation is emitted across thespectrum, almost equally, from X-rays to the far-infrared with a peak in the ultraviolet-optical bands, with somequasars also being strong sources of radio emission and of gamma-rays. In early optical images, quasars looked likesingle points of light (i.e. point sources), indistinguishable from stars, except for their peculiar spectra. With infraredtelescopes and the Hubble Space Telescope, the "host galaxies" surrounding the quasars have been identified in somecases.[2] These galaxies are normally too dim to be seen against the glare of the quasar, except with these specialtechniques. Most quasars cannot be seen with small telescopes, but 3C 273, with an average apparent magnitude of12.9, is an exception. At a distance of 2.44 billion light-years, it is one of the most distant objects directly observablewith amateur equipment.Some quasars display changes in luminosity which are rapid in the optical range and even more rapid in the X-rays.This implies that they are small (Solar System sized or less) because an object cannot change faster than the time ittakes light to travel from one end to the other; but relativistic beaming of jets pointed nearly directly toward usexplains the most extreme cases. The highest redshift known for a quasar (as of December 2007) is 6.43,[3] whichcorresponds to a proper distance of approximately 28 billion light-years from Earth.Quasars are believed to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, making these luminous versions of the general class of objects known as active galaxies. Since light cannot escape the super massive black holes that are at the centre of quasars, the escaping energy is actually generated outside the event horizon by gravitational stresses and immense friction on the incoming material.[4] Large central masses (106 to 109 Solar masses) have been measured in quasars using 'reverberation mapping'. Several dozen nearby large galaxies, with no sign of a quasar nucleus, have been shown to contain a similar central black hole in their nuclei, so it is thought that all large galaxies have one, but only a small fraction emit powerful radiation and so

Quasar 119

are seen as quasars. The matter accreting onto the black hole is unlikely to fall directly in, but will have some angularmomentum around the black hole that will cause the matter to collect in an accretion disc. Quasars may also beignited or re-ignited from normal galaxies when infused with a fresh source of matter. In fact, it has been theorizedthat a quasar could form as the Andromeda galaxy collides with our own Milky Way galaxy in approximately 3–5billion years.[4] [5] [6]

Properties of quasarsMore than 200,000 quasars are known, most from the Sloan Digital Sky Survey. All observed quasar spectra haveredshifts between 0.06 and 6.5. Applying Hubble's law to these redshifts, it can be shown that they are between 780million and 28 billion light-years away (in terms of proper distance). Because of the great distances to the furthestquasars and the finite velocity of light, we see them and their surrounding space as they existed in the very earlyuniverse.Most quasars are known to be farther than three billion light-years away. Although quasars appear faint when viewedfrom Earth, the fact that they are visible from so far away means that quasars are the most luminous objects in theknown universe. The quasar that appears brightest in the sky is 3C 273 in the constellation of Virgo. It has anaverage apparent magnitude of 12.8 (bright enough to be seen through a medium-size amateur telescope), but it hasan absolute magnitude of −26.7. From a distance of about 33 light-years, this object would shine in the sky about asbrightly as our sun. This quasar's luminosity is, therefore, about 2 trillion (2 × 1012) times that of our sun, or about100 times that of the total light of average giant galaxies like our Milky Way. However, this assumes the quasar isradiating energy in all directions. An active galactic nucleus can be associated with a powerful jet of matter andenergy; it need not be radiating in all directions. In a universe containing hundreds of billions of galaxies, most ofwhich had active nuclei billions of years ago and would be seen located billions of light-years away, it is statisticallycertain that thousands of energy jets are pointed toward us, some more directly than others. In many cases it is likelythat the brighter the quasar, the more directly its jet is aimed at us.The hyperluminous quasar APM 08279+5255 was, when discovered in 1998, given an absolute magnitude of −32.2,although high resolution imaging with the Hubble Space Telescope and the 10 m Keck Telescope revealed that thissystem is gravitationally lensed. A study of the gravitational lensing in this system suggests that it has beenmagnified by a factor of ~10. It is still substantially more luminous than nearby quasars such as 3C 273.Quasars were much more common in the early universe. This discovery by Maarten Schmidt in 1967 was earlystrong evidence against the Steady State cosmology of Fred Hoyle, and in favor of the Big Bang cosmology. Quasarsshow where massive black holes are growing rapidly (via accretion). These black holes grow in step with the mass ofstars in their host galaxy in a way not understood at present. One idea is that the jets, radiation and winds fromquasars shut down the formation of new stars in the host galaxy, a process called 'feedback'. The jets that producestrong radio emission in some quasars at the centers of clusters of galaxies are known to have enough power toprevent the hot gas in these clusters from cooling and falling down onto the central galaxy.Quasars are found to vary in luminosity on a variety of time scales. Some vary in brightness every few months, weeks, days, or hours. This means that quasars generate and emit their energy from a very small region, since each part of the quasar would have to be in contact with other parts on such a time scale to coordinate the luminosity variations. As such, a quasar varying on the time scale of a few weeks cannot be larger than a few light-weeks across. The emission of large amounts of power from a small region requires a power source far more efficient than the nuclear fusion which powers stars. The release of gravitational energy by matter falling towards a massive black hole is the only process known that can produce such high power continuously. (Stellar explosions - Supernovas and gamma-ray bursts - can do so, but only for a few weeks.) Black holes were considered too exotic by some astronomers in the 1960s, and they suggested that the redshifts arose from some other (unknown) process, so that the quasars were not really so distant as the Hubble law implied. This 'redshift controversy' lasted for many years. Many lines of evidence (seeing host galaxies, finding 'intervening' absorption lines, gravitational lensing) now demonstrate

Quasar 120

that the quasar redshifts are due to the Hubble expansion, and quasars are as powerful as first thought.[7]

Quasars have all the same properties as active galaxies, but are more powerful: Their radiation is partially'nonthermal' (i.e. not due to a black body), and some (~10%) are observed to also have jets and lobes like those ofradio galaxies that also carry significant (but poorly known) amounts of energy in the form of high energy (i.e.rapidly moving, close to the speed of light) particles (either electrons and protons or electrons and positrons).Quasars can be detected over the entire observable electromagnetic spectrum including radio, infrared, optical,ultraviolet, X-ray and even gamma rays. Most quasars are brightest in their rest-frame near-ultraviolet (near the 1216angstrom (121.6 nm) Lyman-alpha emission line of hydrogen), but due to the tremendous redshifts of these sources,that peak luminosity has been observed as far to the red as 9000 angstroms (900 nm or 0.9 µm), in the near infrared.A minority of quasars show strong radio emission, which originates from jets of matter moving close to the speed oflight. When looked at down the jet, these appear as a blazar and often have regions that appear to move away fromthe center faster than the speed of light (superluminal expansion). This is an optical illusion due to the properties ofspecial relativity.Quasar redshifts are measured from the strong spectral lines that dominate their optical and ultraviolet spectra. Theselines are brighter than the continuous spectrum, so they are called 'emission' lines. They have widths of severalpercent of the speed of light. These widths are due to Doppler shifts caused by the high speeds of the gas emitting thelines. Fast motions strongly indicate a large mass. Emission lines of hydrogen (mainly of the Lyman series andBalmer series), helium, carbon, magnesium, iron and oxygen are the brightest lines. The atoms emitting these linesrange from neutral to highly ionized, i.e. many of the electrons are stripped off the ion, leaving it highly charged.This wide range of ionization shows that the gas is highly irradiated by the quasar, not merely hot, and not by stars,which cannot produce such a wide range of ionizationIron quasars show strong emission lines resulting from low ionization iron (FeII), such as IRAS 18508-7815.

Quasar emission generation

This view, taken with infrared light, is afalse-color image of a quasar-starburst tandemwith the most luminous starburst ever seen in

such a combination.

Since quasars exhibit properties common to all active galaxies, theemissions from quasars can be readily compared to those of smallactive galaxies powered by supermassive black holes. To create aluminosity of 1040 W, or Joules per second, (the typical brightness of aquasar), a super-massive black hole would have to consume thematerial equivalent of 10 stars per year. The brightest known quasarsdevour 1000 solar masses of material every year. The largest known isestimated to consume matter equivalent to 600 Earths per minute.Quasars 'turn on' and off depending on their surroundings, and sincequasars cannot continue to feed at high rates for 10 billion years, aftera quasar finishes accreting the surrounding gas and dust, it becomes anordinary galaxy.

Quasars also provide some clues as to the end of the Big Bang'sreionization. The oldest quasars (redshift ≥ 6) display a Gunn-Petersontrough and have absorption regions in front of them indicating that theintergalactic medium at that time was neutral gas. More recent quasars show no absorption region but rather theirspectra contain a spiky area known as the Lyman-alpha forest. This indicates that the intergalactic medium hasundergone reionization into plasma, and that neutral gas exists only in small clouds.

One other interesting characteristic of quasars is that they show evidence of elements heavier than helium, indicating that galaxies underwent a massive phase of star formation, creating population III stars between the time of the Big Bang and the first observed quasars. Light from these stars may have been observed in 2005 using NASA's Spitzer

Quasar 121

Space Telescope,[8] although this observation remains to be confirmed.

Quasar as an X-ray source

A view of 4C 71.07 from observations by the Burst andTransient Source Experiment. This helped convince

scientists that they were studying data from the quasar andnot some other source in the neighborhood.

In visible light, 4C 71.07 is less than impressive, just adistant speck of light. It's in radio and in X-rays - and now,gamma rays - that this object really shines. 4C 71.07 is its

designation in the 4th Cambridge University catalog ofradio sources. 4C 71.07 has a red shift of z=2.17, putting itabout 11 billion years away in a 12 to 15-billion year-old

universe (using z=1 as 5 billion light years).

QSO 0836+7107 is a Quasi-Stellar Object that emits bafflingamounts of radio energy. The radio signal is caused byelectrons spiraling along the magnetic fields. These electronscan also interact with visible light emitted by the disk aroundthe AGN or the black hole at its center, and that pumps them toemit X- and gamma-radiation.

On board the Compton Gamma Ray Observatory (CGRO) isthe Burst and Transient Source Experiment (BATSE) whichdetects in the 20 keV to 8 MeV range. QSO 0836+7107 or 4C71.07 was detected by BATSE as a source of soft gamma raysand hard X-rays. "What BATSE has discovered is that it can bea soft gamma-ray source", McCollough said. QSO 0836+7107is the faintest and most distant object to be observed in softgamma rays. It has already been observed in gamma rays bythe Energetic Gamma Ray Experiment Telescope (EGRET)also aboard the Compton Gamma Ray Observatory.[9]

Quasar 122

The Chandra X-ray image is of the quasar PKS 1127-145, ahighly luminous source of X-rays and visible light about 10

billion light years from Earth. An enormous X-ray jetextends at least a million light years from the quasar. Image

is 60 arcsec on a side. RA 11h 30m 7.10s Dec -14° 49' 27" inCrater. Observation date: May 28, 2000. Instrument: ACIS.

Credit:NASA/CXC/A.Siemiginowska(CfA)/J.Bechtold(U.Arizona).

The Chandra X-ray Observatory has imaged the quasar PKS1127-145, a highly luminous source of X-rays and visiblelight about 10 billion light years from Earth. The jet exhibitedin X-rays coming from PKS 1127-145 is likely due to thecollision of a beam of high-energy electrons with microwavephotons.

History of quasar observation

The first quasars were discovered with radio telescopes in thelate 1950s. Many were recorded as radio sources with nocorresponding visible object. Using small telescopes and theLovell Telescope as an interferometer, they were shown tohave a very small angular size.[10] Hundreds of these objectswere recorded by 1960 and published in the Third CambridgeCatalogue as astronomers scanned the skies for the opticalcounterparts. In 1960, radio source 3C 48 was finally tied toan optical object. Astronomers detected what appeared to be afaint blue star at the location of the radio source and obtainedits spectrum. Containing many unknown broad emission lines,the anomalous spectrum defied interpretation — a claim by John Bolton of a large redshift was not generallyaccepted.

In 1962 a breakthrough was achieved. Another radio source, 3C 273, was predicted to undergo five occultations bythe moon. Measurements taken by Cyril Hazard and John Bolton during one of the occultations using the ParkesRadio Telescope allowed Maarten Schmidt to optically identify the object and obtain an optical spectrum using the200-inch Hale Telescope on Mount Palomar. This spectrum revealed the same strange emission lines. Schmidtrealized that these were actually spectral lines of hydrogen redshifted at the rate of 15.8 percent. This discoveryshowed that 3C 273 was receding at a rate of 47,000 km/s.[11] This discovery revolutionized quasar observation andallowed other astronomers to find redshifts from the emission lines from other radio sources. As predicted earlier byBolton, 3C 48 was found to have a redshift of 37% the speed of light.

The term quasar was coined by Chinese-born U.S. astrophysicist Hong-Yee Chiu in 1964, in Physics Today, todescribe these puzzling objects:

So far, the clumsily long name 'quasi-stellar radio sources' is used to describe these objects. Because the natureof these objects is entirely unknown, it is hard to prepare a short, appropriate nomenclature for them so thattheir essential properties are obvious from their name. For convenience, the abbreviated form 'quasar' will beused throughout this paper.

– Hong-Yee Chiu in Physics Today, May, 1964Later it was found that not all (actually only 10% or so) quasars have strong radio emission (are 'radio-loud'). Hencethe name 'QSO' (quasi-stellar object) is used (in addition to 'quasar') to refer to these objects, including the'radio-loud' and the 'radio-quiet' classes.One great topic of debate during the 1960s was whether quasars were nearby objects or distant objects as implied by their redshift. It was suggested, for example, that the redshift of quasars was not due to the expansion of space but rather to light escaping a deep gravitational well. However a star of sufficient mass to form such a well would be unstable and in excess of the Hayashi limit.[12] Quasars also show unusual spectral emission lines which were previously only seen in hot gaseous nebulae of low density, which would be too diffuse to both generate the

Quasar 123

observed power and fit within a deep gravitational well.[13] There were also serious concerns regarding the idea ofcosmologically distant quasars. One strong argument against them was that they implied energies that were far inexcess of known energy conversion processes, including nuclear fusion. At this time, there were some suggestionsthat quasars were made of some hitherto unknown form of stable antimatter and that this might account for theirbrightness. Others speculated that quasars were a white hole end of a wormhole. However, when accretion discenergy-production mechanisms were successfully modeled in the 1970s, the argument that quasars were tooluminous became moot and today the cosmological distance of quasars is accepted by almost all researchers.In 1979 the gravitational lens effect predicted by Einstein's General Theory of Relativity was confirmedobservationally for the first time with images of the double quasar 0957+561.[14]

In the 1980s, unified models were developed in which quasars were classified as a particular kind of active galaxy,and a consensus emerged that in many cases it is simply the viewing angle that distinguishes them from otherclasses, such as blazars and radio galaxies. The huge luminosity of quasars results from the accretion discs of centralsupermassive black holes, which can convert on the order of 10% of the mass of an object into energy as comparedto 0.7% for the p-p chain nuclear fusion process that dominates the energy production in sun-like stars.This mechanism also explains why quasars were more common in the early universe, as this energy production endswhen the supermassive black hole consumes all of the gas and dust near it. This means that it is possible that mostgalaxies, including our own Milky Way, have gone through an active stage (appearing as a quasar or some otherclass of active galaxy depending on black hole mass and accretion rate) and are now quiescent because they lack asupply of matter to feed into their central black holes to generate radiation.In 2006 Radio astronomers at the University of Manchester’s Jodrell Bank Observatory discovered a strange newobject in a nearby galaxy, and question if it could be the closest micro-quasar yet discovered. If this object is anextragalactic micro-quasar, it would be the first that has been detected at radio wavelengths. The very highluminosity suggests that it is likely to be associated with a massive black hole system of some type; however this andits longevity imply that this type of object is extremely unusual and has not yet been seen within our Galaxy.[15]

Further reading• Melia, Fulvio, The Edge of Infinity. Supermassive Black Holes in the Universe 2003, Cambridge University Press,

ISBN 978-0-521-81405-8 (Cloth)• The fine-structure constant and the nature of the universe-The Economist [16]

See also• Active galactic nuclei (AGN)• Blazar• List of quasars• Optically Violently Variable (OVV) quasars• Supermassive black hole• M-sigma relation• Microquasar• BL Lac object• Star• Multiply imaged quasar

Quasar 124

References[1] Grupen, Claus; Cowan, Glen (2005). Astroparticle physics. Springer. pp. 11–12. ISBN 3540253122.[2] Hubble Surveys the "Homes" of Quasars (http:/ / hubblesite. org/ newscenter/ archive/ releases/ 1996/ 35/ image/ a/ ) Hubblesite News

Archive, 1996-35[3] Chris J. Willott et al. (2007). "Four Quasars above Redshift 6 Discovered by the Canada-France High-z Quasar Survey" (http:/ / www. iop.

org/ EJ/ abstract/ 1538-3881/ 134/ 6/ 2435). The Astronomical Journal 134: 2435–2450. doi:10.1086/522962. .[4] http:/ / www. jstor. org/ pss/ 3971408[5] http:/ / www. galaxydynamics. org/ papers/ GreatMilkyWayAndromedaCollision. pdf[6] www.cfa.harvard.edu/~tcox/localgroup/lg.pdf[7] Keel, William C. (October 2009). "Alternate Approaches and the Redshift Controversy" (http:/ / www. astr. ua. edu/ keel/ galaxies/ arp.

html). The University of Alabama. . Retrieved 2010-09-27.[8] NASA Goddard Space Flight Center: News of light that may be from population III stars (http:/ / www. nasa. gov/ centers/ goddard/ news/

topstory/ 2005/ universe_objects. html)[9] Dooling D. "BATSE finds most distant quasar yet seen in soft gamma rays Discovery will provide insight on formation of galaxies" (http:/ /

science. nasa. gov/ NEWHOME/ HEADLINES/ ast24nov99_1. htm). .[10] "The MKI and the discovery of Quasars" (http:/ / www. jb. man. ac. uk/ public/ story/ mk1quasars. html). Jodrell Bank Observatory. .

Retrieved 2006-11-23.[11] Schmidt Maarten (1963). "3C 273: a star-like object with large red-shift" (http:/ / adsabs. harvard. edu/ cgi-bin/

nph-bib_query?bibcode=1963Natur. 197. 1040S& amp;db_key=AST& amp;data_type=HTML& amp;format=&amp;high=4521318e0232118). Nature 197: 1040–1040. doi:10.1038/1971040a0. .

[12] S. Chandrasekhar (1964). "The Dynamic Instability of Gaseous Masses Approaching the Schwarzschild Limit in General Relativity".Astrophysical Journal 140 (2): 417–433. doi:10.1086/147938.

[13] J. Greenstein and M. Schmidt (1964). "The Quasi-Stellar Radio Sources 3C 48 and 3C ". Astrophysical Journal 140 (1): 1–34.doi:10.1086/147889.

[14] Active Galaxies and Quasars - Double Quasar 0957+561 (http:/ / www. astr. ua. edu/ keel/ agn/ q0957. html)[15] http:/ / www. ras. org. uk/ index. php?option=com_content& task=view& id=1747& Itemid=2[16] http:/ / www. economist. com/ node/ 16930866

External links• Formation of quasars (Heymann, 2010) (http:/ / fr. calameo. com/ books/ 000145333186209bbc0f5)• 3C 273: Variable Star Of The Season (http:/ / www. aavso. org/ vstar/ vsots/ )• SKY-MAP.ORG SDSS image of quasar 3C 273 (http:/ / www. sky-map. org/ ?object=3C 273& zoom=12&

img_source=SDSS)• Expanding Gallery of Hires Quasar Images (http:/ / www. perseus. gr/ Astro-DSO-Quasars. htm)• Gallery of Quasar Spectra from SDSS (http:/ / www. sdss. org/ gallery/ gal_zqso. html)• SDSS Advanced Student Projects: Quasars (http:/ / cas. sdss. org/ dr6/ en/ proj/ advanced/ quasars/ default. asp)• Black Holes: Gravity's Relentless Pull (http:/ / www. hubblesite. org/ go/ blackholes) Award-winning interactive

multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute• Research Sheds New Light On Quasars (http:/ / www. spacedaily. com/ reports/

Research_Sheds_New_Light_On_Quasars_999. html) (SpaceDaily) July 26, 2006• Audio: Fraser Cain/Pamela L. Gay - Astronomy Cast. Quasars - July 2008 (http:/ / www. astronomycast. com/

astronomy/ galaxies/ ep-98-quasars/ )

Radio galaxy 125

Radio galaxyRadio galaxies and their relatives, radio-loud quasars and blazars, are types of active galaxy that are very luminousat radio wavelengths (up to 1039 W between 10 MHz and 100 GHz). The radio emission is due to the synchrotronprocess. The observed structure in radio emission is determined by the interaction between twin jets and the externalmedium, modified by the effects of relativistic beaming. The host galaxies are almost exclusively large ellipticalgalaxies. Radio-loud active galaxies are interesting not only in themselves, but also because they can be detected atlarge distances, making them valuable tools for observational cosmology. Recently, much work has been done on theeffects of these objects on the intergalactic medium, particularly in galaxy groups and clusters.

Emission processes

False-colour image of the nearby radio galaxyCentaurus A, showing radio (red), 24-micrometre

infrared (green) and 0.5-5 keV X-ray emission(blue). The jet can be seen to emit synchrotron

emission in all three wavebands. The lobes onlyemit in the radio frequency range, and so appear

red. Gas and dust in the galaxy emits thermalradiation in the infrared. Thermal X-ray radiation

from hot gas and non-thermal emission fromrelativistic electrons can be seen in the blue

'shells' around the lobes, particularly to the south(bottom).

The radio emission from radio-loud active galaxies is synchrotronemission, as inferred from its very smooth, broad-band nature andstrong polarization. This implies that the radio-emitting plasmacontains, at least, electrons with relativistic speeds (Lorentz factors of~104) and magnetic fields. Since the plasma must be neutral, it mustalso contain either protons or positrons. There is no way ofdetermining the particle content directly from observations ofsynchrotron radiation. Moreover, there is no way of determining theenergy densities in particles and magnetic fields from observation (thatis, the same synchrotron emissivity may be a result of a few electronsand a strong field, or a weak field and many electrons, or something inbetween). It is possible to determine a minimum energy conditionwhich is the minimum energy density that a region with a givenemissivity can have,[1] but for many years there was no particularreason to believe that the true energies were anywhere near theminimum energies.

A sister process to synchrotron radiation is the inverse-Comptonprocess, in which the relativistic electrons interact with ambientphotons and Thomson scatter them to high energies. Inverse-Comptonemission from radio-loud sources turns out to be particularly importantin X-rays,[2] and, because it depends only on the density of electrons (and on the density of photons, which isknown), a detection of inverse-Compton scattering allows a (somewhat model-dependent) estimate of the energydensities in the particles and magnetic fields. This has been used to argue that many powerful sources are actuallyquite near the minimum-energy condition.

Synchrotron radiation is not confined to radio wavelengths: if the radio source can accelerate particles to highenough energies, features which are detected in the radio may also be seen in the infrared, optical, ultraviolet or evenX-ray, though in the latter case the electrons responsible must have energies in excess of 1 TeV in typical magneticfield strengths. Again, polarization and continuum spectrum are used to distinguish synchrotron radiation from otheremission processes. Jets and hotspots (see below) are the usual sources of high-frequency synchrotron emission. It ishard to distinguish observationally between synchrotron and inverse-Compton radiation, and there is ongoingdisagreement about what processes we are seeing in some objects, particularly in the X-ray.

The process(es) that produce the population of relativistic, non-thermal particles that give rise to synchrotron andinverse-Compton radiation are collectively known as particle acceleration. Fermi acceleration is one plausibleparticle acceleration process in radio-loud active galaxies.

Radio galaxy 126

Radio structures

Pseudo-colour image of the large-scale radiostructure of the FRII radio galaxy 3C98. Lobes,

jet and hotspot are labelled.

Radio galaxies (and, to a lesser extent, radio-loud quasars) display awide range of structures in radio maps. The most common large-scalestructures are called lobes: these are double, often fairly symmetrical,roughly ellipsoidal structures placed on either side of the activenucleus. A significant minority of low-luminosity sources exhibitstructures usually known as plumes which are much more elongated.Some radio galaxies show one or two long narrow features known asjets (the most famous example being the giant galaxy M87 in the Virgocluster) coming directly from the nucleus and going to the lobes. Sincethe 1970s,[3] [4] the most widely accepted model has been that the lobesor plumes are powered by beams of high-energy particles and magneticfield coming from close to the active nucleus. The jets are believed tobe the visible manifestations of the beams, and often the term jet isused to refer both to the observable feature and to the underlying flow.

Pseudo-colour image of the large-scale radiostructure of the FRI radio galaxy 3C31. Jets and

plumes are labelled.

In 1974, radio sources were divided by Fanaroff and Riley into twoclasses, now known as Fanaroff and Riley Class I (FRI), and Class II(FRII).[5] The distinction was originally made based on themorphology of the large-scale radio emission (the type was determinedby the distance between the brightest points in the radio emission): FRIsources were brightest towards the centre, while FRII sources werebrightest at the edges. Fanaroff and Riley observed that there was areasonably sharp divide in luminosity between the two classes: FRIswere low-luminosity, FRIIs were high luminosity.[5] With moredetailed radio observations, the morphology turns out to reflect themethod of energy transport in the radio source. FRI objects typicallyhave bright jets in the centre, while FRIIs have faint jets but brighthotspots at the ends of the lobes. FRIIs appear to be able to transportenergy efficiently to the ends of the lobes, while FRI beams areinefficient in the sense that they radiate a significant amount of theirenergy away as they travel.

In more detail, the FRI/FRII division depends on host-galaxyenvironment in the sense that the FRI/FRII transition appears at higherluminosities in more massive galaxies.[6] FRI jets are known to bedecelerating in the regions in which their radio emission is brightest,[7]

and so it seems that the FRI/FRII transition reflects whether a jet/beam

Radio galaxy 127

can propagate through the host galaxy without being decelerated to sub-relativistic speeds by interaction with theintergalactic medium. From analysis of relativistic beaming effects, the jets of FRII sources are known to remainrelativistic (with speeds of at least 0.5c) out to the ends of the lobes. The hotspots that are usually seen in FRIIsources are interpreted as being the visible manifestations of shocks formed when the fast, and therefore supersonic,jet (the speed of sound cannot exceed c/√3) abruptly terminates at the end of the source, and their spectral energydistributions are consistent with this picture.[8] Often multiple hotspots are seen, reflecting either continued outflowafter the shock or movement of the jet termination point: the overall hotspot region is sometimes called the hotspotcomplex.Names are given to several particular types of radio source based on their radio structure:• Classical double refers to an FRII source with clear hotspots.• Wide-angle tail normally refers to a source intermediate between standard FRI and FRII structure, with efficient

jets and sometimes hotspots, but with plumes rather than lobes, found at or near the centres of clusters.• Narrow-angle tail or Head-tail source describes an FRI that appears to be bent by ram pressure as it moves

through a cluster.• Fat doubles are sources with diffuse lobes but neither jets nor hotspots. Some such sources may be relics whose

energy supply has been permanently or temporarily turned off.

Life cycles and dynamicsThe largest radio galaxies have lobes or plumes extending to megaparsec scales (more in the case of giant radiogalaxies like 3C236), implying a timescale for growth of the order of tens to hundreds of millions of years. Thismeans that, except in the case of very small, very young sources, we cannot observe radio source dynamics directly,and so must resort to theory and inferences from large numbers of objects. Clearly radio sources must start small andgrow larger. In the case of sources with lobes, the dynamics are fairly simple[3] : the jets feed the lobes, the pressureof the lobes increases, and the lobes expand. How fast they expand depends on the density and pressure of theexternal medium. The highest-pressure phase of the external medium, and thus the most important phase from thepoint of view of the dynamics, is the X-ray emitting diffuse hot gas. For a long time it was assumed that powerfulsources would expand supersonically, pushing a shock through the external medium. However, X-ray observationsshow that the internal lobe pressures of powerful FRII sources are often close to the external thermal pressures(e.g.[9] ) and not much higher than the external pressures, as would be required for supersonic expansion. The onlyunambiguously supersonically expanding system known consists of the inner lobes of the low-power radio galaxyCentaurus A [10] (see figure) which are probably a result of a comparatively recent outburst of the active nucleus.

Host galaxies and environmentsRadio galaxies are almost universally found hosted by elliptical galaxies. (The only well-documented exception isreported by [11] .) Some Seyfert galaxies show weak, small radio jets, but they are not radio-luminous enough to beclassified as radio-loud. Such information as we have about the host galaxies of radio-loud quasars and blazarssuggests that they are also hosted by elliptical galaxies.There are several possible reasons for this very strong preference for ellipticals. One is that ellipticals generallycontain the most massive black holes, and so are capable of powering the most luminous active galaxies (seeEddington luminosity). Another is that ellipticals generally inhabit richer environments, providing a large-scaleintergalactic medium to confine the radio source. It may also be that the larger amounts of cold gas in spiral galaxiesin some way disrupts or stifles a forming jet. To date there is no compelling single explanation for the observations.

Radio galaxy 128

Unified modelsThe different types of radio-loud active galaxies are linked by unified models (see active galaxy). The keyobservation that led to the adoption of unified models for powerful radio galaxies and radio-loud quasars was that allquasars appear to be beamed towards us, showing superluminal motion in the cores[12] and bright jets on the side ofthe source nearest to us (the Laing-Garrington effect:[13] [14] ). If this is the case, there must be a population ofobjects not beamed towards us, and, since we know the lobes are not affected by beaming, they would appear asradio galaxies, provided that the quasar nucleus is obscured when the source is seen side-on. It is now accepted thatat least some powerful radio galaxies have 'hidden' quasars, though it is not clear whether all such radio galaxieswould be quasars if viewed from the right angle. In a similar way, low-power radio galaxies are a plausible parentpopulation for BL Lac objects.

Uses of radio galaxies

Distant sourcesRadio galaxies and radio-loud quasars have been widely used, particularly in the 80s and 90s, to find distantgalaxies: by selecting based on radio spectrum and then observing the host galaxy it was possible to find objects athigh redshift at modest cost in telescope time. The problem with this method is that hosts of active galaxies may notbe typical of galaxies at their redshift. Similarly, radio galaxies have in the past been used to find distant X-rayemitting clusters, but unbiased selection methods are now preferred.

Standard rulersSome work has been done attempting to use radio galaxies as standard rulers to determine cosmological parameters.This method is fraught with difficulty because a radio galaxy's size depends on both its age and its environment (seeabove). When a model of the radio source is used, though, methods based on radio galaxies can give good agreementwith other cosmological observations (e.g.[15] ).

Effects on environmentWhether a radio source is expanding supersonically or not (see above), it must do work against the external mediumin expanding, and so it puts energy into heating and lifting the external plasma. The minimum energy stored in thelobes of a powerful radio source might be 1053 J. The lower limit on the work done on the external medium by sucha source is several times this. A good deal of the current interest in radio sources focusses on the effect they musthave at the centres of clusters at the present day, e.g.[16] . Equally interesting is their likely effect on structureformation over cosmological time: it is thought that they may provide a feedback mechanism to slow the formationof the most massive objects.

TerminologyWidely used terminology is awkward now that it is generally accepted that quasars and radio galaxies are the sameobjects (see above). The acronym DRAGN (for 'Double Radiosource Associated with Galactic Nucleus') has beencoined [17] . but has not yet taken off. Extragalactic radio source is common but can lead to confusion, since manyother extragalactic objects are detected in radio surveys, notably starburst galaxies. Radio-loud active galaxy isunambiguous, and so is often used in this article.

Radio galaxy 129

See also• Active galaxy• Quasar• Blazar• Black hole• Relativistic jet• C-shaped radio galaxy• S-shaped radio galaxy• X-shaped radio galaxy• Z-shaped radio galaxy• M-sigma relation• Death Star Galaxy

References[1] Burbidge, G (1956). "On synchrotron radiation from Messier 87". Astrophysical Journal 124: 416. doi:10.1086/146237.[2] Croston JH, Hardcastle MJ, Harris DE, Belsole E, Birkinshaw M, Worrall DM (2005). "An X-ray study of magnetic field strengths and

particle content in FRII radio sources" (http:/ / arxiv. org/ abs/ astro-ph/ 0503203v1). Astrophysical Journal 626: 733–47.doi:10.1086/430170. . Retrieved 2008-08-24.

[3] Scheuer, PAG (1974). "Models of extragalactic radio sources with a continuous energy supply from a central object". Monthly Notices of theRoyal Astronomical Society 166: 513.

[4] Blandford RD, Rees MJ (1974). "A 'twin-exhaust' model for double radio sources". Monthly notices of the Royal Astronomical Society 169:395.

[5] Fanaroff, Bernard L., Riley Julia M. (May 1974). "The morphology of extragalactic radio sources of high and low luminosity" (http:/ / adsabs.harvard. edu/ abs/ 1974MNRAS. 167P. . 31F). Monthly Notices of the Royal Astronomical Society 167: 31P–36P. . Retrieved 2008-08-24.

[6] Owen FN, Ledlow MJ (1994). "The FRI/II Break and the Bivariate Luminosity Function in Abell Clusters of Galaxies". In G.V. Bicknell,M.A. Dopita, and P.J. Quinn, (Eds.). The First Stromlo Symposium: The Physics of Active Galaxies. ASP Conference Series,. 54. AstronomicalSociety of the Pacific Conference Series. pp. 319. ISBN 0-937707-73-2.

[7] Laing RA, Bridle AH (2002). "Relativistic models and the jet velocity field in the radio galaxy 3C31" (http:/ / arxiv. org/ abs/ astro-ph/0206215). Monthly Notices of the Royal Astronomical Society 336: 328–57. doi:10.1046/j.1365-8711.2002.05756.x. . Retrieved 2008-08-24.

[8] Meisenheimer K, Röser H-J, Hiltner PR, Yates MG, Longair MS, Chini R, Perley RA (1989). "The synchrotron spectra of radio hotspots".Astronomy and Astrophysics 219: 63–86.

[9] Hardcastle MJ., Birkinshaw M, Cameron RA, Harris DE, Looney LW, Worrall DM (2003). "Magnetic field strengths in the hotspots andlobes of three powerful FRII radio sources". Astrophysical Journal 581: 948. doi:10.1086/344409.

[10] Kraft RP, Vázquez S, Forman WR, Jones C, Murray SS, Hardcastle MJ, Worrall DM (2003). "X-ray emission from the hot ISM and SWradio lobe of the nearby radio galaxy Centaurus A". Astrophysical Journal 592: 129. doi:10.1086/375533.

[11] Ledlow MJ, Owen FN, Keel WC (1998). "An Unusual Radio Galaxy in Abell 428: A Large, Powerful FR I Source in a Disk-dominatedHost" (http:/ / adsabs. harvard. edu/ abs/ 1998ApJ. . . 495. . 227L). Astrophysical Journal 495: 227. doi:10.1086/305251. . Retrieved2008-08-24.

[12] Barthel PD (1989). "Is every quasar beamed?". Astrophysical Journal 336: 606. doi:10.1086/167038.[13] Laing RA (1988). "The sidedness of jets and depolarization in powerful extragalactic radio sources". Nature 331: 149.

doi:10.1038/331149a0.[14] Garrington S, Leahy JP, Conway RG, Laing RA (1988). "A systematic asymmetry in the polarization properties of double radio sources".

Nature 331: 147. doi:10.1038/331147a0.[15] Daly RA, Djorgovski SG (2003). "A Model-Independent Determination of the Expansion and Acceleration Rates of the Universe as a

Function of Redshift and Constraints on Dark Energy" (http:/ / adsabs. harvard. edu/ abs/ 2003ApJ. . . 597. . . . 9D). Astrophysical Journal597: 9. doi:10.1086/378230. . Retrieved 2008-08-24.

[16] "Perseus Cluster: Chandra "Hears" a Supermassive Black Hole in Perseus" (http:/ / chandra. harvard. edu/ photo/ 2003/ perseus/ ). .Retrieved 2008-08-24.

[17] Leahy JP (1993). "DRAGNs". In Röser, H-J, Meisenheimer, K (Eds.). Jets in Extragalactic Radio Sources. Springer-Verlag.

Radio galaxy 130

External links• Atlas of DRAGNs (http:/ / www. jb. man. ac. uk/ atlas/ ) A collection of radio images of the 3CRR catalogue of

radio-loud active galaxies.• Radio and optical images of radio galaxies and quasars (http:/ / www. cv. nrao. edu/ ~abridle/ images. htm)• The on-line 3CRR catalogue of radio sources (http:/ / 3crr. extragalactic. info/ )

Ring galaxy

Hoag's Object, a ring galaxy.

A ring galaxy is a galaxy with a ring-like appearance. The ringconsists of massive, relatively young blue stars, which are extremelybright. The central region contains relatively little luminous matter.Some astronomers believe that ring galaxies are formed when a smallergalaxy passes through the center of a larger galaxy. Because most of agalaxy consists of empty space, this "collision" rarely results in anyactual collisions between stars. However, the gravitational disruptionscaused by such an event could cause a wave of star formation to movethrough the larger galaxy. Others think that rings are formed aroundsome galaxies when external accretion takes place. Star formationwould then take place in the accreted material because of the shocksand compressions of the accreted material. There might be a linkbetween ring galaxies and polar-ring galaxies.

Hoag's Object, discovered by Art Hoag in 1950, is an example of a ring galaxy.

See also• Interacting galaxy• Cartwheel galaxy• AM 0644-741• Hoag's Object

External links• Hoag's Object [1] at Astronomy Picture of the Day.

References[1] http:/ / antwrp. gsfc. nasa. gov/ apod/ ap100822. html

Seyfert galaxy 131

Seyfert galaxy

The Circinus Galaxy, a Seyfert 2 galaxy. Credit:A. S. Wilson, P. L. Shopbell, C. Simpson, T.

Storchi-Bergmann, F. K. B. Barbosa, M. J. Ward,WFPC2, HST, NASA.

Seyfert galaxies are a class of galaxies with nuclei that producespectral line emission from highly ionized gas,[1] named after CarlKeenan Seyfert, the astronomer who first identified the class in 1943.[2]

The centres of Seyfert galaxies form a subclass of active galactic nuclei(AGN), and are thought to contain supermassive black holes[1] withmasses between 107 and 108 solar masses.[3]

Characteristics

Seyfert galaxies are characterized by extremely bright nuclei, andspectra which have very bright emission lines of hydrogen, helium,nitrogen, and oxygen. These emission lines exhibit strong Dopplerbroadening, which implies velocities from 500 to 4000 km/s, and arebelieved to originate near an accretion disk surrounding the centralblack hole.[4]

These emission lines may come from the surface of the accretion diskitself, or may come from clouds of gas illuminated by the central engine in an ionization cone. The exact geometryof the emitting region is difficult to determine due to poor resolution. However, each part of the accretion disk has adifferent velocity relative to our line of sight, and the faster the gas is rotating around the black hole, the broader theline will be. Similarly, an illuminated disc wind also has a position-dependent velocity.

The narrow lines are believed to originate from the outer part of the AGN where velocities are lower, while the broadlines originate closer to the black hole. This is confirmed by the fact that the narrow lines do not vary detectably,which implies that the emitting region is large, contrary to the broad lines which can vary on relatively shorttimescales. Reverberation mapping is a technique which uses this variability to try to determine the location andmorphology of the emitting region.Seyfert galaxies also show strong emission in the infrared, ultraviolet, and X-ray parts of the spectrum, whereas onlyless than 5% are radio loud. The radio emission is believed to be synchrotron emission from the jet. The infraredemission is due to radiation in other bands being reprocessed by dust near the nucleus. The highest energy photonsare believed to be created by inverse compton scattering by a high temperature corona near the black hole.[5]

ClassificationSeyferts were first classified as Type 1 or 2, depending upon whether the spectra show both narrow and broademission lines (Type 1), or only narrow lines (Type 2). They are now given a fractional classification dependingupon the relative strengths of the narrow and broad components (e.g. Type 1.5 or Type 1.9).[4] It is believed thatType 1 and Type 2 galaxies are in essence the same, and they only differ due to the angle at which they are observed.This is known as Seyfert Unification theory. In Type 2 Seyferts it is believed that the broad component is obscuredby dust and/or by our viewing angle on the galaxy. In some Type 2 Seyfert galaxies, the broad component can beobserved in polarized light; it is believed that light from the broad-line region is scattered by a hot, gaseous halosurrounding the nucleus, allowing us to view it indirectly. This effect was first discovered by Antonucci and Millerin the Type 2 Seyfert NGC 1068.

Seyfert galaxy 132

See also• Low-ionization nuclear emission-line region, another class of galaxies that contain AGN

References[1] L. S. Sparke, J. S. Gallagher III (2007). Galaxies in the Universe: An Introduction. Cambridge: Cambridge University Press.

ISBN 0-521-67186-6.[2] C. K. Seyfert (1943). "Nuclear Emission in Spiral Nebulae" (http:/ / adsabs. harvard. edu/ abs/ 1943ApJ. . . . 97. . . 28S). Astrophysical

Journal 97: 28–40. doi:10.1086/144488. .[3] Osterbrock, Donald E. and Ferland, Gary J. (2006). Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (2nd ed.). University

Science Books. p. 390. ISBN 1-891389-34-3.[4] Donald E. Osterbrock, Gary J. Ferland (2006). Astrophysics of Gaseous Nebulae and Active Galactic Nuclei. Sausalito, CA: University

Science Books. ISBN 1-891389-34-3.[5] Haardt, F., & Maraschi, L. (1991). ""A two-phase model for the X-ray emission from Seyfert galaxies"" (http:/ / adsabs. harvard. edu/ abs/

1991ApJ. . . 380L. . 51H). Astrophysical Journal 380: L51–L54. doi:10.1086/186171. .

External links• Seyfert Galaxies (http:/ / www. seyfertgalaxies. com)

Spiral galaxy

An example of a spiral galaxy, the PinwheelGalaxy (also known as Messier 101 or NGC

5457)

A spiral galaxy is a certain kind of galaxy originally described byEdwin Hubble in his 1936 work The Realm of the Nebulae[1] and, assuch, forms part of the Hubble sequence. Spiral galaxies consist of aflat, rotating disk containing stars, gas and dust, and a centralconcentration of stars known as the bulge. These are surrounded by amuch fainter halo of stars, many of which reside in globular clusters.

Spiral galaxies are named for the (usually two-armed) spiral structuresthat extend from the center into the disk. The spiral arms are sites ofongoing star formation and are brighter than the surrounding diskbecause of the young, hot OB stars that inhabit them. Roughly half ofall spirals are observed to have an additional component in the form ofa bar-like structure, extending from the central bulge, at the ends ofwhich the spiral arms begin. Our own Milky Way has recently (in the 1990s) been confirmed to be a barred spiral,although the bar itself is difficult to observe from our position within the Galactic disk.[2] The most convincingevidence for its existence comes from a recent survey [3], performed by the Spitzer Space Telescope, of stars in theGalactic center.[4]

Together with irregulars, spiral galaxies make up approximately 60% of galaxies in the local Universe.[5] They aremostly found in low-density regions and are rare in the centers of galaxy clusters.[6]

Spiral galaxy 133

StructureSpiral galaxies consist of four distinct components:• A flat, rotating disc of (mostly newly created) stars and interstellar matter• A central stellar bulge of mainly older stars, which resembles an elliptical galaxy• A near-spherical halo of stars, including many in globular clusters• A supermassive black hole at the very center of the central bulgeThe relative importance, in terms of mass, brightness and size, of the different components varies from galaxy togalaxy.

Spiral arms

NGC 1300 in infrared light.

Spiral arms are regions of stars that extend from the center of spiraland barred spiral galaxies. These long, thin regions resemble a spiraland thus give spiral galaxies their name. Naturally, differentclassifications of spiral galaxies have distinct arm-structures. Sc andSBc galaxies, for instance, have very "loose" arms, whereas Sa andSBa galaxies tightly wrapped arms (with reference to the Hubblesequence). Either way, spiral arms contain a great many young, bluestars (due to the high mass density and the high rate of star formation),which make the arms so remarkable.

Galactic bulge

A bulge is a huge, tightly packed group of stars. The term commonlyrefers to the central group of stars found in most spiral galaxies.

Using the Hubble classification, the bulge of Sa galaxies is usually composed of population II stars, that is old, redstars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Scand SBc galaxies are a great deal lesser, and are composed of young, blue, Population I stars. Some bulges havesimilar properties to those of elliptical galaxies (scaled down to lower mass and luminosity), and others simplyappear as higher density centers of disks, with properties similar to disk galaxies.Many bulges are thought to host a supermassive black hole at their center. Such black holes have never been directlyobserved, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believedto be a supermassive black hole. There is a tight correlation between the mass of the black hole and the velocitydispersion of the stars in the bulge, the M-sigma relation.

Galactic spheroidThe bulk of the stars in a spiral galaxy are located either close to a single plane (the Galactic plane) in more or lessconventional circular orbits around the center of the galaxy (the galactic centre), or in a spheroidal galactic bulgearound the galactic core.However, some stars inhabit a spheroidal halo or galactic spheroid. The orbital behaviour of these stars is disputed,but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may beacquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius DwarfElliptical Galaxy is in the process of merging with the Milky Way and observations show that some stars in the haloof the Milky Way have been acquired from it.Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc (but

Spiral galaxy 134

similar to those in the galactic bulge). The galactic halo also contains many globular clusters.The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarf stars closeto the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to theirirregular movement around the centre of the galaxy—if they do so at all—these stars often display unusually highproper motion.

Origin of the spiral structureThe pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925.He realized that the idea of stars arranged permanently in a spiral shape was untenable due to the "windingdilemma". Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy(via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved asthe galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around thegalaxy ever tighter. This is called the winding problem. Measurements in the late 1960s showed that the orbitalvelocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher thanexpected from Newtonian dynamics but still cannot explain the stability of the spiral structure.There are two leading hypotheses or models for the spiral structures of galaxies:• Star formation caused by density waves in the galactic disk of the galaxy.• The SSPSF model - Star formation caused by shock waves in the interstellar medium.These different hypotheses do not have to be mutually exclusive, as they may explain different types of spiral arms.

Density waves modelBertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowlythan the galaxy’s stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of whichare short-lived blue stars that light the arms.

Explanation of spiral galaxy arms.

This idea was developed into density wave theory by C. C. Lin andFrank Shu in 1964.[7] They suggested that the spiral arms weremanifestations of spiral density waves, attempting to explain thelarge-scale structure of spirals in terms of a small-amplitude wavepropagating with fixed angular velocity, that revolves around thegalaxy at a speed different from that of the galaxy's gas and stars.

Historical theory of Lin and Shu

The first acceptable theory for the spiral structure was devised by C. C.Lin and Frank Shu in 1964.• They suggested that the spiral arms were manifestations of spiral density waves.• They assumed that the stars travel in slightly elliptical orbits and that the orientations of their orbits is correlated

i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from thegalactic centre. This is illustrated in the diagram. It is clear that the elliptical orbits come close together in certainareas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, butpass through the arms as they travel in their orbits.

Spiral galaxy 135

Star formation caused by density waves

The following hypotheses exist for star formation caused by density waves:• As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse

(the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and formstars.

• As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.• As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas,

which in turn causes the gas to collapse and form stars.

The bright galaxy NGC 3810 demonstratesclassical spiral structure in this very detailedimage from Hubble. Credit: ESA/Hubble and

NASA.

More young stars in spiral arms

The arms appear brighter because there are more young stars (hencemore massive, bright stars). These massive, bright stars also die outquickly, which would leave just the (darker) background stellardistribution behind the waves, hence making the waves visible.While stars, therefore, do not remain forever in the position that wenow see them in, they also do not follow the arms. The arms simplyappear to pass through the stars as the stars travel in their orbits.

Alignment of spin axis with cosmic voidsRecent results suggest that the orientation of the spin axis of spiral galaxies is not a chance result, but instead theyare preferentially aligned along the surface of cosmic voids.[8] That is, spiral galaxies tend to be oriented at a highangle of inclination relative to the large-scale structure of the surroundings. They have been described as lining uplike "beads on a string," with their axis of rotation following the filaments around the edges of the voids.[9]

Spiral nebula“Spiral nebula” is an old term for a spiral galaxy. Until the early 20th century, most astronomers believed that objectslike the Whirlpool Galaxy were just one more form of nebula that were within our own Milky Way galaxy. The ideathat they might instead be other galaxies, independent of the Milky Way, was the subject of The Great Debate of1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mt. Wilson Observatory. In 1926, EdwinHubble[10] observed Cepheid variables in several spiral nebulae, including the Andromeda Galaxy, proving that theyare, in fact, entire galaxies outside our own. The term “spiral nebula” has since fallen into disuse.

The Milky WayThe Milky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the MilkyWay is a barred spiral galaxy in the 1990s.[11] Their suspicions were confirmed by the Spitzer Space Telescopeobservations in 2005[12] which showed the galaxy's central bar to be larger than previously suspected.

Famous examples

• Triangulum Galaxy• Whirlpool Galaxy

• Andromeda Galaxy • Sunflower Galaxy• Pinwheel Galaxy

Spiral galaxy 136

See also

Components

• Galactic disk• Bulge (astronomy)

• Galactic halo • Galactic corona

Classification

• Galaxy color-magnitude diagram • Dwarf galaxy • Lenticular galaxy• Galaxy morphological classification • Dwarf elliptical galaxy • Ring galaxy• Hubble sequence • Dwarf spheroidal galaxy • Starburst galaxy• Disc galaxy • Elliptical galaxy • Seyfert galaxy• Active galaxy • Grand design spiral galaxy • Unbarred spiral galaxy• Barred spiral galaxy • Intermediate spiral galaxy

• Irregular galaxy

Other

• Galactic coordinate system• Galaxy formation and evolution• Groups and clusters of galaxies

• List of galaxies• List of nearest galaxies

• Timeline of galaxies, clusters of galaxies, and large scale structure• Tully-Fisher relation

References[1] Hubble, E. P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. ISBN 0300025009.[2] Ripples in a Galactic Pond (http:/ / www. sciamdigital. com/ index. cfm?fa=Products. ViewIssuePreview&

ARTICLEID_CHAR=3BC08F0C-2B35-221B-67A9F2AE04AFC79A), Scientific American, October 2005[3] http:/ / www. astro. wisc. edu/ sirtf/[4] Benjamin, R. A. et al.; Churchwell, E.; Babler, B. L.; Indebetouw, R.; Meade, M. R.; Whitney, B. A.; Watson, C.; Wolfire, M. G. et al.

(September 2005). "First GLIMPSE Results on the Stellar Structure of the Galaxy." (http:/ / www. journals. uchicago. edu/ doi/ full/ 10. 1086/491785). The Astrophysical Journal Letters 630 (2): L149–L152. doi:10.1086/491785. . Retrieved 2007-09-21.

[5] Loveday, J. (February 1996). "The APM Bright Galaxy Catalogue." (http:/ / articles. adsabs. harvard. edu/ full/ 1996MNRAS. 278. 1025L).Monthly Notices of the Royal Astronomical Society 278 (4): 1025–1048. . Retrieved 2007-09-15.

[6] Dressler, A. (March 1980). accessdate= 2007-09-15 "Galaxy morphology in rich clusters — Implications for the formation and evolution ofgalaxies." (http:/ / articles. adsabs. harvard. edu/ full/ 1980ApJ. . . 236. . 351D). The Astrophysical Journal 236: 351–365.doi:10.1086/157753. accessdate= 2007-09-15.

[7] Lin, C. C.; Shu, F. H. (August 1964). "On the spiral structure of disk galaxies." (http:/ / articles. adsabs. harvard. edu/ full/ 1964ApJ. . . 140. .646L). The Astrophysical Journal 140: 646–655. doi:10.1086/147955. . Retrieved 2007-09-26.

[8] Trujillo, I.; Carretero, C.; Patiri, S.G. (2006). "Detection of the Effect of Cosmological Large-Scale Structure on the Orientation of Galaxies"(http:/ / adsabs. harvard. edu/ abs/ 2005astro. ph. 11680T). The Astrophysical Journal 640 (2): L111–L114. doi:10.1086/503548. .

[9] Alder, Robert (2006). "Galaxies like necklace beads" (http:/ / www. astronomy. com/ asy/ default. aspx?c=a& id=4215). Astronomymagazine. . Retrieved 2006-08-10.

[10] Hubble, E. P. (May 1926). "A spiral nebula as a stellar system: Messier 33." (http:/ / articles. adsabs. harvard. edu/ full/ 1926ApJ. . . . 63. .236H). The Astrophysical Journal 63: 236–274. doi:10.1086/142976. . Retrieved 2007-09-21.

[11] Chen, W.; Gehrels, N.; Diehl, R.; Hartmann, D. (1996). "On the spiral arm interpretation of COMPTEL ^26^Al map features" (http:/ /adsabs. harvard. edu/ abs/ 1996A& AS. . 120C. 315C). Space Science Reviews 120: 315–316. . Retrieved 2007-03-14.

[12] McKee, Maggie (August 16, 2005). "Bar at Milky Way's heart revealed" (http:/ / www. newscientist. com/ article/dn7854--bar-at-milky-ways-heart-revealed. html). New Scientist. . Retrieved 2009-06-17.

Spiral galaxy 137

External links• Giudice, G.F.; Mollerach, S.; Roulet, E. (1994). "Can EROS/MACHO be detecting the galactic spheroid instead

of the galactic halo?" (http:/ / arxiv. org/ abs/ astro-ph/ 9312047). Physical Review D 50: 2406–2413.doi:10.1103/PhysRevD.50.2406. Retrieved 2007-02-04.

• Stephens, Tim (March 6, 2007). "AEGIS survey reveals new principle governing galaxy formation and evolution"(http:/ / www. ucsc. edu/ news_events/ press_releases/ text. asp?pid=1080). UC Santa Cruz. Retrieved2006-05-24.

• Spiral Galaxies @ SEDS Messier pages (http:/ / www. seds. org/ messier/ spir. html)• SpiralZoom.com (http:/ / spiralzoom. com/ Science/ spiralgalaxies/ SpiralGalaxies. html), an educational website

about Spiral Galaxies and other spiral formations found in nature. For high school & general audience.• Spiral Structure explained (http:/ / burro. cwru. edu/ Academics/ Astr222/ Galaxies/ Spiral/ spiral. html)

Starburst galaxy

The Antennae Galaxies are an example of a very high starburst galaxyoccurring from the collision of NGC 4038/NGC 4039. Credit: NASA/ESA

A starburst galaxy is a galaxy in the process ofan exceptionally high rate of star formation,compared to the usual star formation rate seen inmost galaxies. Galaxies are often observed tohave a burst of star formation after a collision orclose encounter between two galaxies. The rateof star formation is so great for a galaxyundergoing a starburst that, if the rate wassustained, the gas reservoirs from which stars areformed would be used up on timescales muchshorter than that of the galaxy. For this reason, itis presumed that starbursts are temporary.Well-known starburst galaxies include M82,NGC 4038/NGC 4039 (the Antennae Galaxies),and IC 10.

Starburst definitions

Several definitions of the term starburst galaxyexist and there isn't really a strict definition onwhich all astronomers agree. However, many generally agree that the definition must in some way be related to thesethree factors:

1. the rate at which the galaxy is currently converting gas into stars (the star-formation rate, or SFR)2. the available quantity of gas from which stars can be formed3. comparison of the timescale of star formation with the age or rotation period of the galaxy.Commonly used definitions include:

Starburst galaxy 138

Starburst activity in the central region of nearby dwarf galaxy NGC 1569(Arp 210). Taken by Hubble Space Telescope

• Continued star-formation with the currentSFR would exhaust the available gasreservoir in much less than the age of theUniverse (the Hubble Time). This issometimes referred to as a "true" starburst.

• Continued star-formation with the currentSFR would exhaust the available gasreservoir in much less than the dynamicaltimescale of the galaxy (perhaps one rotationperiod in a disk type galaxy).

• The current SFR, normalised by thepast-averaged SFR is much greater than unity.This ratio is referred to as the birthrateparameter.

Starburst triggering mechanisms

As viewed from our position 12.2 billion light years away, the Baby BoomGalaxy is seen to be creating 4,000 stars per year. Credit: NASA

Essentially to ignite a starburst, it is necessary toconcentrate a large amount of cool molecular gasin a small volume. Such concentrations andperturbations are strongly suspected to causeglobal starburst phenomena in major galaxymergers, although the exact mechanisms are notfully understood.Observational surveys have long since shownthat there is often a burst of disk star-formationin merging and interacting pairs of galaxies. It isalso currently believed that nearby interactionsbetween galaxies that don't actually merge cantrigger unstable rotation modes, such as the barinstability, that cause gas to be funneled towardsthe nucleus, igniting bursts of star formation nearthe galactic nucleus.

Types of starburst

Classifying the starburst category itself isn't easysince starburst galaxies don't represent a specifictype in themselves. Starbursts can occur in disk galaxies, and irregular galaxies often exhibit knots of starburst, oftenspread throughout the irregular galaxy. However, several different subtypes of starburst are currently underdiscussion among galactic astronomers:

• Blue compact galaxies (BCGs). These galaxies are often low mass, low metallicity, dust-free objects. Becausethey are dust-free and contain a large number of hot, young stars, they are often blue in optical and ultravioletcolours. It was initially thought that BCGs were genuinely young galaxies in the process of forming their first

Starburst galaxy 139

generation of stars, thus explaining their low metal content. However old stellar populations have been found inmost BCGs and it is thought that efficient mixing may explain the apparent lack of dust and metals. Most BCGsshow signs of recent mergers and/or close interactions. Well-studied BCGs include IZw18 (the most metal poorgalaxy known), ESO338-IG04 and Haro11.• Blue compact dwarf galaxies (BCD galaxies) are small compact galaxies• Pea galaxy (Pea galaxies) are small compact galaxies resembling primordial starbursts. They were found by

citizen scientists taking part in the Galaxy Zoo project.• Luminous infrared galaxies (LIRGs)

• Ultra-luminous Infrared Galaxies (ULIRGs). These galaxies are generally extremely dusty objects. Theultraviolet radiation produced by the obscured star-formation is absorbed by the dust and reradiated in theinfrared spectrum at wavelengths of around 100 micrometres. This explains the extreme red colours associatedwith ULIRGs. It is not known for sure that the UV radiation is produced purely by star-formation and someastronomers believe ULIRGs to be powered (at least in part) by active galactic nuclei (AGN). X-rayobservations of many ULIRGs that penetrate the dust suggest that many starburst are double cored systems,lending support to the hypothesis that ULIRGs are powered by star-formation triggered by major mergers.Well-studied ULIRGs include Arp 220.

• Hyperluminous Infrared galaxies (HLIRGs)• Wolf-Rayet galaxies (WR galaxies), galaxy where a large portion of the bright stars are Wolf-Rayet stars.

The ingredients of a starburstFirstly, a starburst must have a large supply of gas available to form stars. The burst itself may be triggered by aclose encounter with another galaxy (such as M81/M82), a collision with another galaxy (such as the Antennae), orby another process which forces material into the center of the galaxy (such as a stellar bar).Inside the starburst is quite an extreme environment. The large amounts of gas mean that very massive stars areformed. Young, hot stars ionize the gas (mainly hydrogen) around them creating H II regions. Groups of very hotstars are known as OB associations. These stars burn very bright and very fast, and are quite likely to explode at theend of their lives as supernovae.After the supernova explosion, the ejected material expands and becomes a supernova remnant. These remnantsinteract with the surrounding environment within the starburst (the interstellar medium) and can be the site ofnaturally occurring masers.Studying nearby starburst galaxies can help us determine the history of galaxy formation and evolution. Largenumbers of the very distant galaxies seen, for example, in the Hubble Deep Field are known to be starbursts, but theyare too far away to be studied in any detail. Observing nearby examples and exploring their characteristics can giveus an idea of what was happening in the early universe as the light we see from these distant galaxies left them whenthe universe was much younger (see redshift). Unfortunately however, starburst galaxies seem to be quite rare in ourlocal universe, and are more common further away - indicating that there were more of them billions of years ago.All galaxies were closer together then, and therefore more likely to be influenced by each other's gravity. Morefrequent encounters produced more starbursts as galactic forms evolved with the expanding universe.

Well-known starburstsM82 is the archetypal starburst galaxy. Its high level of star formation is due to a close encounter with the nearby spiral M81. Maps of the regions made with radio telescopes show large streams of neutral hydrogen [1] connecting the two galaxies, also as a result of the encounter. Radio images of the central regions of M82 also show a large number of young supernova remnants, left behind when the more massive stars created in the starburst came to the end of their lives. The Antennae is another well-known starburst system, made famous by a stunning Hubble picture

Starburst galaxy 140

[2], released in 1997.

Sources• "Chandra :: Field Guide to X-ray Sources :: Starburst Galaxies" [3]. chandra.harvard.edu. Retrieved 2007-12-29.

See also• Active galaxy• Baby Boom Galaxy• Blue compact dwarf galaxy• Messier 82• Pea galaxy• Starburst

References[1] http:/ / articles. adsabs. harvard. edu/ cgi-bin/ nph-build_image?bg=%23FFFFFF& / seri/ A%2BA. . / 0075/ 600/ 0000106. 000&

db_key=AST& bits=4& res=100& filetype=. gif[2] http:/ / hubblesite. org/ newscenter/ newsdesk/ archive/ releases/ 1997/ 34/[3] http:/ / chandra. harvard. edu/ xray_sources/ starburst. html

Type-cD galaxyThe type-cD galaxy[1] (also cD-type galaxy[2] , cD galaxy[3] ) is a galaxy morphology classification, a subtype oftype-D giant elliptical galaxy and have a large halo of stars[4] . They can be found near the centres of some richgalaxy clusters.[5] They are also known as supergiant ellipticals[6] or central dominant galaxies[7] .

cD galaxiesThe cD-type is a classification in the Yerkes galaxy classification scheme, one of two Yerkes classifications still incommon use, along with D-type.[8] The "c" in "cD" refers to the fact that the galaxies are very large, hencesupergiant, while the "D" refers to the fact that the galaxies appear diffuse.[9] A backformation of "cD" is frequentlyused to mean central Dominant galaxy.[7] cD's are also frequently considered the largest galaxies around.[10] [11]

cD galaxies are similar to lenticular galaxies (S0) or elliptical galaxies (E#), but many times larger, some havingenvelopes that exceed one million lightyears in radius.[12] They appear elliptical-like, with large low surfacebrightness envelopes.[13] It is currently thought that cD's are the result of galaxy mergers.[14] Some cD's havemultiple galactic nuclei.[15] cD galaxies are one of the types frequently found to be the Brightest cluster galaxy(BCG) of a cluster.[16] Many fossil group galaxies are similar to cD BCG galaxies, leading some to theorize that thecD results from the creation of a fossil group, and then the new cluster accumulating around the fossil group.[17]

However, cD's themselves are not found as field galaxies, unlike fossil groups.[13] cD's form around 20% ofBCGs.[13]

Type-cD galaxy 141

GrowthcD galaxies are believed to grow via mergers of galaxies that spiral in to the center of a galaxy cluster, a theory firstproposed by Herbert J. Rood in 1965.[18] This "cannibalistic" mode of growth leads to the overwhelming diameterand luminosity of the cD's.[19] The second-brightest galaxy in the cluster is usually under-luminous, a consequenceof its having been "eaten".[20] Remains of "eaten" galaxies sometimes appear as a diffuse halo of gas and dust.[19]

This halo can be up to 3 million light years in diameter.[14]

Dynamical frictionDynamical friction is believed to play an important role in the formation of cD galaxies at the centres of galaxyclusters.[21] This process begins when the motion of a large galaxy in a cluster attracts smaller galaxies and darkmatter into a wake behind it. This over-density follows behind the larger galaxy and exerts a constant gravitationalforce on it, causing it to slow down. As it loses kinetic energy, the large galaxy gradually spirals toward the centre ofthe cluster. Once there, the stars, gas, dust and dark matter of the large galaxy and its trailing galaxies will join withthose of other galaxies who preceded them in the same fate.[22] A giant or supergiant diffuse or elliptical galaxy willresult from this accumulation.[23] The centers of merged or merging galaxies can remain recognizable for long times,appearing as multiple "nuclei" of the cD galaxy.[24]

cD clustersType-cD galaxies are also used to define clusters. A galaxy cluster with a cD at its centre is termed a "cD cluster" or"cD galaxy cluster" or "cD cluster of galaxies".[25]

Examples of cD galaxies• Perseus A [26]

• NGC 6166 [27]

• IC 1101 — the largest known galaxy, in terms of diameter (around 6 million light years) [28] [29] [30]

• Messier 87, the central galaxy in the Virgo Cluster.• NGC 1399 in the Fornax Cluster

References[1] Sidereal Times, June 2002, page 3[2] Proceedings of PATRAS 2008, page 59[3] Galaxy Clusters, Jan Hartlap, page 3[4] Surface Photometry and the Structure of Elliptical Galaxies, "Chapter 11. cD and Brightest Cluster Galaxies" (http:/ / nedwww. ipac. caltech.

edu/ level5/ Sept01/ Kormendy/ Kormendy11. html), John Kormendy, S. Djorgovski, 1989[5] A Dictionary of Astronomy, "cD galaxy" (http:/ / www. highbeam. com/ doc/ 1O80-cDgalaxy. html) (accessed 14 April 2010)[6] encyclopedia.com "supergiant elliptical" (http:/ / www. encyclopedia. com/ doc/ 1O80-supergiantelliptical. html)[7] "Uncertainties on Clusters of Galaxies Distances", C. Adami, M.P. Ulmer, 18 July 2000, arXiv:astro-ph/0007265 (accessed 14 April 2010)[8] An Atlas of DRAGNs, "Glossary" (http:/ / www. jb. man. ac. uk/ atlas/ gloss. html#cD), J. P. Leahy, 15 March 1997 (accessed 14 April

2010)[9] Global Telescope Network, "Types of Galaxies" (http:/ / gtn. sonoma. edu/ resources/ normal_galaxies/ types. php), Kevin McLin, 14 April

2010 (accessed 14 April 2010)[10] Universe Today, "What is the Largest Galaxy?" (http:/ / www. universetoday. com/ guide-to-space/ galaxies/ what-is-the-largest-galaxy/ ),

Fraser Cain (accessed 14 April 2010)[11] EurekAlert, "Scientists observe largest explosion in space" (http:/ / www. eurekalert. org/ pub_releases/ 2005-01/ ou-sol010505. php),

Andrea Gibson, 5 January 2005 (accessed 15 April 2010)[12] Encyclopedia Britannica, "cD-galaxy" (http:/ / www. britannica. com/ EBchecked/ topic/ 100915/ cD-galaxy) (accessed 14 April 2010)[13] Monthly Notices of the Royal Astronomical Society, "Intracluster light and the extended stellar envelopes of cD galaxies: an analytical

description", Marc S. Seigar, Alister W. Graham, Helmut Jerjen, July 2007, Volume 378, Issue 4, pp. 1575-1588,doi:10.1111/j.1365-2966.2007.11899.x, Bibcode: 2007MNRAS.378.1575S, arXiv:astro-ph/0612229v2 (accessed 15 April 2010)

Type-cD galaxy 142

[14] COSMOS - The SAO Encyclopedia of Astronomy, "CD Galaxies" (http:/ / astronomy. swin. edu. au/ cosmos/ C/ CD+ Galaxies),Swinburne University of Technology (accessed 14 April 2010)

[15] Internet Encyclopedia of Science, "D galaxy" (http:/ / www. daviddarling. info/ encyclopedia/ D/ D_galaxy. html), David Darling (accessed14 April 2010)

[16] IAU Symposium 245, "Star Formation in Bulges from GALEX", Sukyoung K. Yi, 5 September 2007, doi:10.1017/S174392130801819X,arXiv:0709.0177 (accessed 14 April 2010)

[17] Universe Today, "How Do Fossil Galaxy Clusters Form so Quickly?" (http:/ / www. universetoday. com/ 2006/ 04/ 27/how-do-fossil-galaxy-clusters-form-so-quickly/ ), Fraser Cain, 27 April 2006 (accessed 15 April 2010)

[18] Rood, H. J. (1965). The Dynamics of the Coma Cluster of Galaxies (http:/ / adsabs. harvard. edu/ abs/ 1965PhDT. . . . . . . . . 3R). (PhDthesis). The University of Michigan.

[19] "Curious About Astronomy?" (http:/ / curious. astro. cornell. edu/ ). . Retrieved 28 March 2007.[20] Hausman, M. J.; Ostriker, J. P. (November 1977). "Cannibalism among the galaxies - Dynamically produced evolution of cluster luminosity

functions" (http:/ / adsabs. harvard. edu/ abs/ 1977ApJ. . . 217L. 125O). The Astrophysical Journal Letters 217: L125-L128. .[21] Merritt, David (January 1983). "Relaxation and tidal stripping in rich clusters of galaxies. I. Evolution of the mass distribution" (http:/ /

adsabs. harvard. edu/ abs/ 1983ApJ. . . 264. . . 24M). The Astrophysical Journal 264: 24–48. .[22] Merritt, David (January 1984). "Relaxation and tidal stripping in rich clusters of galaxies. II. Evolution of the luminosity distribution" (http:/

/ adsabs. harvard. edu/ abs/ 1984ApJ. . . 276. . . 26M). The Astrophysical Journal 276: 26–37. .[23] Merritt, David (February 1985). "Relaxation and tidal stripping in rich clusters of galaxies. III. Growth of a massive central galaxy" (http:/ /

adsabs. harvard. edu/ abs/ 1985ApJ. . . 289. . . 18M). The Astrophysical Journal 289: 18–32. .[24] Merritt, David (May 1984). "The nature of multiple-nucleus cluster galaxies" (http:/ / adsabs. harvard. edu/ abs/ 1984ApJ. . . 280L. . . 5M).

The Astrophysical Journal 280: L5–8. .[25] PDF (http:/ / articles. adsabs. harvard. edu/ cgi-bin/ nph-iarticle_query?1971PASP. . . 83. . 313R& amp;data_type=PDF_HIGH&

amp;whole_paper=YES& amp;type=PRINTER& amp;filetype=. pdf), "'Tuning Fork' Classification of Rich Clusters of Galaxies", HerbertJ.Rood, Gummuluru N. Sastry, June 1971, doi:10.1086/129128, Bibcode: 1971PASP...83..313R (accessed 14 April 2010)

[26] Nature, "FIGURE 4. Optical, radio and X-ray images of the Perseus cluster." (http:/ / www. nature. com/ nature/ journal/ v460/ n7252/fig_tab/ nature08135_F4. html) 9 July 2009, ISSN 0028-0836 ; EISSN 1476-4687 ; (accessed 15 April 2010)

[27] Nature, "FIGURE 3. The entropy of the intracluster medium in spherical shells of radius r." (http:/ / www. nature. com/ nature/ journal/v460/ n7252/ fig_tab/ nature08135_F3. html) 9 July 2009, ISSN 0028-0836 ; EISSN 1476-4687 ; (accessed 15 April 2010)

[28] Science, "The Central Galaxy in Abell 2029: An Old Supergiant" (http:/ / www. sciencemag. org/ cgi/ content/ abstract/ 250/ 4980/ 539),Juan M. Uson, Stephen P. Boughn, and Jeffrey R. Kuhn, 26 October 1990, Vol. 250, no. 4980, pp.539-540,doi:10.1126/science.250.4980.539

[29] Ellensburg Daily Record, "Galaxy Found", United Press International, 27 October 1990, p.16[30] Lodi News-Sentinel, "Giant Galaxy Discovered", UPI, 26 October 1990, pg.9

Further reading• PDF (http:/ / nedwww. ipac. caltech. edu/ level5/ Morgan2/ paper. pdf) - "A Preliminary Classification of the

Forms of Galaxies According to Their Stellar Population" (http:/ / nedwww. ipac. caltech. edu/ level5/ Morgan2/frames. html), W. W. Morgan, Yerkes Obeservatory, 1958, doi:10.1086/127415, Bibcode: 1959PASP...71..394M(PASP 70)

• PDF (http:/ / arxiv. org/ pdf/ astro-ph/ 0612229v2) - "Intracluster light and the extended stellar envelopes of cDgalaxies: an analytical description", Marc S. Seigar, Alister W. Graham, Helmut Jerjen, July 2007doi:10.1111/j.1365-2966.2007.11899.x, Bibcode: 2007MNRAS.378.1575S, arXiv:astro-ph/0612229v2 (MNRAS07/2007)

Type-cD galaxy 143

See also• Giant elliptical galaxy (gE)• Giant galaxy• Elliptical galaxy (E#)• Lenticular galaxy (S0, SA0, SAB0, SB0, E9)• Type-D galaxy

Unbarred lenticular galaxy

An example of this type, imaged by the Hubble Space Telescope

An unbarred lenticular galaxy is alenticular version of an unbarred spiralgalaxy. They have the Hubble type of SA0

An example of this is the Galaxy AM0644-741. For other examples seeCategory:Unbarred lenticular galaxies.

See also

• Barred lenticular galaxy

Unbarred spiral galaxy 144

Unbarred spiral galaxy

The Whirlpool Galaxy and its companionsatellite. The Whirlpool is an unbarred spiral

galaxy

An unbarred spiral galaxy[1] [2] is a type of spiral galaxy without acentral bar, or one that is not a barred spiral galaxy. It is designatedwith an SA in the galaxy morphological classification scheme.

The Sombrero Galaxy is an unbarred spiral galaxy.Barless spiral galaxies are one of three general types of spiral galaxiesunder the de Vaucouleurs system classification system, the other twobeing intermediate spiral galaxy and barred spiral galaxy. Under theHubble tuning fork, it is one of two general types of spiral galaxy, theother being barred spirals.

Grades

Under the de Vaucouleurs classification system, SA-galaxies are one of threetypes of spiral galaxy

Unbarred spiral galaxy 145

Example Type Image Information Notes

SA0- SA0- is a type of lenticular galaxy

SA0 SA0 is a type of lenticular galaxy

SA0+ SA0+ is a type of lenticular galaxy

NGC 3593 SA0/a SA0/a can also be considered a type of unbarred lenticulargalaxy

NGC 3593 is actually an"SA(s)0/a"

NGC 3169 SAa NGC 3169 is actually an "SA(s)apec"

Messier 81 SAab M81 is actually an "SA(s)ab"

Messier 88 SAb M88 is actually an "SA(rs)b"

NGC 3949 SAbc NGC 3949 is actually an "SA(s)bc"

NGC 4414 SAc NGC 4414 is actually an "SA(rs)c"

TriangulumGalaxy

SAcd Triangulum is actually an"SA(s)cd"

NGC 300 SAd NGC 300 is actually an "SA(s)d"

NGC 45 SAdm SAdm can also be considered a type of unbarred Magellanicspiral

NGC 45 is actually an "SA(s)dm"

NGC 4395 SAm SAm is a type of Magellanic spiral (Sm) NGC 4395 is actually an "SA(s)m"

References[1] Astronomical Journal, "Near-infrared surface photometry and morphology in virgo cluster spiral galaxy nuclear regions", Bernard J.

Rauscher, April 1995, Bibcode: 1995AJ....109.1608R, doi:10.1086/117389[2] Astronomy Pictures, "M99" (http:/ / www. astronomy-pictures. com/ 2008/ ST-2000_m99-2008. htm) (accessed 18 April 2010)

146

Appendix

Brightest cluster galaxy

This image from NASA's Hubble SpaceTelescope shows the galaxy cluster Abell S0740that is over 450 million light-years away in thedirection of the constellation Centaurus. The

giant elliptical galaxy ESO 325-G004 looms largeat the cluster's center. This BCG is as massive as

100 billion of our suns.

Brightest cluster galaxy (BCG) is defined as the brightest galaxy in acluster of galaxies. BCGs include the most massive galaxies in theuniverse. They are generally elliptical galaxies which lie close to thegeometric and kinematical center of their host galaxy cluster, hence atthe bottom of the cluster potential well. They are also generallycoincident with the peak of the cluster X-ray emission.

Their brightness, coupled with their small variance in luminosity, makethem excellent standard candles for distance determination.

Formation scenarios for BCGs include:• Cooling flow—Star formation from the central cooling flow in high

density cooling centers of X-ray cluster halos.

The study of accretion populations in BCGs [1] has cast doubt over thistheory and astronomers have seen no evidence of cooling flows inradiative cooling clusters[2] . The two remaining theories exhibithealthier prospects.

• Galactic cannibalism—Galaxies sink to the center of the cluster dueto dynamical friction and tidal stripping[3] .

• Galactic merger—Rapid galactic mergers between several galaxiestake place during cluster collapse[4] .

It is possible to differentiate the cannibalism model from the merging model by considering the formation period ofthe BCGs. In the cannibalism model, there are numerous small galaxies present in the evolved cluster, whereas in themerging model, a hierarchical cosmological model is expected due to the collapse of clusters. The merging model isnow generally accepted as the most likely one.[5]

BCGs are divided into various classes of galaxies: giant ellipticals (gE), D galaxies and cD galaxies [6] . cD and Dgalaxies both exhibit an extended diffuse envelope surrounding an elliptical-like nucleus akin to regular ellipticalgalaxies. The light profiles of BCGs are well described by a Sersic surface brightness law.

Brightest cluster galaxy 147

References[1] McNamara and O’Connell (1989), Star formation in cooling flows in clusters of galaxies (http:/ / adsabs. harvard. edu/ abs/ 1989AJ. . . . . 98.

2018M)[2] Motl et al. (2004), Formation of Cool Cores in Galaxy Clusters via Hierarchical Mergers (http:/ / adsabs. harvard. edu/ abs/ 2004ApJ. . . 606. .

635M)[3] J. Ostriker and M. Hausman (1977), Cannibalism among the galaxies - Dynamically produced evolution of cluster luminosity functions (http:/

/ adsabs. harvard. edu/ abs/ 1977ApJ. . . 217L. 125O)[4] D. Merritt (1984), Relaxation and tidal stripping in rich clusters of galaxies. II - Evolution of the luminosity distribution (http:/ / adsabs.

harvard. edu/ abs/ 1984ApJ. . . 276. . . 26M)[5] J. Dubinski (1998), The Origin of the Brightest Cluster Galaxies (http:/ / adsabs. harvard. edu/ abs/ 1998ApJ. . . 502. . 141D)[6] Matthews, T. A., Morgan, W. W. and Schmidt, M. (1964), A Discussion of Galaxies Identified with Radio Sources (http:/ / adsabs. harvard.

edu/ abs/ 1964ApJ. . . 140. . . 35M)

See also• Fossil group

Galaxy color-magnitude diagram

A mock-up of the galaxy color-magnitude diagram with three populations: the redsequence, the blue cloud, and the green valley.

The Galaxy color-magnitude diagramshows the relationship between absolutemagnitude, luminosity, and mass ofgalaxies. A preliminary description of thethree areas of this diagram was made in2003 by Eric F. Bell et al. from theCOMBO-17 survey[1] that clarified thebimodal distribution of red and blue galaxiesas seen in analysis of Sloan Digital SkySurvey data[2] and even in de Vaucouleurs'1961 analyses of galaxy morphology[3]

Noticed in this diagram are three mainfeatures: the red sequence, the green valley,and the blue cloud. The red sequenceincludes most red galaxies which aregenerally elliptical galaxies. The blue cloudincludes most blue galaxies which aregenerally spirals. In between the twodistributions is an underpopulated spaceknown as the green valley which includes anumber of red spirals. Unlike thecomparable HR diagram for stars, galaxy properties are not necessarily completely determined by their location onthe color-magnitude diagram. The diagram also shows considerable evolution through time. The red sequence earlierin evolution of the universe was more constant in color across magnitudes and the blue cloud was not as uniformlydistributed but showed sequence progression.

Galaxy color-magnitude diagram 148

References[1] Bell, Eric F. et al. Nearly 5000 Distant Early‐Type Galaxies in COMBO‐17: A Red Sequence and Its Evolution since z=1, The Astrophysical

Journal, 608:752–767, 2004 June 20. (http:/ / adsabs. harvard. edu/ abs/ 2004ApJ. . . 608. . 752B)[2] Strateva, I., et al. Color Separation of Galaxy Types in the Sloan Digital Sky Survey Imaging Data, 2001, The Astronomical Journal, 122,

1861 (http:/ / www. journals. uchicago. edu/ servlet/ linkout?suffix=rf95& dbid=64& doi=10. 1086/ 420778& key=2001AJ. . . . 122. 1861S)[3] de Vaucouleurs, G. Integrated Colors of Bright Galaxies in the u, b, V System. 1961, The Astrophysical Journal Supplement Series, 5, 233.

(http:/ / adsabs. harvard. edu/ abs/ 1961ApJS. . . . 5. . 233D)

List of galaxies

The Hubble Ultra Deep Field shows over 10,000galaxies in a mere 0.000024% of the sky

This is a list of notable galaxies.

List of galaxies

Galaxy Notes

M82 This is the prototype starburst galaxy.

M87 This is the central galaxy of the Virgo Cluster, the central cluster of the Local Supercluster.[1]

M102 This galaxy cannot be definitively identified, with the most likely candidate being NGC 5866, and a good chance of it being amisidentification of M101. Other candidates have also been suggested.

NGC 2770 NGC 2770 is referred to as the Supernova Factory due to three recent supernovae occurring within it.

NGC 3314

NGC3314a

NGC3314b

This is a pair of spiral galaxies, one superimposed on another, at two separate and distinct ranges, and unrelated to each other. It isa rare chance visual alignment.

ESO 137-001 Lying in the galaxy cluster Abell 3627, this galaxy is being stripped of its gas by the pressure of the intracluster medium (ICM),due to its high speed traversal through the cluster, and is leaving a high density tail with large amounts of star formation. The tailfeatures the largest amount of star formation outside of a galaxy seen so far. The galaxy has the appearance of a comet, with thehead being the galaxy, and a tail of gas and stars.[2] [3] [4] [5]

Comet Galaxy Lying in galaxy cluster Abell 2667, this spiral galaxy is being tidally stripped of stars and gas through its high speed traversalthrough the cluster, having the appearance of a comet.

List of galaxies 149

List of named galaxiesThis is a list of galaxies that are well known by something other than an entry in a catalog or list, or a set ofcoordinates, or a systematic designation.

Galaxy Origin of name Notes

Milky WayGalaxy

This is the galaxy that contains Earth, it is named after the nebulosityin the night sky that marks the densest concentration of stars of ourgalaxy in the sky, which appears to blur together into a faint glow,called the Milky Way.

Andromeda Commonly just Andromeda, this, called the Andromeda Galaxy,Andromeda Nebula, Great Andromeda Nebula, Andromeda SpiralNebula, and such, has been traditionally called Andromeda, after theconstellation in which it lies.

Bode's Galaxy Named for Johann Elert Bode who discovered this galaxy in 1774.

CartwheelGalaxy

Its visual appearance is similar to that of a spoked cartwheel.

Cigar Galaxy Appears similar in shape to a cigar.

Comet Galaxy This galaxy is named after its unusual appearance, looking like acomet.

The comet effect is caused by tidal stripping by its galaxycluster, Abell 2667.

Hoag's Object This is named after Art Hoag, who discovered this ring galaxy. It is of the subtype Hoag-type galaxy, and may in fact be apolar-ring galaxy with the ring in the plane of rotation ofthe central object.

LargeMagellanicCloud

Named after Ferdinand Magellan This is the fourth largest galaxy in the Local Group, andforms a pair with the SMC, and from recent research, maynot be part of the Milky Way system of satellites at all.

SmallMagellanicCloud

Named after Ferdinand Magellan This forms a pair with the LMC, and from recent research,may not be part of the Milky Way system of satellites atall.

Mayall'sObject

This is named after Nicholas U. Mayall, of the Lick Observatory, whodiscovered it.[6] [7] [8]

Also called VV 32 and Arp 148, this is a very peculiarlooking object, and is likely to be not one galaxy, but twogalaxies undergoing a collision. Event in images is aspindle shape and a ring shape.

PinwheelGalaxy

Similar in appearance to a pinwheel (toy).

SombreroGalaxy

Similar in appearance to a sombrero.

SunflowerGalaxy

TadpoleGalaxy

The name comes from the resemblance of the galaxy to a tadpole. This shape resulted from tidal interaction that drew out along tidal tail.

WhirlpoolGalaxy

From the whirlpool appearance this gravitationally disturbed galaxyexhibits.

List of galaxies 150

List of naked-eye galaxiesThis is a list of galaxies that are visible to the naked-eye, for at the very least, keen-eyed observers in a very dark-skyenvironment that is high in altitude, during clear and stable weather.

Naked-eye Galaxies

Galaxy ApparentMagnitude

Distance Notes

Milky Way Galaxy -26.74 (theSun)

0 This is our galaxy, most things visible to the naked-eye in the sky are part of it,including the Milky Way composing the zone of avoidance.[9]

Large MagellanicCloud

0.9 160 kly (50kpc) Visible only from the southern hemisphere. It is also the brightest patch of nebulosity inthe sky.[9] [10] [11]

Small MagellanicCloud (NGC292)

2.7 200 kly (60kpc) Visible only from the southern hemisphere.[9] [12]

Andromeda Galaxy(M31 , NGC224)

3.4 2.5 Mly(780kpc)

Once called the Great Andromeda Nebula, it is situated in the Andromedaconstellation.[9] [13]

Omega Centauri(NGC5139)

3.7 18 kly (5.5kpc) Once thought to be a star and later a globular cluster, Omega Centauri was confirmed ashaving a black hole at its center and thus its status has been changed to being a dwarfgalaxy as of April 2010. [14]

Triangulum Galaxy(M33 , NGC598)

5.7 2.9 Mly (900kpc)

Being a diffuse object, its visibility is strongly affected by even small amounts of lightpollution, ranging from easily visible in direct vision in truly dark skies to a difficultaverted vision object in rural/suburban skies.[15]

Centaurus A (NGC5128)

7.8 13.7 ± 0.9 Mly(4.2 ± 0.3 Mpc)

Centaurus A has been spotted with the naked eye by Stephen James O'Meara[16]

Bode's Galaxy (M81, NGC3031)

7.89 12 Mly(3.6Mpc)

Highly experienced amateur astronomers may be able to see Messier 81 underexceptional observing conditions. [17] [18] [19]

Sculptor Galaxy(NGC 253)

8.0 11.4 ± 0.7 Mly(3.5 ± 0.2 Mpc)

According to Brian A. Skiff, the naked- ey visibility of this galaxy is discussed in an oldSky & Telescope letter or note from the late 1960s or early 1970s.[20]

Messier 83 (NGC5236)

8.2 14.7 Mly (4.5Mpc)

M83 has reportedly been seen with the naked eye.[21]

• Sagittarius Dwarf Elliptical Galaxy is not listed, because it is not discernible as being a separate galaxy in the sky.

Firsts

Galactic Firsts

First Galaxy Date

Notes

First galaxy Milky WayGalaxy &AndromedaGalaxy

1923 Edwin Hubble determined the distance to the Andromeda Nebula, and found that it could notbe part of the Milky Way, so defining that Milky Way was not the entire universe, andmaking the two separate objects, and two galaxies. However, the first galaxies seen would beall of the naked-eye galaxies, but they were not identified as such until the 20th century.

First radio galaxy Cygnus A 1952 Of several items, then called radio stars, Cygnus A was identified with a distant galaxy,being the first of many radio stars to become a radio galaxy.[22]

List of galaxies 151

First quasar 3C2733C48

19621960

3C273 was the first quasar with its redshift determined, and by some considered the firstquasar. 3C48 was the first "radio-star" with an unreadable spectrum, and by othersconsidered the first quasar.

First Seyfert galaxy NGC 1068 (M77) 1908 The characteristics of Seyfert galaxies were first observed in M77 in 1908, however, Seyfertswere defined as a class in 1943.[23]

First discovered object,later identified to be acannibalized galaxy

Omega Centauri Omega Centauri is considered the core of a disrupted dwarf spheroidal galaxy cannibalizedby the Milky Way, and was originally catalogued in 1677 as a nebula. It is currentlycatalogued as a globular cluster.

First superluminalgalactic jet

3C279 1971 The jet is emitted by a quasar

First superluminal jetfrom a Seyfert

III Zw 2 2000 [24]

First spiral galaxy Whirlpool Galaxy 1845 Lord William Parsons, Earl of Rosse discovered the first spiral nebula from observing theM51 white nebula.[25]

PrototypesThis is a list of galaxies that became prototypes for a class of galaxies.

Prototype Galaxies

Class Galaxy Date Notes

BL Lac object BL Lacertae (BLLac)

This AGN was originally catalogued as a variable star, and "stars" of its type are considered BLLac objects.

Hoag-typeGalaxy

Hoag's Object This is the prototype Hoag-type Ring Galaxy

ExtremesThis list is incomplete.

Title Galaxy Data Notes

Least separation between binary centralblack holes

4C37.11

24 ly(7.3pc)

OJ 287 has an inferred pair with a 12 year orbital period, and thus would be muchcloser than 4C 37.11's pair.

Distances

List of galaxies 152

Title Galaxy Distance Notes

Closest neighbouringgalaxy

Canis Major Dwarf 0.025Mly

Discovered in 2003, a satellite of the Milky Way, slowlybeing cannibalized by it.

Most distant galaxy UDFy-38135539 z=8.55 Discovered in 2010, it became the most remote objectknown, exceeding GRB 090423.[26]

Closest quasar 3C 273 z=0.158 First identified quasar, this is the most commonly acceptednearest quasar.

Most distant quasar CFHQS J2329-0301 z=6.43 Discovered in 2007.

Closest radio galaxy Centaurus A (NGC 5128 , PKS 1322-427) 13.7 Mly [27]

Most distant radiogalaxy

TN J0924-2201 z=5.2

Closest Seyfert galaxy Circinus Galaxy 13 Mly This is also the closest Seyfert 2 galaxy. The closest Seyfert1 galaxy is NGC 4151.

Most distant Seyfertgalaxy

z=

Closest blazar Markarian 421 (Mrk 421, Mkn 421, PKS1101+384, LEDA 33452)

z=0.030 This is a BL Lac object.[28] [29]

Most distant blazar Q0906+6930 z=5.47 This is a flat spectrum radio-loud quasar type blazar.[30][31]

Closest BL Lac object Markarian 421 (Mkn 421, Mrk 421, PKS1101+384, LEDA 33452)

z=0.030 [28] [29]

Most distant BL Lacobject

z=

Closest LINER

Most distant LINER z=

Closest LIRG

Most distant LIRG z=

Closest ULIRG IC 1127 (Arp 220 , APG 220) z=0.018 [32]

Most distant ULIRG z=

Closest starburstgalaxy

Cigar Galaxy (M82 , Arp 337/APG 337 , 3C231 , Ursa Major A)

3.2Mpc [33] [34]

Most distant starburstgalaxy

z=

Brightness and power

List of galaxies 153

Title Galaxy Data Notes

Apparently brightest galaxy Baby Boom Galaxy Starburst galaxy located in the very distant universe.

Apparently faintest galaxy Apparent magnitude

Intrinsically brightestgalaxy

Absolute magnitude Markarian 231 is the most luminous nearby galaxy (~590Mly;apmag 13.8).

Intrinsically faintest galaxy Boötes Dwarf Galaxy (BoodSph)

Absolute magnitude-6.75

This does not include dark galaxies.

Highest surface brightnessgalaxy

Lowest surface brightnessgalaxy

Andromeda IX

Visually brightest galaxy Large Magellanic Cloud Apparent magnitude0.6

This galaxy has high surface brightness combined with highapparent brightness.

Visually faintest galaxy This galaxy has low surface brightness combined with lowapparent brightness.

Mass

Title Galaxy Mass Notes

Least massive galaxy Willman 1 [35]

Most massive galaxy Messier 87 (M87, NGC 4486, VirgoA)

[36]

Most massive spiral galaxy ISOHDFS 27 The preceding most massive spiral was UGC12591[37]

Least massive galaxy with globularcluster(s)

Andromeda I [38]

Dimension

Title Galaxy Size Notes

Most expansive galaxy IC 1101 5-6 million light-years

Least expansive galaxy

Closest galaxies

List of galaxies 154

5 Closest Galaxies

Rank Galaxy Distance

1 Milky Way Galaxy 0 This is our galaxy, as such, we are part of it.

2 Omega Centauri 0.0183 Mly

3 Canis Major Dwarf 0.025 Mly

4 Virgo Stellar Stream 0.030 Mly

5 Sagittarius Dwarf Elliptical Galaxy 0.081 Mly

6 Large Magellanic Cloud 0.163 Mly

• Mly represents millions of light-years, a measure of distance.• Distances are measured from Earth, with Earth being at zero.

Nearest Galaxies by Type

Title Galaxy Date Distance Notes

Nearest galaxy Milky Way always 0 This is our galaxy

Nearest galaxy to our own Canis Major Dwarf 2003 0.025 Mly

Nearest dwarf galaxy Canis Major Dwarf 2003 0.025 Mly

Nearest large galaxy to our own Andromeda Galaxy always 2.54 Mly First identified as a separate galaxy in 1923

Nearest giant galaxy Centaurus A 12 Mly

Nearest Neighbouring Galaxy Title-holder

Galaxy Date Distance Notes

Canis Major Dwarf 2003 - 0.025Mly

Sagittarius DwarfElliptical Galaxy

1994 − 2003 0.081Mly

Large MagellanicCloud

antiquity − 1994 0.163Mly

This is the upper bound, as it is nearest galaxy observable with the naked-eye.

Small MagellanicCloud

1913 - 1914 This was the first intergalactic distance measured. In 1913, Ejnar Hertzsprung measures thedistance to SMC using Cepheid variables. In 1914, he did it for LMC.

Andromeda Galaxy 1923 This was the first galaxy determined to be not part of the Milky Way.

• Mly represents millions of light-years, a measure of distance.• Distances are measured from Earth, with Earth being at zero.

• Omega Centauri does not appear on this list because is not currently considered a galaxy, per se, it is considered aformer galaxy, and all that remains of one that was cannibalized by the Milky Way.

List of galaxies 155

Farthest galaxies

Most Remote Galaxies by Type

Title Galaxy Date Distance Notes

Most remote galaxy UDFy-38135539 2010 z=8.55 [26]

Most remote normal galaxy UDFy-38135539 2010 z=8.55 [26]

Most remote quasar CFHQS J2329-0301 2007 z=6.43 This is the undisputed most remote quasar of anytype

Most distant non-quasarSMG

Baby Boom Galaxy (EQJ100054+023435)

2008 z=4.547 [39]

• z represents redshift, a measure of recessional velocity and inferred distance due to cosmological expansion

Most Remote Galaxy Record-holders

Galaxy Date Distance Notes

UDFy-38135539 2010 - z=8.55 This was the remotest object known at time of discovery. It exceeded thedistance of IOK-1 and GRB 090423 [26]

IOK-1 2006 − 2010 z=6.96 This was the remotest object known at time of discovery. In 2009, gammaray burst GRB 090423 was discovered at z=8.2, taking the title of mostdistant object. The next galaxy to hold the title also succeeded GRB090423, that being UDFy-38135539.[40] [41] [26]

SDF J132522.3+273520 2005 − 2006 z=6.597 This was the remotest object known at time of discovery.[41] [42]

SDF J132418.3+271455 2003 − 2005 z=6.578 This was the remotest object known at time of discovery.[42] [43] [44] [45]

HCM-6A 2002 − 2003 z=6.56 This was the remotest object known at time of discovery. The galaxy islensed by galaxy cluster Abell 370. This was the first galaxy, as opposed toquasar, found to exceed redshift 6. It exceeded the redshift of quasar SDSSpJ103027.10+052455.0 of z=6.28[43] [44] [46] [47] [48] [49]

SSA22−HCM1 1999 − 2002 z=5.74 This was the remotest object known at time of discovery. In 2000, thequasar SDSSp J104433.04-012502.2 was discovered at z=5.82, becomingthe most remote object in the universe known. This was followed by anotherquasar, SDSSp J103027.10+052455.0 in 2001, the first object exceedingredshift 6, at z=6.28[50] [51]

HDF 4-473.0 1998 − 1999 z=5.60 This was the remotest object known at the time of discovery.[51]

RD1 (0140+326 RD1) 1998 z=5.34 This was the remotest object known at time of discovery. This was the firstobject found beyond redshift 5.[51] [52] [53] [54] [55]

CL 1358+62 G1 &CL 1358+62 G2

1997 − 1998 z=4.92 These were the remotest objects known at the time of discovery. The pair ofgalaxies were found lensed by galaxy cluster CL1358+62 (z=0.33). Thiswas the first time since 1964 that something other than a quasar held therecord for being the most distant object in the universe. It exceeded themark set by quasar PC 1247-3406 at z=4.897[51] [53] [54] [56] [57] [58]

From 1964 to 1997, the title of most distant object in the universe were held by a succession of quasars.[58] That list is available at list of quasars.

List of galaxies 156

8C 1435+63 1994 − 1997 z=4.25 This is a radio galaxy. At the time of its discovery, quasar PC 1247-3406 atz=4.73, discovered in 1991 was the most remote object known. This was thelast radio galaxy to hold the title of most distant galaxy. This was the firstgalaxy, as opposed to quasar, that was found beyond redshift 4.[59] [60][51] [61]

4C 41.17 1990 − 1994 z=3.792 This is a radio galaxy. At the time of its discovery, quasar PC 1158+4635,discovered in 1989, was the most remote object known, at z=4.73 In 1991,quasar PC 1247-3406, became the most remote object known, atz=4.897[51] [60] [61] [62] [63]

1 Jy 0902+343 (GB6B0902+3419 , B20902+34)

1988 − 1990 z=3.395 This is a radio galaxy. At the time of discovery, quasar Q0051-279 atz=4.43, discovered in 1987, was the most remote object known. In 1989,quasar PC 1158+4635 was discovered at z=4.73, making it the most remoteobject known. This was the first galaxy discovered above redshift 3. It wasalso the first galaxy found above redshift 2.[51] [63] [64] [65] [66]

3C 256 1984 − 1988 z=1.819 This is a radio galaxy. At the time, the most remote object was quasar PKS2000-330, at z=3.78, found in 1982.[51] [67]

3C 241 1984 z=1.617 This is a radio galaxy. At the time, the most remote object was quasar PKS2000-330, at z=3.78, found in 1982.[68] [69]

3C 324 1983 − 1984 z=1.206 This is a radio galaxy. At the time, the most remote object was quasar PKS2000-330, at z=3.78, found in 1982.[51] [68] [70]

3C 65 1982 − 1983 z=1.176 This is a radio galaxy. At the time, the most remote object was quasarOQ172, at z=3.53, found in 1974. In 1982, quasar PKS 2000-330 at z=3.78became the most remote object.

3C 368 1982 z=1.132 This is a radio galaxy. At the time, the most remote object was quasarOQ172, at z=3.53, found in 1974.[51]

3C 252 1981 − 1982 z=1.105 This is a radio galaxy. At the time, the most remote object was quasarOQ172, at z=3.53, found in 1974.

3C 6.1 1979 - z=0.840 This is a radio galaxy. At the time, the most remote object was quasarOQ172, at z=3.53, found in 1974.[51] [71]

3C 318 1976 - 0.752 This is a radio galaxy. At the time, the most remote object was quasarOQ172, at z=3.53, found in 1974.[51]

3C 411 1975 - 0.469 This is a radio galaxy. At the time, the most remote object was quasarOQ172, at z=3.53, found in 1974.[51]

From 1964 to 1997, the title of most distant object in the universe were held by a succession of quasars.[58] That list is available at list of quasars.

3C 295 1960 - z=0.461 This is a radio galaxy. This was the remotest object known at time ofdiscovery of its redshift. This was the last non-quasar to hold the title ofmost distant object known until 1997. In 1964, quasar 3C 147 became themost distant object in the universe known.[51] [58] [72] [73] [74]

LEDA 25177(MCG+01-23-008)

1951 − 1960 z=0.2(V=61000km/s)

This galaxy lies in the Hydra Supercluster. It is located at B1950.008h 55m 4s +03° 21′ and is the BCG of the fainter Hydra Cluster Cl0855+0321 (ACO 732).[51] [74] [75] [76] [77] [68] [78]

LEDA 51975(MCG+05-34-069)

1936 - z=0.13(V=39000km/s)

The brightest cluster galaxy of the Bootes cluster (ACO 1930), an ellipticalgalaxy at B1950.0 14h 30m 6s +31° 46′ apparent magnitude 17.8, was foundby Milton L. Humason in 1936 to have a 40,000 km/s recessional redshiftvelocity.[68] [79] [80]

List of galaxies 157

LEDA 20221(MCG+06-16-021)

1932 - z=0.075(V=23000km/s)

This is the BCG of the Gemini Cluster (ACO 568) and was located atB1950.0 07h 05m 0s +35° 04′[79] [81]

BCG of WMH Christie'sLeo Cluster

1931 − 1932 z=(V=19700km/s)

[81] [82] [83] [84]

BCG of Baede's UrsaMajor Cluster

1930 − 1931 z=(V=11700km/s)

[84] [85]

NGC 4860 1929 − 1930 z=0.026(V=7800km/s)

[86] [87]

NGC 7619 1929 z=0.012(V=3779km/s)

Using redshift measurements, NGC 7619 was the highest at the time ofmeasurement. At the time of announcement, it was not yet accepted as ageneral guide to distance, however, later in the year, Edwin Hubbledescribed redshift in relation to distance, leading to a seachange, and havingthis being accepted as an inferred distance.[86] [88] [89]

NGC 584 (Dreyer nebula584)

1921 − 1929 z=0.006(V=1800km/s)

At the time, nebula had yet to be accepted as independent galaxies.However, in 1923, galaxies were generally recognized as external to theMilky Way.[68] [86] [88] [90] [91] [92] [52]

M104 (NGC 4594) 1913 − 1921 z=0.004(V=1180km/s)

This was the second galaxy whose redshift was determined; the first beingAndromeda - which is approaching us and thus cannot have its redshift usedto infer distance. Both were measured by Vesto Melvin Slipher. At thistime, nebula had yet to be accepted as independent galaxies. NGC 4594 wasoriginally measured as 1000 km/s, then refined to 1100, and then to 1180 in1916.[86] [90] [52]

M81 antiquity - 20thcentury

antiquity -1913 (basedon redshift)

antiquity -1930 (basedon Cepheids)

11.8 Mly(z=-0.10)

This is the lower bound, as it is remotest galaxy observable with thenaked-eye. It is 12 million light-years away. Redshift cannot be used toinfer distance, because it's moving toward us faster than cosmologicalexpansion.

Messier 101 1930 - Using the pre-1950's Cepheid measurements, M101 was one of the mostdistant so measured.

Triangulum Galaxy 1924 - 1930 In 1924, Edwin Hubble announced the distance to M33 Triangulum.

Andromeda Galaxy 1923 - 1924 In 1923, Edwin Hubble measured the distance to Andromeda, and settledthe question whether there were galaxies, or was everything in the MilkyWay.

Small Magellanic Cloud 1913 - 1923 This was the first intergalactic distance measured. In 1913, EjnarHertzsprung measures the distance to SMC using Cepheid variables.

• z represents redshift, a measure of recessional velocity and inferred distance due to cosmological expansion• quasars and other AGN are not included on this list, since they are only galactic cores, unless the host galaxy was observed when it was most

distant

[51]

• A1689-zD1, discovered in 2008, with z=7.6, does not appear on this list because it has not been confirmed with aspectroscopic redshift.

• Abell 68 c1 and Abell 2219 c1, discovered in 2007, with z=9, do not appear on this list because they have notbeen confirmed.[93]

• IOK4 and IOK5, discovered in 2007, with z=7, do not appear on this list because they have not been confirmedwith a spectroscopic redshift.

List of galaxies 158

• Abell 1835 IR1916, discovered in 2004, with z=10.0, does not appear on this list because its claimed redshift isdisputed. Some follow-up observations have failed to find the object at all.

• STIS 123627+621755, discovered in 1999, with z=6.68, does not appear on this list because its redshift was basedon an erroneous interpretation of an oxygen emission line as a hydrogen emission line.[94] [95] [96]

• BR1202-0725 LAE, discovered in 1998 at z=5.64 does not appear on the list because it was not definitivelypinned. BR1202-0725 (QSO 1202-07) refers to a quasar that the Lyman alpha emitting galaxy is near. The quasaritself lies at z=4.6947[52] [55]

• BR2237-0607 LA1 and BR2237-0607 LA2 were found at z=4.55 while investigating around the quasarBR2237-0607 in 1996. Neither of these appear on the list because they were not definitively pinned down at thetime. The quasar itself lies at z=4.558[97] [98]

• Two absorption dropouts in the spectrum of quasar BR 1202-07 (QSO 1202-0725, BRI 1202-0725, BRI1202-07)were found, one in early 1996, another later in 1996. Neither of these appear on the list because they were notdefinitively pinned down at the time. The early one was at z=4.38, the later one at z=4.687, the quasar itself lies atz=4.695[51] [99] [100] [101] [102]

• In 1986, a gravitationally lensed galaxy forming a blue arc was found lensed by galaxy cluster CL 2224-02(C12224 in some references). However, its redshift was only determined in 1991, at z=2.237, by which time, itwould no longer be the most distant galaxy.[103] [104]

• An absorption drop was discovered in 1985 in the light spectrum of quasar PKS 1614+051 at z=3.21 This doesnot appear on the list because it was not definitively fixed down. At the time, it was claimed to be the firstnon-QSO galaxy found beyond redshift 3. The quasar itself is at z=3.197[51] [105]

• In 1975, 3C 123 was incorrectly determined to lie at z=0.637 (actually z=0.218)[106] [107]

• From 1964 to 1997, the title of most distant object in the universe were held by a succession of quasars.[58] Thatlist is available at list of quasars.

• In 1958, cluster Cl 0024+1654 and Cl 1447+2619 were estimated to have redshifts of z=0.29 and z=0.35respectively. However, no galaxy was spectroscopically determined.[74]

Field galaxies

List of field galaxies

Galaxy Data Notes

NGC 4555

Interacting galaxies

List of galaxies 159

List of galaxies in tidal interaction

Galaxies Data

Notes

• Milky Way Galaxy• Large Magellanic Cloud• Small Magellanic Cloud

The Magellanic Clouds are being tidally disrupted by the Milky Way Galaxy, resulting in the MagellanicStream drawing a tidal tail away from the LMC and SMC, and the Magellanic Bridge drawing material fromthe clouds to our galaxy.

• Messier 51 (Arp 85)

• Whirlpool Galaxy(NGC 5194, M51a)

• NGC 5195 (M51b)

The smaller galaxy NGC 5195 is tidally interacting with the larger Whirlpool Galaxy, creating its granddesign spiral galaxy architecture.

• M81• M82• NGC 3077

These three galaxies interact with each other and draw out tidal tails, which are dense enough to form starclusters. The bridge of gas between these galaxies is known as Arp's Loop.[108]

• NGC 6872 and IC 4970

• NGC 6872• IC 4970

NGC 6872 is a barred spiral galaxy with a grand design spiral nucleus, and distinct well-formed outerbarred-spiral architecture, caused by tidal interaction with satellite galaxy IC 4970.

Tadpole Galaxy The Tadpole Galaxy tidally interacted with another galaxy in a close encounter, and remains slightlydisrupted, with a long tidal tail.

List of galaxies in non-merger significant collision

Galaxies Data Notes

Arp 299 (NGC 3690 & IC 694) These two galaxies have recently collided and are now both barred irregular galaxies.

List of galaxies disrupted post significant non-merger collisions

Galaxies Data Notes

Mayall's Object This is a pair of galaxies, one which punched through the other, resulting in a ring galaxy.

Galaxy mergers

List of galaxies undergoing near-equal merger

Galaxies Data Notes

Antennae Galaxies (Ringtail Galaxy,NGC 4038 & NGC 4039, Arp 244)

2galaxies

Two spiral galaxies currently starting a collision, tidally interacting, and in the process ofmerger.

Butterfly Galaxies (Siamese TwinsGalaxies, NGC 4567 & NGC 4568)

2galaxies

Two spiral galaxies in the process of starting to merge.

Mice Galaxies (NGC 4676, NGC4676A & NGC 4676B, IC 819 & IC820, Arp 242)

2galaxies

Two spiral galaxies currently tidally interacting and in the process of merger.

NGC 520 2galaxies

Two spiral galaxies undergoing collision, in the process of merger.

NGC 2207 and IC 2163 (NGC 2207 &IC 2163)

2galaxies

These are two spiral galaxies starting to collide, in the process of merger.

List of galaxies 160

NGC 5090 and NGC 5091 (NGC 5090& NGC 5091)

2galaxies

These two galaxies are in the process of colliding and merging.

NGC 7318 (Arp 319, NGC 7318A &NGC 7318B)

2galaxies

These are two starting to collide

Four galaxies in CL0958+4702 4galaxies

These four near-equals at the core of galaxy cluster CL 0958+4702 are in the process ofmerging.[109]

Galaxy protocluster LBG-2377 z=3.03 This was announced as the most distant galaxy merger ever discovered. It is expected that thisproto-cluster of galaxies will merge together to form a brightest cluster galaxy, and become thecore of a larger galaxy cluster.[110] [111]

List of recently merged galaxies of near-equals

Galaxy Data Notes

Starfish Galaxy (NGC 6240, IC 4625) This recently coalesced galaxy still has two prominent nuclei.

List of galaxies undergoing disintegration by cannibalization

Disintegrating Galaxy Consuming Galaxy Notes

Canis Major Dwarf Galaxy Milky Way Galaxy The Monoceros Ring is thought to be the tidal tail of the disrupted CMa dg.

Virgo Stellar Stream Milky Way Galaxy This is thought to be a completely disrupted dwarf galaxy.

Sagittarius Dwarf Elliptical Galaxy Milky Way Galaxy M54 is thought to the be core of this dwarf galaxy.

List of objects considered destroyed galaxies

DefunctGalaxy

Galaxy Notes

OmegaCentauri

Milky WayGalaxy

This is now categorized a globular cluster of the Milky Way. However, it is considered the core of a dwarfgalaxy that the Milky Way cannibalized.[14]

Mayall II AndromedaGalaxy

This is now categorized a globular cluster of Andromeda. However, it is considered the core of a dwarf galaxythat Andromeda cannibalized.

List of objects mistakenly identified as galaxies

"Galaxy" Object Data Notes

G350.1-0.3 Supernova remnant Due to its unusual shape, it was originally misidentified as a galaxy.

Lists of galaxies• Local Group• List of nearest galaxies• List of polar-ring galaxies• List of spiral galaxies• List of quasars

List of galaxies 161

References[1] Hayden Planetarium, Local Large-Scale Structure (http:/ / haydenplanetarium. org/ universe/ duguide/ exgt_local_structure. php)[2] Sky and Telescope, New Stars in a Galaxy's Wake (http:/ / www. skyandtelescope. com/ community/ skyblog/ newsblog/ 10003481. html), 28

September 2007[3] NASA, 'Orphan' Stars Found in Long Galaxy Tail (http:/ / www. nasa. gov/ mission_pages/ chandra/ news/ 07-103. html), 09.20.07[4] arXiv, H-alpha tail, intracluster HII regions and star-formation: ESO137-001 in Abell 3627 (http:/ / xxx. lanl. gov/ abs/ 0706. 1220), Fri, 8

June 2007 17:50:48 GMT[5] Universe Today, Galaxy Leaves New Stars Behind in its Death Plunge (http:/ / www. universetoday. com/ 2007/ 09/ 20/

galaxy-leaves-news-stars-behind-in-its-death-plunge/ ) ; September 20th, 2007[6] Publications of the Astronomical Society of the Pacific, Vol. 53, No. 313, p.187 ; The Radial Velocity of a Peculiar Nebula (http:/ / articles.

adsabs. harvard. edu/ full/ 1941PASP. . . 53. . 187S) ; 1941PASP...53..187S[7] Astrophysical Journal, vol. 140, p.1617 ; The Strange Extragalactic Systems: Mayall's Object and IC 883 (http:/ / articles. adsabs. harvard.

edu/ full/ 1964ApJ. . . 140. 1617B) ; 1964ApJ...140.1617B[8] Astrophysical Journal, vol. 119, p.215 ; On the Indentification of Radio Sources (http:/ / adsabs. harvard. edu/ abs/ 1954ApJ. . . 119. . 215B) ;

01/1954 ; 1954ApJ...119..215B[9] Karen Masters (December 2003). "Curious About Astronomy: Can any galaxies be seen with the naked eye?" (http:/ / curious. astro. cornell.

edu/ question. php?number=590). Curious.astro.cornell.edu. . Retrieved 2008-11-01.[10] Astronomy Knowledge Base, Magellanic Cloud (http:/ / www. site. uottawa. ca:4321/ astronomy/ index. html#MagellanicCloud), UOttawa[11] SEDS, The Large Magellanic Cloud, LMC (http:/ / seds. org/ MESSIER/ xtra/ ngc/ lmc. html)[12] SEDS, The Small Magellanic Cloud, SMC (http:/ / www. seds. org/ messier/ xtra/ ngc/ smc. html)[13] SEDS, Messier 31 (http:/ / www. seds. org/ messier/ m/ m031. html)[14] UPI, Black hole found in Omega Centauri (http:/ / www. upi. com/ Science_News/ 2008/ 04/ 10/ Black_hole_found_in_Omega_Centauri/

UPI-67471207850855/ ) ,April 10, 2008 at 2:07 PM[15] http:/ / www. skyandtelescope. com/ resources/ darksky/ 3304011. html[16] http:/ / astronomy-mall. com/ Adventures. In. Deep. Space/ aintno. htm[17] "Farthest Naked Eye Object" (http:/ / www. uitti. net/ stephen/ astro/ essays/ farthest_naked_eye_object. shtml). Uitti.net. . Retrieved

2008-11-01.[18] SEDS, Messier 81 (http:/ / www. seds. org/ messier/ m/ m081. html)[19] S. J. O'Meara (1998). The Messier Objects. Cambridge: Cambridge University. ISBN 0-521-55332-6.[20] http:/ / messier. obspm. fr/ xtra/ supp/ m81naked. txt[21] http:/ / www. springerlink. com/ content/ q1696565458u3286/[22] Astrophysical Journal, Centennial Issue, Vol. 525C, p. 569 ; Baade & Minkowski's Identification of Radio Sources (http:/ / adsabs. harvard.

edu/ full/ 1999ApJ. . . 525C. 569B) ; 1999ApJ...525C.569B[23] SEDS, Seyfert Galaxies (http:/ / www. seds. org/ ~spider/ spider/ ScholarX/ seyferts. html)[24] Astronomy and Astrophysics, v.357, p.L45-L48 (2000) III Zw 2, the first superluminal jet in a Seyfert galaxy (http:/ / adsabs. harvard. edu/

abs/ 2000A& A. . . 357L. . 45B) ; 2000A&A...357L..45B[25] SEDS, Lord Rosse's drawings of M51, his "Question Mark" "Spiral Nebula" (http:/ / seds. lpl. arizona. edu/ messier/ more/ m051_rosse.

html)[26] Lehnert, M.D.; Nesvadba, N.P.H. (October 2010). ""Spectroscopic confirmation of a galaxy at redshift z = 8.6"". Nature (467): 940-942.

doi:10.1038/nature09462. Bibcode: 2010arXiv1010.4312L. arXiv:1010.4312v1[27] Sub-parsec-scale structure and evolution in Centaurus A Introduction (http:/ / dsnra. jpl. nasa. gov/ research/ theses/ Tingay/ node41. html) ;

Tue November 26 15:27:29 PST 1996[28] The 2006 Giant Flare in PKS 2155-304 and Unidentified TeV Sources (http:/ / www. gravity. psu. edu/ events/ tev_workshop/ talks/

justin_fink_tevunid_flare. pdf)[29] Julie McEnery. "Time Variability of the TeV Gamma-Ray Emission from Markarian 421" (http:/ / www. iac. es/ blazars/ mcenery. html).

Iac.es. . Retrieved 2008-11-01.[30] bNet, Ablaze from afar: astronomers may have identified the most distant "blazar" yet (http:/ / findarticles. com/ p/ articles/ mi_m1134/

is_7_113/ ai_n6249016), Sept, 2004[31] arXiv, Q0906+6930: The Highest-Redshift Blazar (http:/ / aps. arxiv. org/ abs/ astro-ph/ 0406252), 9 June 2004[32] Monthly Notices of the Royal Astronomical Society, Volume 384, Issue 3, pp. 875-885 ; Optical spectroscopy of Arp220: the star formation

history of the closest ULIRG (http:/ / adsabs. harvard. edu/ abs/ 2008MNRAS. 384. . 875R) ; 03/2008 ; 2008MNRAS.384..875R[33] Chandra Proposal ID #01700041 ; ACIS Imaging of the Starburst Galaxy M82 (http:/ / adsabs. harvard. edu/ abs/ 1999cxo. . prop. . 362M) ;

09/1999 ; 1999cxo..prop..362M[34] Starburst Galaxies: Proceedings of a Workshop (page 27) (http:/ / books. google. com/ books?id=uy_RqxIJoJMC& pg=PA27& lpg=PA27&

dq=closest+ starburst+ galaxy& source=web& ots=P2cI6c6Lxo& sig=iv0Oav85cc1vcZx8_TtDhLJeYdk& hl=en& sa=X& oi=book_result&resnum=27& ct=result#PPA27,M1) ; 2001 ; ISBN 354041472X

[35] New Scientist, "Smallest galaxy hints at hidden population" (http:/ / www. newscientist. com/ article/dn11980-smallest-galaxy-hints-at-hidden-population. html), David Shiga, 4 June 2007 (accessed 9 September 2010)

List of galaxies 162

[36] The Independent, Science & technology: WHAT ON EARTH? (http:/ / findarticles. com/ p/ articles/ mi_qn4158/ is_20040218/ai_n12772100), 28 February 2004

[37] ESO Press Release 25/00 , Most Massive Spiral Galaxy Known in the Universe (http:/ / www. eso. org/ public/ outreach/ press-rel/ pr-2000/pr-25-00. html) , 8 December 2000

[38] arXiv, Star Clusters in Local Group Galaxies (http:/ / arxiv. org/ abs/ astro-ph/ 9912529v2), 28 December 1999[39] arXiv, Spectroscopic Confirmation Of An Extreme Starburst At Redshift 4.547 (http:/ / fr. arxiv. org/ abs/ 0806. 0657), Tue, 3 June 2008

22:59:35 GMT; doi:10.1086/590555 Bibcode: 2008ApJ...681L..53C[40] Nature 443, 186-188 (14 September 2006), A galaxy at a redshift z = 6.96 (http:/ / www. nature. com/ nature/ journal/ v443/ n7108/ abs/

nature05104. html), doi:10.1038/nature05104;[41] arXiv, Star Forming Galaxies at z > 5 (http:/ / arxiv. org/ abs/ 0804. 0644v1) , Fri, 4 April 2008[42] PASJ: Publ. Astron. Soc. Japan 57, 165-182, February 25, 2005; The SUBARU Deep Field Project: Lymanα Emitters at a Redshift of 6.6

(http:/ / pasj. asj. or. jp/ v57/ n1/ 570114/ 570114-frame. html)[43] BBC News, Most distant galaxy detected (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 2884411. stm), Tuesday, 25 March, 2003, 14:28

GMT[44] SpaceRef, Subaru Telescope Detects the Most Distant Galaxy Yet and Expects Many More (http:/ / www. spaceref. com/ news/ viewpr.

html?pid=11046), Monday, March 24, 2003[45] arXiv, The Discovery of Two Lyman$\alpha$ Emitters Beyond Redshift 6 in the Subaru Deep Field (http:/ / arxiv. org/ abs/ astro-ph/

?0301096), 28 February 2003[46] New Scientist, New record for Universe's most distant object (http:/ / www. newscientist. com/ article/

dn2046-new-record-for-universes-most-distant-object. html), 17:19 14 March 2002[47] BBC News, Far away stars light early cosmos (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 1871043. stm), Thursday, 14 March, 2002,

11:38 GMT[48] The Astrophysical Journal Letters, 568:L75–L79, April 1, 2002 ; A Redshift z = 6.56 Galaxy behind the Cluster Abell 370 (http:/ / www.

journals. uchicago. edu/ doi/ abs/ 10. 1086/ 340424) ; DOI: 10.1086/340424[49] K2.1 HCM 6A — Discovery of a redshift z = 6.56 galaxy lying behind the cluster Abell 370 (http:/ / hera. ph1. uni-koeln. de/ ~heintzma/ U/

Lens. htm)[50] The Astrophysical Journal Letters, 522:L9–L12, September 1, 1999, An Extremely Luminous Galaxy at z = 5.74 (http:/ / www. journals.

uchicago. edu/ doi/ full/ 10. 1086/ 312205)[51] Publications of the Astronomical Society of the Pacific, 111: 1475-1502, 1999 December; SEARCH TECHNIQUES FOR DISTANT

GALAXIES; INTRODUCTION (http:/ / nedwww. ipac. caltech. edu/ level5/ Sept04/ Stern/ Stern1. html)[52] New York Times, NEBULA DREYER BREAKS ALL SKY SPEED RECORDS; Portion of the Constellation of Cetus Is Rushing Along at

Rate of 1,240 Miles a Second. (http:/ / query. nytimes. com/ gst/ abstract. html?res=9F06E1DB153CE533A2575BC1A9679C946095D6CF) ;January 18, 1921, Tuesday

[53] Astronomy Picture of the Day, A Baby Galaxy (http:/ / apod. nasa. gov/ apod/ ap980324. html), March 24, 1998[54] arXiv, A Galaxy at z = 5.34 (http:/ / arxiv. org/ pdf/ astro-ph/ 9803137)PDF (209 KB), 11 March 1998[55] A New Most Distant Object: z = 5.34 (http:/ / www. astro. ucla. edu/ ~wright/ old_new_cosmo. html#12Mar98)[56] Astrophysical Journal Letters v.486, p.L75 ; 09/1997, A Pair of Lensed Galaxies at z=4.92 in the Field of CL 1358+62 (http:/ / adsabs.

harvard. edu/ abs/ 1997ApJ. . . 486L. . 75F) ; 1997ApJ...486L..75F ; 10.1086/310844[57] Astronomy Picture of the Day, Behind CL1358+62: A New Farthest Object (http:/ / apod. nasa. gov/ apod/ ap970731. html), July 31, 1997[58] "Astrophysics and Space Science" 1999, 269/270, 165-181 ; GALAXIES AT HIGH REDSHIFT - 8. Z > 5 GALAXIES (http:/ / nedwww.

ipac. caltech. edu/ level5/ Illingworth/ Ill8. html) ; Garth Illingworth[59] arXiv, Ultra-Steep Spectrum Radio Galaxies at Hy Redshifts (http:/ / arxiv. org/ abs/ astro-ph/ 9910311), 18 October 1999[60] The Astrosynthetic Journal, 999:L1-L4, February 31, 1994 ; KECK OBSERVATIONS OF THE MOST DISTANT GALAXY: 8C1435+63

AT z=4.25 (http:/ / arxiv. org/ pdf/ astro-ph/ 9411007)PDF (181 KB)[61] New Scientist, Galaxy hunters close to the edge (http:/ / www. newscientist. com/ article/ mg14419502.

500-galaxy-hunters-close-to-the-edge. html), 5 November 1994[62] Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 401, no. 2, p. L69-L73 ; Hubble Space Telescope imaging of distant galaxies

- 4C 41.17 at Z = 3.8 (http:/ / adsabs. harvard. edu/ full/ 1992ApJ. . . 401L. . 69M) ; 1992ApJ...401L..69M[63] Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 363, November 1, 1990, p. 21-39 ; 4C 41.17 - A radio galaxy at a redshift of 3.8

(http:/ / adsabs. harvard. edu/ full/ 1990ApJ. . . 363. . . 21C) ; 1990ApJ...363...21C[64] Science News, Farthest galaxy is cosmic question - 0902+34 (http:/ / findarticles. com/ p/ articles/ mi_m1200/ is_2002_June_17/

ai_6672257) April 23, 1988[65] Science News, Two distant galaxies provide new puzzles - 4c 41.17, B2 09021+34 (http:/ / findarticles. com/ p/ articles/ mi_m1200/

is_n20_v142/ ai_12917867), November 14, 1992[66] arXiv, DUST IN HIGH REDSHIFT RADIO GALAXIES ANDTHE EARLY EVOLUTION OF SPHEROIDAL GALAXIES (http:/ / arxiv.

org/ pdf/ astro-ph/ 9509108)PDF (119 KB), 21 September 1995[67] Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X), vol. 324, January 1, 1988, p. L1-L3. Peculiar morphology of the

high-redshift radio galaxies 3C 13 and 3C 256 in subarcsecond seeing (http:/ / articles. adsabs. harvard. edu/ / full/ 1988ApJ. . . 324L. . . 1L/L000001. 000. html) ; 1988ApJ...324L...1L

List of galaxies 163

[68] Royal Astronomical Society, Monthly Notices (ISSN 0035-8711), vol. 211, December 15, 1984, p. 833-855 ; Stellar populations in distantradio galaxies (http:/ / adsabs. harvard. edu/ full/ 1984MNRAS. 211. . 833L) ; 1984MNRAS.211..833L

[69] Journal of the British Astronomical Association, vol.94, no.3, p.97-103 ; The Most Distant Galaxies (http:/ / adsabs. harvard. edu/ full/1984JBAA. . . 94. . . 97L) ; 1984JBAA...94...97L

[70] Sky and Telescope V. 65, P. 321, 1983 ; 3C324 - Most Distant Galaxy (http:/ / adsabs. harvard. edu/ abs/ 1983S& T. . . . 65. . 321S) ;1983S&T....65..321S

[71] Astrophysical Journal, Part 1, vol. 231, July 15, 1979, p. 307-311 ; Spectrophotometry of three high-redshift radio galaxies - 3C 6.1, 3C 265,and 3C 352 (http:/ / adsabs. harvard. edu/ cgi-bin/ bib_query?1979ApJ. . . 231. . 307S) ; 07/1979 ; 1979ApJ...231..307S ; doi 10.1086/157194

[72] The Discovery of Radio Galaxies and Quasars (http:/ / www. astro. caltech. edu/ ~george/ ay21/ qso. txt)[73] Annual Review of Astronomy and Astrophysics Vol. 31: 639-688 (September 1993) ; High Redshift Radio Galaxies (http:/ / arjournals.

annualreviews. org/ doi/ abs/ 10. 1146/ annurev. aa. 31. 090193. 003231) ; (doi:10.1146/annurev.aa.31.090193.003231)[74] Astrophysical Journal, vol. 133, p.355 ; The Ability of the 200-INCH Telescope to Discriminate Between Selected World Models (http:/ /

articles. adsabs. harvard. edu/ full/ 1961ApJ. . . 133. . 355S) ; 1961ApJ...133..355S[75] Monthly Notices of the Royal Astronomical Society, Vol. 113, p.658 ; The law of red shifts (George Darwin Lecture) Hubble, E. P. (http:/ /

articles. adsabs. harvard. edu/ full/ 1953MNRAS. 113. . 658H) ; 1953MNRAS.113..658H[76] OBSERVATIONAL TESTS OF WORLD MODELS; 6.1. Local Tests for Linearity of the Redshift-Distance Relation (http:/ / nedwww.

ipac. caltech. edu/ level5/ Sept01/ Sandage/ Sand6. html) ; Annu. Rev. Astron. Astrophys. 1988. 26: 561-630[77] Astron. J., 61, 97-162 (1956) ; Redshifts and magnitudes of extragalactic nebulae (http:/ / articles. adsabs. harvard. edu/ full/ 1956AJ. . . . .

61. . . 97H) ; 1956AJ.....61...97H[78] Astronomical Society of the Pacific Leaflets, Vol. 7, p.393 ; From Atoms to Galaxies (http:/ / articles. adsabs. harvard. edu/ full/ 1958ASPL.

. . . 7. . 393M) ; 1958ASPL....7..393M[79] Astrophysical Journal, vol. 83, p.10 ; The Apparent Radial Velocities of 100 Extra-Galactic Nebulae (http:/ / adsabs. harvard. edu/ full/

1936ApJ. . . . 83. . . 10H) ; 1936ApJ....83...10H[80] THE FIRST 50 YEARS AT PALOMAR: 1949-1999 ; The Early Years of Stellar Evolution, Cosmology, and High-Energy Astrophysics;

5.2.1. The Mount Wilson Years (http:/ / nedwww. ipac. caltech. edu/ level5/ Sept03/ Sandage/ Sandage5_2. html) ; Annu. Rev. Astron.Astrophys. 1999. 37: 445-486

[81] Journal of the Royal Astronomical Society of Canada, Vol. 26, p.180 ; Notes and Queries (Doings at Mount Wilson-Ritchey's PhotographicTelescope-Infra-red Photographic Plates) (http:/ / articles. adsabs. harvard. edu/ full/ 1932JRASC. . 26. . 180C) ; 1932JRASC..26..180C

[82] Astrophysical Journal, vol. 74, p.35 ; Apparent Velocity-Shifts in the Spectra of Faint Nebulae (http:/ / adsabs. harvard. edu/ cgi-bin/bib_query?1931ApJ. . . . 74. . . 35H) ; 07/1931 ; 1931ApJ....74...35H

[83] Astrophysical Journal, vol. 74, p.43 ; The Velocity-Distance Relation among Extra-Galactic Nebulae (http:/ / articles. adsabs. harvard. edu/full/ 1931ApJ. . . . 74. . . 43H) ; 1931ApJ....74...43H

[84] Astronomical Society of the Pacific Leaflets, Vol. 1, p.149 ; The Large Apparent Velocities of Extra-Galactic Nebulae (http:/ / adsabs.harvard. edu/ full/ 1931ASPL. . . . 1. . 149H) ; 1931ASPL....1..149H

[85] Astrophys. J., 71, 351-356 (1930) The Rayton short-focus spectrographic objective. (http:/ / adsabs. harvard. edu/ full/ 1930ApJ. . . . 71. .351H) 1930ApJ....71..351H

[86] Publications of the Astronomical Society of the Pacific, v.108, p.1073-1082 ; H_0: The Incredible Shrinking Constant, 1925-1975 (http:/ /articles. adsabs. harvard. edu/ full/ seri/ PASP. / 0108/ 0001073. 000. html) ; 1996PASP..108.1073T

[87] Publications of the Astronomical Society of the Pacific, Vol. 41, No. 242, p.244 ; The Berkeley Meeting of the Astronomical Society of thePacific, June 20-21, 1929 (http:/ / adsabs. harvard. edu/ full/ 1929PASP. . . 41. . 244. ) ; 1929PASP...41..244

[88] From the Proceedings of the National Academy of Sciences; Volume 15 : March 15, 1929 : Number 3 ; THE LARGE RADIAL VELOCITYOF N. G. C. 7619 (http:/ / antwrp. gsfc. nasa. gov/ diamond_jubilee/ d_1996/ hum_1929. html) ; January 17, 1929

[89] THE JOURNAL OF THE ROYAL ASTRONOMICAL SOCIETY OF CANADA / JOURNAL DE LA SOCIÉTÉ ROYALED'ASTRONOMIE DU CANADA; Vol. 83, No.6 December 1989 Whole No. 621 ; EDWIN HUBBLE 1889-1953 (http:/ / antwrp. gsfc. nasa.gov/ diamond_jubilee/ d_1996/ sandage_hubble. html)

[90] National Academy of Sciences; Biographical Memoirs: V. 52 (http:/ / books. google. com/ books?id=h9xnzIV_zQYC) - VESTO MELVINSLIPHER; ISBN 0309030994

[91] Harvard College Observatory Bulletin No. 739, pp.1-1 ; Nebula with Highest Receding Velocity (http:/ / articles. adsabs. harvard. edu/ full/1920BHarO. 739. . . . 1B) ; 1920BHarO.739....1B

[92] New York Times, DREYER NEBULA NO. 584 INCONCEIVABLY DISTANT; Dr. Slipher Says the Celestial Speed Champion Is 'ManyMillions of Light Years' Away. (http:/ / query. nytimes. com/ gst/ abstract. html?res=9906E2DA153CE533A2575AC1A9679C946095D6CF) ;January 19, 1921, Wednesday

[93] New Scientist, Baby galaxies sighted at dawn of universe (http:/ / space. newscientist. com/ article/ dn12233), 22:34 10 July 2007[94] Lawrence Livermore National Laboratory, Lab scientists revoke status of space object (https:/ / www. llnl. gov/ str/ March01/ NewsMar01.

html)[95] arXiv, The Unusual Spectral Energy Distribution of a Galaxy Previously Reported to be at Redshift 6.68 (http:/ / arxiv. org/ abs/ astro-ph/

0011558), 30 November 2000[96] BBC News, Hubble spies most distant object (http:/ / news. bbc. co. uk/ 2/ low/ science/ nature/ 319812. stm), Thursday, April 15, 1999[97] arXiv, Detection of Lyman-alpha Emitting Galaxies at Redshift z=4.55 (http:/ / arxiv. org/ abs/ astro-ph/ 9606135), 21 June 1996

List of galaxies 164

[98] 31/01/02 ; DAZLE NEAR IR NARROW BAND IMAGER (http:/ / www. aao. gov. au/ dazle/ science. pdf)PDF (570 KB) ;DAZLE-IoA-Doc-0002

[99] ESO Press Release 11/95, ESO Astronomers Detect a Galaxy at the Edge of the Universe (http:/ / www. eso. org/ public/ outreach/ press-rel/pr-1995/ pr-11-95. html), 15 September 1995

[100] New Scientist, Trouble at the edge of time (http:/ / www. newscientist. com/ article/ mg14820002. 600-trouble-at-the-edge-of-time. html),21 October 1995

[101] Astronomy and Astrophysics, v.316, p.33-42, High resolution observations of the QSO BR 1202-0725: deuterium and ionic abundances atredshifts above z=4 (http:/ / adsabs. harvard. edu/ full/ 1996A& A. . . 316. . . 33W), 1996A&A...316...33W

[102] Astrophysical Journal Letters v.456, p.L13, A Redshift 4.38 MG II Absorber toward BR 1202-0725 (http:/ / adsabs. harvard. edu/ full/1996ApJ. . . 456L. . 13E), 1996ApJ...456L..13E

[103] R.A.S. MONTHLY NOTICES V.263, NO. 3/AUG1, P. 628, 1993 ; The Nature of Star Formation in Lensed Galaxies at High Redshift(http:/ / adsabs. harvard. edu/ full/ 1993MNRAS. 263. . 628S) ; 1993MNRAS.263..628S

[104] Gravitational Lenses II: Galaxy Clusters as Lenses (http:/ / www. astro. uni-bonn. de/ ~peter/ Poster2e. html)[105] Astronomical Journal (ISSN 0004-6256), vol. 93, June 1987, p. 1318-1325 ; A galaxy at a redshift of 3.215 - Further studies of the PKS

1614+051 system (http:/ / adsabs. harvard. edu/ cgi-bin/ nph-bib_query?bibcode=1987AJ. . . . . 93. 1318D& db_key=AST) ;1987AJ.....93.1318D

[106] NED, Searching NED for object "3C 123" (http:/ / nedwww. ipac. caltech. edu/ cgi-bin/ nph-objsearch?objname=3c123& extend=no&out_csys=Equatorial& out_equinox=J2000. 0& obj_sort=RA+ or+ Longitude& of=pre_text& zv_breaker=30000. 0& list_limit=5&img_stamp=YES)

[107] Astrophys. J., Lett., Vol. 199, p. L3 - L4 3C 123: a distant first-ranked cluster galaxy at z = 0.637 (http:/ / adsabs. harvard. edu/ full/1975ApJ. . . 199L. . . 3S) 1975ApJ...199L...3S

[108] Sky and Telescope, Stars in the Middle of Nowhere (http:/ / www. skyandtelescope. com/ community/ skyblog/ newsblog/ 13685257.html), 10 January 2008

[109] Sky and Telescope, Galaxy Monster Mash (http:/ / www. skyandtelescope. com/ community/ skyblog/ newsblog/ 9053516. html), 9 August2007

[110] ABC News, Found! Oldest galaxy pile-up (http:/ / www. abc. net. au/ science/ articles/ 2008/ 04/ 09/ 2211965. htm), Wednesday, 9 April2008

[111] The Astrophysical Journal Letters, 681:L57–L60, July 10, 2008 ; A Candidate Brightest Protocluster Galaxy at z = 3.03 (http:/ / www.journals. uchicago. edu/ doi/ abs/ 10. 1086/ 590406)

External links• Wolfram Research: Scientific Astronomer Documentations - Brightest Galaxies (http:/ / documents. wolfram.

com/ applications/ astronomer/ Atlas/ BrightestGalaxies. html)• 1956 Catalogue of Galaxy Redshifts: Redshifts and magnitudes of extragalactic nebulae (http:/ / articles. adsabs.

harvard. edu/ full/ 1956AJ. . . . . 61. . . 97H) by Milton L. Humason, Nicholas U. Mayall, Allan Sandage• 1936 Catalogue of Galaxy Redshifts: The Apparent Radial Velocities of 100 Extra-Galactic Nebulae (http:/ /

adsabs. harvard. edu/ full/ 1936ApJ. . . . 83. . . 10H) by Milton L. Humason• 1925 Catalogue of Galaxy Redshifts: [ ] by Vesto Slipher• (1917) First Catalogue of Galaxy Redshifts: Nebulae (http:/ / articles. adsabs. harvard. edu/ / full/ 1917PAPhS. .

56. . 403S/ 0000405. 000. html) by Vesto Slipher

See also• Galaxy• Milky Way Galaxy• Local Group• Galaxy groups and clusters

• List of galaxy clusters• Local Supercluster• Supercluster

• List of galaxy superclusters

Fossil group 165

Fossil groupFossil Galaxy Groups, fossil Groups, or fossil clusters are believed to be the end-result of galaxy merging within anormal galaxy group, leaving behind the X-ray halo. Galaxies within a group interact and merge. The physicalprocess behind this galaxy-galaxy merger is dynamical friction. The time-scales for dynamical friction on luminous(or L*) galaxies suggest that fossil groups are old, undisturbed systems that have seen little infall of L* galaxiessince their initial collapse. Fossil groups are thus an important laboratory for studying the formation and evolution ofgalaxies and the intragroup medium in an isolated system.

External links• Fossil galaxies 'eat neighbours' [1]

• Concentrated Dark Matter at the Cores of Fossil Galaxies [2]

• A fossil galaxy cluster [3]

See also• Brightest Cluster Galaxy• Type-cD galaxy

References[1] http:/ / news. bbc. co. uk/ 1/ hi/ sci/ tech/ 4423651. stm[2] http:/ / chandra. harvard. edu/ press/ 05_releases/ press_040705. html[3] http:/ / www. esa. int/ esaCP/ SEMCFFOFGLE_index_0. html

Article Sources and Contributors 166

Article Sources and ContributorsGalaxy  Source: http://en.wikipedia.org/w/index.php?oldid=395834950  Contributors: -- April, 05jdunn, 2D, A Train, ABF, ACSE, Abce2, Abhimat.gautam, Adamwang, AdjustShift,Adrianisgood, Afita, Ageekgal, Ahoerstemeier, Aitias, Aka042, Akhil.aggarwal2, Alansohn, Aldren kenji, AlexPlank, AlexiusHoratius, Alfio, Alienware9955, Alison, Allstarecho,Andattaca2010, Andres, AndrewWTaylor, Andrewrp, Andrij Kursetsky, Andy17061993, Animum, Anomalocaris, Antandrus, Aranherunar, Ardric47, Argo Navis, Arpingstone, Art LaPella,Artaxiad, Arthana, Aruton, Arvindn, Asdfdsafg, AshLin, Ashcraft, Ashton1983, AstroNomer, Astrotwitch, Athenean, AugPi, Auquacutie, Avenue, Avoided, Avprslayer, AxelBoldt, B4hand,Banes, Beland, Ben-Zin, Bencherlite, Bender235, Bentley4, Betacommand, Big Bird, BigMar992, Bilboon, Billypancho, Binary TSO, Birkett, Blah master man, Bletch, Bluerasberry, Blurpeace,Bobak, Bobo192, Bobsteel09, Bogey97, Bongwarrior, Boo2u89, BoomerAB, Borislav, Brainmuncher, Brighterorange, Bruce89, Brynpttrsn, Bsadowski1, CJLL Wright, Cactus.man,Calabraxthis, Caltas, CameronsAshley, Can't sleep, clown will eat me, CanOfWorms, CapitalR, Captmondo, CardinalDan, Carmichael95, Caseyisgay, Cbraga, Cburb13, Cedrus-Libani, Centrx,Ceoil, Chad Hennings, Chelseamarie322, Chiggen, Chinese baybay, Chinneeb, Chrislintott, Clarince63, Clementi, Clementina, Closedmouth, Cobaltbluetony, Cocomeco, Cocytus, ColinJohnston, Cometstyles, CommonsDelinker, Concorde4950, Conti, Conversion script, Cool3, Coolerguy101, Cosmo0, Courcelles, Crimson chin7, Crimsonvalor, Crispmuncher, Crum375, Crystalwhacker, Ctjf83, Curps, Cwilliamsdog, Cyde, D6, DIUZOMA, DMPalmer, DSRH, DVD R W, DVdm, DW40, Da monster under your bed, Dabomb87, Dadude3320, DamianFinol, DanMS,Daniel Bush, Danielratiu, DannyZ, DarkAudit, DavidLevinson, Dawn Bard, Dbmag9, DeadEyeArrow, Deathlie, Deor, DerHexer, Derek Ross, DetlevSchm, Diderot, DimaY2K, Dimosvki, Dims,Dlohcierekim, Dmcq, Docu, Dominicanpapi82, Dooflotchie, Download, Dputig07, Dr. Submillimeter, Drkencarter, Drtgjhjiddf, Dylan620, Dysepsion, E946, Edwinstearns, El C, Elassint, Ellywa,Elm-39, Epbr123, EricandHolli, Escape Orbit, Essexmutant, Euku, Euryalus, Everyguy, Evlekis, Excirial, FC190, Falcon8765, Farquharsons, Fasttiger100, Fbs. 13, Felix Dance, Fellwalker57,Flehmen, Fleung, Foober, Fox, FrancisTyers, Franz123, Frecklefoot, Fribbler, Friginator, Fumitol, Funnyfarmofdoom, Fyyer, Fæ, Gaff, Gail, Gareth Wyn, Garion96, Gdo01, Gene Nygaard, Gfes,GhostPirate, Gifðas, Ginosal, Glenn, Gogo Dodo, GoingBatty, Gonzonoir, Gordo1717, Graeme Bartlett, Grafen, Gran2, Greatorix, Green meklar, GregorB, Grim23, Gurch, Gz33, H2g2bob,Halfblue, Hanberke, Harald Khan, Harloshaply, Harry the Dirty Dog, Hattar393, Hbkrishnan, Hdt83, Headbomb, Henning Makholm, HenryLi, HereToHelp, Heroesrule17, Hiddenfromview,Hjb26, Hogghogg, Hojimachong, Holofect, Homie07, Horselover Frost, Hqb, Hu12, Husky2002, Hut 8.5, Hypocrite9901, Ialsofedthisup, Iantresman, Iazz, Icairns, Ideogram, Igatsios, Imran,Inductiveload, Infrogmation, Ioeth, Irbisgreif, Iridescent, Ispy1981, Ixfd64, Iyragaura, J.delanoy, JHMM13, JMK, JNW, Jack of ages, Jackol, Jacob Hand, Jagged 85, Jakethakid, JamesHoadley,Jan5899, Janejellyroll, Japeo, Jarry1250, Jeff3000, Jeffdunhamfan123, Jehochman, Jeremyb, Jesse0986, Jhsounds, Jim Birkenshaw, Jiy, Jll, Jmencisom, JoanneB, JodyB, Joe Jarvis, JohannWolfgang, John D. Croft, John Vandenberg, Johnchiu, Jojhutton, Jojit fb, Jordanhorn, Jose77, Joseph Dwayne, Jossi, Jovianeye, Jpk, Jsponge96, Junglecat, Jyril, Kaci8567, Kaffi, Karol Langner,Kashiera, Katalaveno, Katieh5584, Kbdank71, Keenan Pepper, Keenanmeboy, Keilana, Kesac, Kestasjk, Khan singh, King Bee, King of Hearts, Kingpin13, KirinX, Kitch, Kittins floating in thesky yay, Kiwipeel, Korg, Kosebamse, Kozuch, Krash, Kukini, Kurtroscillo, Kuru, Kvantti, L Kensington, Lars Lindberg Christensen, Latinquasar, Latitude0116, Lazulilasher, LeaveSleaves, LeeM, Lemchesvej, LeoNomis, Lfastrup, Liftarn, Lightmouse, Ligulem, Linnell, Little Mountain 5, Livajo, Lkatkinsmith, Lmiller777, Logical2u, LonelyMarble, Looxix, Lucinos, Luna Santin,Lyricmac, MER-C, MHD, MPerel, Macy, Madhero88, Maedin, Magickmonkey54, Magnus Manske, Mailer diablo, Malachi007, Mani1, Marasama, Marshall Stax, MartinezMD,Martinwilke1980, Marvinandmilo, Master of Pies, Materialscientist, Mateuszica, Math Champion, Mathew Carrier, Matthewhayes, Matticus78, Mav, Maxis ftw, Mbc362, Mbz1, Mean as custard,Melicans, Menchi, Metalhead94, Mhardcastle, Michael C Price, Midgrid, Mike Rosoft, Mike Storm, Mike s, Minesweeper, Mirv, MisfitToys, Mlpearc, MoRsE, Moey1, MojoTas, Monotonehell,Morenooso, Moxy, Mr. Lefty, Mr.Z-man, Mr.wang, MrJuancho03, Mschel, Murgh, Mygerardromance, Mysdaao, NHRHS2010, NLUT, Natl1, NatureA16, Naturespace, NawlinWiki,Necromancer44, Neko-chan, Neurophyre, Nick C, Nicksallama, Nikai, Nikolay94, Njaelkies Lea, Not me, Notjake13, NuclearWarfare, Nyh, Obradovic Goran, Ohms law, Oliphaunt, Olivier,Omicronpersei8, OnePt618, Palica, Panser Born, Patilsagar09, Pdcook, Pedia wiki, Peter Delmonte, Peter Isotalo, PeterisP, Petr Kopač, Phaedriel, Phantomsteve, Philip Trueman, PhySusie,Pie053, Pinkadelica, Piotrus, Placeneck, Plek, Polluxian, Possum, Prodego, Profoss, Protonk, Psyche825, Purnajitphukon, Pyxelator, Qlwinsor, Quaoar, Qxz, RJHall, RJaguar3, RL0919, RL579,Random astronomer, Ranveig, Raven4x4x, Recognizance, Reconsider the static, RepodudexDXDxD, Reverendgraham, RexNL, Rgbower, Rich Farmbrough, Richmond96, Ritchiemate,Rjwilmsi, Rmrfstar, RobertG, Robma, Robprain, Rock4arolla, Rocket71048576, Romanm, Rominandreu, Ronkilburn, Rory096, Rrburke, Rst20xx, Rubbrchikin, Rursus, Ryulong, Sade,Salamurai, Salva31, SamForestell, Satori, Sbandrews, Scapler, Sceptre, SchfiftyThree, Schneelocke, SchuminWeb, Scientizzle, Scohoust, Sean D Martin, Sean K, Seaphoto, Semperf, Sengkang,Serendipodous, Seth Ilys, Seth ze, Shanes, Shijualex, Sidonuke, Siroxo, Slayer094, Smallweed, SmartGuy, Smartie960, Snowolf, So9q, Sophia, Spacepotato, Spencer, Spiff, Spitfire,SpookyMulder, Spssbkp, Spud Gun, SqueakBox, Squids and Chips, Sry85, Ste4k, Stephen, Stephenb, Stifynsemons, Stuckinmyhead, Su37amelia, Supersonicstars, SwordSmurf, Sylent, Syrthiss,THEN WHO WAS PHONE?, TNTfan101, Tarheel95, Tariqabjotu, Tasudrty, Teapotgeorge, Tempodivalse, Terrorking101, Tetraedycal, TexasAndroid, Tfine80, The Epopt, The High Fin SpermWhale, The Land, The Thing That Should Not Be, The wub, ThePointblank, There are no names, Thierry Caro, Thingg, Thompson2266, Tide rolls, Tim Q. Wells, Timmytootoo, Timwi, Tiptoety,Tjbvista, Tmobileloverdeluxe, Tobby72, Tolone, Tommy2010, Tonyle, Took, Torchwoodwho, Touch Of Light, Traroth, Trevor MacInnis, Triona, Trippcook, Trueheartless, Tuckerson1,Tyche151, Tyler Oderkirk, UBeR, UberScienceNerd, Ulric1313, Uncle Dick, Uncle Dick2, Unschool, Useight, User27091, Vary, VasilievVV, Vasyatka1, Versus22, Vivio Testarossa,Vndragon4, Vsmith, W3rH3re, WJBscribe, Waachiperchow, Wackywace, Watch37264, Watcharakorn, Watercleanerperson, Wavelength, Wellsy1992, Wiki alf, WikiLaurent, Wikiborg,Wikipelli, WilliamKF, Willking1979, Wimt, Winchelsea, Wjfox2005, Wknight94, Wnt, Wolfgang1018, WolfmanSF, Woohookitty, Workofthedevil, Wwheaton, XJamRastafire, Xerxes314,Xiner, Yonatan, Zachareth, Zanaq, Zhou Yu, Zoz, Петър Петров, 1103 anonymous edits

Galaxy formation and evolution  Source: http://en.wikipedia.org/w/index.php?oldid=390626429  Contributors: -- April, 195.92.168.xxx, 209.2.165.xxx, 212.185.227.xxx, Abtract,Ahoerstemeier, Alastair Haines, Andycjp, Archanamiya, Arpingstone, AstroNomer, Astrotwitch, AxelBoldt, Azcolvin429, Bacteria, Boud, Bruin69, Bryan Derksen, CambridgeBayWeather,Celiviel, Cgingold, Charles Matthews, CharlotteWebb, Chris 73, ColinFrayn, Conversion script, Cosmo0, Crag, Dark jedi requiem, Doc Perel, Dr. Submillimeter, Eric Kvaalen, Evil Monkey,FT2, FlorianMarquardt, Gandalf61, Giftlite, GorgonzolaCheese, Grendelkhan, Gurch, GwydionM, HKL47, Headbomb, Hebb l, Hetar, Hubie59, Hurricane Floyd, IVAN3MAN, Iantresman,Icairns, Ilmari Karonen, Immunize, Jahter, Jitterro, John D. Croft, Jorichoma, Jyril, Karol Langner, Keflavich, Kot Barsik, Kris1284x, Lights, LikeHolyWater, Looxix, Luckypengu07, MJT1331,Megaton, Merovingian, Muad, Nikai, Noisy, Nuno Tavares, Oashi, Olivier, Palica, Paymanpayman, Pika ten10, Pringl123, QuadrivialMind, R6144, RJHall, RetiredUser2, Reuben, RichardNowell, Roadrunner, Robma, Rodasmith, Ruslik0, ScienceApologist, Scog, Sheliak, Shp0ng1e, SqueakBox, Stirling Newberry, Sverdrup, Template namespace initialisation script, Tetracube,Tothebarricades.tk, Trevor MacInnis, TutterMouse, UrukHaiLoR, Vanished User 0001, Viking59, Viriditas, Vsmith, Ward3001, Warut, Wsiegmund, Zigger, 98 anonymous edits

Galaxy merger  Source: http://en.wikipedia.org/w/index.php?oldid=363690617  Contributors: Cornellrockey, Eteq, Fcombes, IVAN3MAN, Incnis Mrsi, Lights, Richard Nowell, Robofish, Scog,Skullers, 5 anonymous edits

Galaxy morphological classification  Source: http://en.wikipedia.org/w/index.php?oldid=391148962  Contributors: Anton Gutsunaev, Argo Navis, Arthena, CRGreathouse, Carcharoth,Cosmo0, Curps, DIUZOMA, Dekimasu, Dr. Submillimeter, El C, Geremia, Harloshaply, Hcagri, Hurricane Devon, Icairns, Ivan T., JGrochow, KGyST, Karl D. Gordon, Karol Langner,KathrynLybarger, Kcordina, Keflavich, Leia, M1ss1ontomars2k4, MIT Trekkie, Marasama, Med, Neilc, Neko-chan, Newone, Northgrove, Octoberasian, Omodaka, Originalwana, Paul venter,Philip Trueman, Philip tao, Pie4all88, Polylepsis, Robma, Rotational, Rothorpe, Rparle, Sam Hocevar, ScienceApologist, Shenme, SimonP, Spiritia, Ste4k, The Mad Genius, WilliamKF,XJamRastafire, Zandperl, Zero sharp, Zoicon5, Zzzzzzzzzzz, 57 anonymous edits

Hubble sequence  Source: http://en.wikipedia.org/w/index.php?oldid=394847232  Contributors: -- April, Ahoerstemeier, Alfio, AstroNomer, AxelBoldt, CielProfond, Cosmo0, Curps, Emijrp,Fvw, Harloshaply, Hurricane Devon, JohnOwens, Josh Grosse, Jyril, Karl D. Gordon, Kcordina, Lightmouse, Looxix, Lumos3, Mav, Midway, Ojay123, Ojigiri, OlEnglish, Poor Yorick,ScienceApologist, Sciurinæ, SimonD, Ste4k, Sverdrup, Swamp Ig, Titanium Dragon, Tlusťa, WilliamKF, XJamRastafire, Xerxes314, Žiedas, 33 anonymous edits

Dark matter halo  Source: http://en.wikipedia.org/w/index.php?oldid=391631276  Contributors: Aeron Daly, Ahoerstemeier, Beno1000, Bm gub, Boud, Brews ohare, Cosmo0, Djxerox,Editfreak66, Gandalf61, Hamiltondaniel, Jeremygoodman, Jtsch, Marasama, Mhatthei, Micasta, Oldnoah, Onebravemonkey, RedBLACKandBURN, Reuben, Rjwilmsi, Robertvan1,ScienceApologist, Scikid, Shanes, WilliamKF, Zurich Astro, 26 anonymous edits

Galactic bulge  Source: http://en.wikipedia.org/w/index.php?oldid=391665993  Contributors: AEVanVogt, Alfio, Amikake3, Astrotwitch, Az29, Bryan Derksen, Cacycle, CommonsDelinker,Computor, Długosz, Edwinhubbel, Eequor, Eteq, EugeneForrester, Gothic2, GregorB, H2g2bob, Hairy Dude, Hcagri, Icairns, Iokseng, It's-is-not-a-genitive, JRGL, Lights, Marasama, Meco,Mnmngb, Mramz88, PMDrive1061, PurpleHz, RJHall, Redrocketboy, Rjwilmsi, Robina Fox, Rory096, Rpyle731, Serendipodous, Shantavira, Slakr, Sury1313, Susanlarsen, The Anome, 27anonymous edits

Galactic corona  Source: http://en.wikipedia.org/w/index.php?oldid=369062008  Contributors: A2Kafir, Boud, Cocytus, Cosmo0, Craigy144, Eroica, Jyril, LrdChaos, Marasama,Prsephone1674, Shanes, Tarnum, Wolfy, 5 anonymous edits

Galactic disc  Source: http://en.wikipedia.org/w/index.php?oldid=384199971  Contributors: Caco de vidro, Emersoni, Marasama, PigFlu Oink, Poppy, RHaworth, Rich Farmbrough, RobertG,Rpyle731, Sax Russell, Zyxwfgh127, Zzzzzzzzzzz, 24 anonymous edits

Galactic halo  Source: http://en.wikipedia.org/w/index.php?oldid=363085902  Contributors: Alfio, Avono, Betacommand, Boleyn3, Boud, Bryan Derksen, Bthv, Canderson7, Cosmo0,Dgrichevsky, Einsidler, Eleferen, Funandtrvl, Gerry Lynch, Hcagri, Iantresman, Icairns, J.delanoy, Jennavecia, Marasama, Morenooso, Naturehead, Onebravemonkey, Palica, RetiredUser2,RexNL, Satori, Scapler, Sgauria, Sintaku, Station1, Trevor MacInnis, Vespristiano, Vinsfan368, Wolfy, Zotel, 24 anonymous edits

Ionization cone  Source: http://en.wikipedia.org/w/index.php?oldid=208164229  Contributors: CaptinJohn, DragonflySixtyseven, GregorB

Low-ionization nuclear emission-line region  Source: http://en.wikipedia.org/w/index.php?oldid=356139818  Contributors: ChristieGera, Dr. Submillimeter, Element16, Mythealias, Parejkoj,Roberto Mura, TheInfinityPoint, WilliamKF, 6 anonymous edits

Article Sources and Contributors 167

Relativistic jet  Source: http://en.wikipedia.org/w/index.php?oldid=388335712  Contributors: 81120906713, Ajaxkroon, Angr, Audriusa, Barrons, Boothy443, Bryan Derksen, Caco de vidro,Christopher Thomas, CosineKitty, Curps, DabMachine, Dar-Ape, DemonThing, Dispenser, Djily, Dr. Submillimeter, Edcolins, Egowiki, Epolk, Eteq, Gaius Cornelius, HieronymousCrowley, I donot exist, IVAN3MAN, IanOsgood, Ixfd64, Ketiltrout, Kheider, Lempel, Liquidat, Mac Davis, Mgiganteus1, Mhardcastle, Mrbrak, N328KF, Nevermore4ever, Orionus, Parejkoj, PauloHelene,RadRafe, Rama, Ravencc, Roberto Mura, SMesser, Sharkbait784, Spacepotato, Tachyonics, Tasudrty, Tcisco, TotoBaggins, Txomin, VoidLurker, Wereon, Xioxox, Ylai, 24 anonymous edits

Supermassive black hole  Source: http://en.wikipedia.org/w/index.php?oldid=394194965  Contributors: 100DashSix, 1to0to-1, 84user, Abyssoft, Adrian.benko, Ahoerstemeier, Alansohn,Alfiboy, Alfie66, Alfio, Andy Dingley, AnthonyQBachler, Anyeverybody, Aranherunar, Archanamiya, Artem Karimov, Arvindn, AstroNomer, Author3, Avidmosh, B, BIL, Bastian964,Benandorsqueaks, Bender235, Bibliomaniac15, Bkell, Blanchardb, BlurTento, Boing! said Zebedee, Bongwarrior, Bornfury, Bryan Derksen, Cacycle, CamXV, Can't sleep, clown will eat me,Captain panda, Chase me ladies, I'm the Cavalry, Christopher Thomas, ClovisHopman, Cmg8462, Coneslayer, Crass Spektakel, Crazysunshine, Curlymeatball38, Curps, Cwilli201, Cyclotis04,Da Joe, Daniels220, Dave6, Deathphoenix, DerHexer, Dr. Submillimeter, DrCrisp, E Wing, EdC, Edwinhubbel, Ekilfeather, El C, El3mentary, Enviroboy, Eroica, EugeneForrester, Everybody'sGot One, Extra999, Falcon8765, Flubeca, Foodman, Fvasconcellos, GHe, Gajamukhu, Gary King, Gerardw, Giftlite, Greg L, Grrow, Hairy Dude, Hbackman, Headbomb, Hifrommike65,Hjgrihfosd, Hugo-cs, ISD, IanOsgood, Ike179, Indon, Interstellar Man, JaGa, JanicePssc, Javert, Jeffq, Jehochman, Jheise, Jleous, Joelholdsworth, John Hyams, Johno000, Jordanfehlen, JuJube,Kazvorpal, Keilana, Kemosobby, Kieff, Knightshield, Kurtan, Kwamikagami, Kyzersawsay, Lightmouse, Lights, Lilac Soul, Little Mountain 5, Looxix, Lysdexia, Maldek, Markjdb, Martin451,Mattweng, MattyB17, Maxis ftw, Megastar, Miguel, Minesweeper, Moeron, Moheezy, Motley Crue Rocks, Mpatel, MrBell, MrWhipple, Msa11usec, NatureA16, NawlinWiki, Neparis,NerdyScienceDude, Newone, Nickersonl, Nymetsfan, Omicronpersei8, Oscar Bravo, Owlbuster, Palica, Parejkoj, Patrick, Patty144, Pauljr231, Permafrost, Perugrl2, Peruvianllama, Peter Isotalo,Philippe, Photobiker, PhySusie, Piano non troppo, Pickhorn, Plingggggg, Pmsyyz, ProfessorPaul, Puzl bustr, Pyxzer, Quaint and curious, RJHall, RUL3R, Rake, Remember, Remurmur, RexNL,Rich Farmbrough, Ringy, Rjwilmsi, Romanm, Roznoni, Rumpuscat, Sacolcor, Saravask, Sarthella, Scepia, Sean D Martin, Seraphim, Sfuerst, Slicky, Slon02, Sonicology, SqueakBox, Steve3849,Submitter to Truth, Super-Magician, SuperStingray, Susanpalwick, Susurrus, Sverdrup, TechnoFaye, Tetracube, That Guy, From That Show!, The Rambling Man, Thehelpfulone, Thejk1026,Theusernameiwantedisalreadyinuse, Thrane, Tide rolls, Timwi, Tommy2010, Tony873004, Trent215, Twaz, UMD NR, Ulric1313, Uriel8, UrukHaiLoR, Vanillaflavouredpinapples, Variable,Versus22, Vipershark22, Vlmastra, Vonspringer, Vsmith, WASD, WandringMinstrel, Wetman, Wikieditor1988, WilyD, Wknight94, Wnt, Work permit, Wtfdontkill, Wwheaton, Xanzzibar,Yamamoto Ichiro, Zahd, Zeldafan237, Zharradan.angelfire, 400 ,ةيناريد دهاجم دابع anonymous edits

Galaxy groups and clusters  Source: http://en.wikipedia.org/w/index.php?oldid=392228405  Contributors: Ahoerstemeier, Alansohn, Alfio, Argo Navis, Arpingstone, Art Carlson, Atrizu,AxelBoldt, Bnord, Bryan Derksen, Caco de vidro, ChristopherWillis, Ckatz, Conversion script, Cosmo jaime, DanMS, Darkwind6000, DavidLevinson, DeadEyeArrow, Decumanus, Dekaels,DocWatson42, Dr. Submillimeter, DrFO.Jr.Tn, Drgonzophd, Enigmaman, Enviroboy, Ericoides, Etacar11, George100, Geremia, Gracefool, Halfblue, Heron, Hjb26, Icairns, JRP, JosephDwayne, Jyril, Kanthoney, Ketiltrout, Kozuch, Lars Lindberg Christensen, Looxix, Magnus Manske, Martynas Patasius, Mathew Carrier, Mhoenig, Michael C Price, Michael Hardy, Mjquinn id,MockTurtle, Motorneuron, Mrh30, NawlinWiki, Nguyen Thanh Quang, Nickshanks, Originalwana, Oth, Polemos, Pyxelator, RJHall, Rcech, Rnt20, SAE1962, Sardanaphalus, Scog, Shamiboy9,Solipsist, Sverdrup, Territory, Tevatron, Themusicgod1, Timwi, Tokenizeman, Tomruen, Valhallasw, WadeSimMiser, Worldtraveller, XJamRastafire, 80 anonymous edits

Galaxy supercluster  Source: http://en.wikipedia.org/w/index.php?oldid=395196450  Contributors: -- April, Ace45954, Alfio, Alro, Argo Navis, AxelBoldt, Azcolvin429, Bobby D. Bryant,Boo2u81, Bryan Derksen, Carbuncle, Cardamon, Chrisjj, Citylover, Conversion script, Cutter, David Latapie, Dispenser, El C, Ellywa, Fram, Harald Khan, Heron, I do not exist, Icairns, Ixfd64,JWB, Jerzy, Jesusjuice08, Keraunos, KnightRider, KnowledgeOfSelf, Mathew Carrier, Michael C Price, MisfitToys, Mnmngb, Mxn, Oth, Patrick1982, Pde, Peaches1955, PiccoloNamek,Pyrospirit, RadicalOne, Rich Farmbrough, Schneelocke, SchuminWeb, Scwlong, Signalhead, Silverxxx, Slakr, Starkiller88, Steven Andrew Scarface220995, StuffOfInterest, Sverdrup, Trv003,Twinsday, Versus, VeryVerily, WilliamKF, Ziggurat, 127 ,לורק לארשי anonymous edits

Galaxy filament  Source: http://en.wikipedia.org/w/index.php?oldid=385355685  Contributors: 122589423KM, Alfio, Allander, Anthony Appleyard, Art LaPella, Atlant, Beland, Bender235,Bryan Derksen, Caco de vidro, Chris the speller, Cyclopia, DabMachine, David Latapie, Dorftrottel, DougsTech, El C, Frostyservant, Giftlite, Iantresman, Icairns, Jyril, Kpjas, Lightmouse,MER-C, Mathias-S, Mendaliv, Mnmngb, Mosesofmason, Motorneuron, Mxn, Njál, Phantom in ca, Pie4all88, Poliocretes, Poulpy, Rjwilmsi, Saulelis, Schneelocke, SchuminWeb,ScienceApologist, Serendipodous, Sheliak, Smack, Starwed, StuffOfInterest, Sverdrup, Tarotcards, Timsdad, Timwi, Tom-, Twinsday, Tyler, William Allen Simpson, 71 anonymous edits

Active galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=395644607  Contributors: Alansohn, Alfio, AnOddName, Andre Engels, ArielGold, Arpingstone, Astrobiologist, Avihu,Beland, BigDukeSix, Boud, Bryan Derksen, Ctj, Dbundy, DinDraithou, Dr. Submillimeter, Dysfunktion, Edwinhubbel, EmersonLowry, Enviroboy, Epbr123, Evanreyes, Falcon8765, FinlayMcWalter, Gene Nygaard, Ghhs, Giftlite, Glenn, Grant76, GutoAndreollo, Headbomb, Hellothere17, IVAN3MAN, Icairns, Jdearden, Jolielegal, Joseph Dwayne, Julianonions, Kat23, Kirx,Krash, Kurgus, Laurascudder, Looxix, Lwinte01, MBK004, Mateuszica, Mattisse, Meno25, Mhardcastle, Michael Hardy, Mnmngb, Moshe Constantine Hassan Al-Silverburg, Moxy, Mrbrak,NaiPiak, Nebulosus, Neko-chan, Newone, OlEnglish, PaddyLeahy, Parejkoj, Pathoschild, Paul Pogonyshev, Postdlf, Privong, RJHall, RNoble21, Richard Arthur Norton (1958- ), Roadrunner,Scog, Sevela.p, Slightsmile, Spacepotato, Tetracube, Tevatron, Thaisk, Tom87020, Topbanana, WookieInHeat, Zzzzzzzzzzz, 94 anonymous edits

Barred lenticular galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=395601694  Contributors: Dr. Submillimeter, Fotaun, Hurricane Devon, OlEnglish, Zzzzzzzzzzz, 5 anonymousedits

Barred irregular galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=369048520  Contributors: DaMatriX, Doprendek, Dr. Submillimeter, Hurricane Devon, LilHelpa, Nohomework,PigFlu Oink, TenaliBorogovy, Zzuuzz, Zzzzzzzzzzz, 2 anonymous edits

Barred spiral galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=394888115  Contributors: A2Kafir, Abb3w, Adriellerner, Ageekgal, Alpha Quadrant, Angr, Arthena, Atakdoug,CanOfWorms, Canuck100, Captmondo, Chaos syndrome, Christopher1968, Clh288, Cpastern, Craigsjones, Da Joe, DaMatriX, Dan East, David R Merritt, Dr. Submillimeter, Dragons flight,Dratman, Etacar11, Fredrik, Fvw, George100, Ginsengbomb, Gogo Dodo, Hairy Dude, Hurricane Devon, Icairns, JorisvS, Joseph Dwayne, KGyST, Keraunos, KnightRider, Knowledge Seeker,Livajo, Marsve, Mhdkandil, Midway, Mike s, Mintleaf, Mnmngb, Modest Genius, Neurophyre, Nihiltres, Nikolay94, Northgrove, Numbo3, OlEnglish, Pauli133, Pikiwyn, Princessliana, Quaeler,RJHall, Rahga, Raskolnikov The Penguin, RichiH, Ringbang, ShaunES, Ste4k, Sweetmoose6, Territory, Tide rolls, WilliamKF, Woohookitty, Wwagner, XJamRastafire, Xandi, Xerxes314,Zzzzzzzzzzz, 103 anonymous edits

Blazar  Source: http://en.wikipedia.org/w/index.php?oldid=395014008  Contributors: Aarchiba, Alfio, Alton, Bgold, Billjefferys, Charles Matthews, Chupon, ConradPino, Curps, Cwenger,Dantheman531, David Gerard, Dekker, Discospinster, Dorftrottel, Dr. Submillimeter, El C, Evil saltine, Gadfium, Gary King, Harmil, Harp, Icairns, Infinoid, JHunterJ, JaGa, Jmacwiki, JoJan,KnightRider, Kurgus, Looxix, M3r3p1, Marcelo-Silva, Michael Hardy, Milstein, Mirv, Monedula, Mrbrak, Nipisiquit, Ondon, Paranoid, Parejkoj, Pjacobi, Plushy, Poor Yorick, Promatrax161,Quantumobserver, Reyk, Rich Farmbrough, Roadrunner, RobertAustin, STHayden, Sam Hocevar, Shappy, Sonicology, Spacepotato, Sukida, Variable, Venny85, Wikiborg, WilliamKF, Wwoods,Zzzzzzzzzzz, 78 anonymous edits

Blue compact dwarf galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=393795570  Contributors: KGyST, NeilN, Rich Farmbrough, Richard Nowell, Spacepotato, 2 anonymous edits

Dark galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=395971177  Contributors: A2Kafir, Allen3, Ataleh, Crum375, Dr. Submillimeter, El C, Gadz, Giftlite, J.delanoy, JHUastro,Karmos, Marasama, Mike Rosoft, Mike s, Mnmngb, NSR, Onebravemonkey, Plumbago, Reyk, Rich Farmbrough, Roberto Mura, ScienceApologist, Silver Spoon, SimonP, Spacefem, St3vo,Stardustdeath, Whiteboycat, Zzzzzzzzzzz, 45 anonymous edits

Disc galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=332257803  Contributors: Axeman89, Beland, Dr. Submillimeter, Emersoni, Jean-François Clet, John Belushi, Kanesue,Mindmatrix, Qwertyus, RHaworth, Tarnum, Zzzzzzzzzzz, 6 anonymous edits

Dwarf elliptical galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=362478274  Contributors: A2Kafir, Alfio, Alpha Quadrant, Barticus88, Brownlee, Captmondo, Chopchopwhitey,ChrisCork, Dr. Submillimeter, Eteq, Gaius Cornelius, Harloshaply, Hynespb, IanOsgood, Icairns, Ilikeverin, Joaquim™ Filho, Lee937, Mrmrbeaniepiece, No1lakersfan, Plato, Rjwilmsi, Rursus,SD6-Agent, Sam Hocevar, Secretlondon, Uber nemo, VeryVerily, WilliamKF, Zzzzzzzzzzz, 23 anonymous edits

Dwarf galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=390751320  Contributors: A2Kafir, Alexander110, Allen McC., Alpha Quadrant, Arthena, Captmondo, Ccchen63, ColoniesChris, Cosmo0, Craigsjones, Da Joe, Dr. Submillimeter, Edgar181, Erud, FKmailliW, GABaker, Gadykozma, Garion96, Grrow, Hurricane Devon, Huwr, Icairns, KGyST, Looxix, Lzz, Meco,Mnmngb, Naturehead, Neko-chan, Neurophyre, Pauli133, Polylepsis, Puzl bustr, RetiredUser2, Richard Nowell, Siroxo, Tarnum, Tonyrex, Uber nemo, Vary, WilliamKF, XJamRastafire,Zzzzzzzzzzz, 28 anonymous edits

Dwarf spheroidal galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=367160509  Contributors: A2Kafir, Alfio, Alpha Quadrant, Captmondo, Da Joe, Dr. Submillimeter, Eteq, Icairns,Igodard, Jackie, Joaquim™ Filho, Joseph Dwayne, KGyST, Pentasyllabic, Safalra, Shashwat986, WilliamKF, Zzzzzzzzzzz, 17 anonymous edits

Dwarf spiral galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=362478631  Contributors: Alpha Quadrant, Ardric47, Dr. Submillimeter, Hurricane Devon, Imasleepviking, Joaquim™Filho, Marcosm21, Roberto Mura, Ste4k, Tarnum, WilliamKF, Zzzzzzzzzzz, 7 anonymous edits

Elliptical galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=394779512  Contributors: 1210Poppy, 2D, AEVanVogt, Alansohn, Alfio, Alpha Quadrant, Argo Navis, Arpingstone, Art LaPella, Arthena, Astrotwitch, Attilios, B.d.mills, Banaticus, Bender235, Bobo192, Captmondo, CardinalDan, Cherlin, Collinp6, Cosmo0, Cremepuff222, Cygnus78, David R Merritt, Dino, Discospinster, Dr. Submillimeter, DrFO.Jr.Tn, Dragons flight, El C, EngineerShorty, Etacar11, Falcorian, Friendlystar, Fvw, GreekAlexander, Harald Khan, Hi IM Bi, Hobartimus, Hurricane

Article Sources and Contributors 168

Devon, Icairns, Igoldste, ImperatorExercitus, J.delanoy, JSpung, Jac16888, Jake Wartenberg, Jeff G., Joanjoc, John D. Croft, Jyril, KGyST, Katalaveno, Kostmo, Kyng, Leia, Leslie Mateus,LilHelpa, Looxix, Magnus Manske, Mallignamius, Marasama, Marcos, Maxis ftw, Mendaliv, Mikeo, Mintleaf, Moshe Constantine Hassan Al-Silverburg, Npk, Palica, Pepsi Lite, Pgk, PhilipTrueman, Philip tao, Pickhorn, Pill, Pingveno, Pmronchi, RJHall, RexNL, Rjwilmsi, Robert Treat, RyanCross, S Schaffter, Scapler, ScienceApologist, SiegeLord, Sleeping123, Snowolf, Ste4k,Sverdrup, Synchronism, Tide rolls, Tom, Trv003, Tzepish, Unapachita, Unyoyega, Versus22, Vsst, Wiikkiiwriter, WilliamKF, Xpegahx, Yakudza, Z-d, Zzzzzzzzzzz, 189 ,ينام anonymous edits

Faint blue galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=385862187  Contributors: A2Kafir, Android79, Charles Matthews, Cherlin, CielProfond, Deirdre, Dr. Submillimeter,Drunken Pirate, FK65, Lomn, Mateuszica, RJHall, Reinyday, Strangelv, Xezbeth, Zzzzzzzzzzz, 6 anonymous edits

Field galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=345071728  Contributors: Dr. Submillimeter, Mohamed Magdy, Zzzzzzzzzzz, 4 anonymous edits

Flocculent spiral galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=387383985  Contributors: Alpha Quadrant, Anthony Appleyard, Headbomb, Rich Farmbrough, Timotheus Canens,7 anonymous edits

Grand design spiral galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=373343134  Contributors: Alton, ArchetypeRyan, BlueMoonlet, Clpo13, Doradus, Fidelia, Jan1nad, Larry V,MisfitToys, Nono64, Oerjan, Safalra, Selfworm, Sonicology, 5 anonymous edits

Host galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=286831234  Contributors: A2Kafir, Bobo192, Curps, Dr. Submillimeter, El C, GK, Gadfium, George100, Johnbobyray, Mrbrak,NatureA16, Zzzzzzzzzzz, 9 anonymous edits

Interacting galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=394763585  Contributors: Arthena, Azcolvin429, Bacteria, Bryan Derksen, Chowbok, Dark dude, DaveRusin, Dorftrottel,Dr. Submillimeter, Emesee, Emijrp, Eteq, Fcombes, Friendlystar, Funnybunny, Harald Khan, Heron, Hurricane Devon, Jeandré du Toit, Jpo, KGyST, Keraunos, Knightshield, Kozuch, Kross,KyuuA4, Martarius, Mboverload, Mtwykstr, Mu301, NatureA16, Neverquick, Nibios, Parejkoj, Piledhigheranddeeper, Proxima Centauri, PuzzletChung, RJHall, Roberto Mura, Rotational, Ryt,Serendipodous, SiliconDioxide, Silly rabbit, Silver Spoon, Sin-man, Slightsmile, Ste4k, Vsmith, Welsh, WilliamKF, Zzzzzzzzzzz, 30 anonymous edits

Intermediate spiral galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=395602010  Contributors: Dr. Submillimeter, Eras-mus, Fotaun, Poulpy, Rich Farmbrough, WilliamKF,XJamRastafire, Zzzzzzzzzzz, 15 anonymous edits

Irregular galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=395281148  Contributors: .:Ajvol:., A2Kafir, Afri, Ahoerstemeier, Alfio, Alpha Quadrant, Argo Navis, Art LaPella,Blablason, Buddharox101, Caltas, Captmondo, CommonsDelinker, Corpx, Cosmo0, Da Joe, Darth Panda, Dr. Submillimeter, Dragons flight, FKmailliW, Fvw, Gilliam, Hobartimus, HurricaneDevon, Icairns, Imasleepviking, J.delanoy, Japo, Joseph Dwayne, Jpo, Jusdafax, Katharineamy, KnightRider, Kuru, Leia, Looxix, Lzz, Midway, Mild Bill Hiccup, Originalwana, Palica,Polylepsis, Praetor alpha, Realist2, Recognizance, RedRollerskate, Robert Weemeyer, RobertG, Rothorpe, Scog, Sheep81, SimonP, Ste4k, The Singing Badger, The Thing That Should Not Be,TomDaemon, Tothebarricades.tk, Wiki1905, WilliamKF, Zzzzzzzzzzz, Алиса Селезньова, 127 anonymous edits

Lenticular galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=394411787  Contributors: A2Kafir, Ahoerstemeier, Alfio, Argo Navis, Arpingstone, Avicennasis, Badgernet,CRGreathouse, Captmondo, Cosmo0, Da Joe, DerHexer, Docu, Dr. Submillimeter, Dragons flight, FKmailliW, Friendlystar, Fvw, Gogo Dodo, Gungey300, Hurricane Devon, Iam on andromeda,Icairns, Joseph Dwayne, KnightRider, Koshyg, Leia, LilHelpa, Mintleaf, Mnmngb, Mtruch, Ojay123, Polylepsis, Radoslaw Ziomber, Schneelocke, SimonP, Snoyes, Spencer, Stan Shebs, Ste4k,Sverdrup, Timwi, Tsiaojian lee, WilliamKF, XJamRastafire, Zzzzzzzzzzz, 42 anonymous edits

Low surface brightness galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=394957079  Contributors: A2Kafir, Ardric47, Art LaPella, Bluemoose, Dr. Submillimeter, El C, Fueled,Greymancer, Icairns, JHUastro, JzG, Kikuyu3, Reuben, Scog, Stevertigo, Wikiborg, Zzzzzzzzzzz, 15 anonymous edits

Luminous infrared galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=389160712  Contributors: Axeman89, Davecl, Dr. Submillimeter, Drumguy8800, Dutchsatellites.com, El C,FKmailliW, Jsurace, Kevin Nelson, Poispois, Quaristice, Roberto Mura, WilliamKF, Zzzzzzzzzzz, 8 anonymous edits

Lyman-alpha emitter  Source: http://en.wikipedia.org/w/index.php?oldid=385010114  Contributors: Alejandr013, Colonies Chris, Headbomb, Imasleepviking, Mnmngb, Originalwana, RJHall,Reedy, The Anome, 11 anonymous edits

Lyman-break galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=356117315  Contributors: DragonflySixtyseven, Ryantrainor, 4 anonymous edits

Magellanic spiral  Source: http://en.wikipedia.org/w/index.php?oldid=357769431  Contributors: MSGJ, Tide rolls, 16 anonymous edits

Pea galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=396132151  Contributors: Chrislintott, Christopher Thomas, Falcorian, Gene Nygaard, Half65, HedgeFundBob, James McBride,Jezhotwells, Kosebamse, Magioladitis, Marasama, Neko-chan, Phil Boswell, Rich Farmbrough, Richard Nowell, Ruhrfisch, Salavat, Scog, Ukexpat, 60 anonymous edits

Peculiar galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=371415515  Contributors: Charles Matthews, Denni, Discospinster, Dr. Submillimeter, Frotz, JYolkowski, Jeandré du Toit,Liveste, Mirek256, Porktober1, Poulpy, Roberto Mura, Tasudrty, Zzzzzzzzzzz, 14 anonymous edits

Polar-ring galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=394889728  Contributors: Acom, Astronomer g, Dr. Submillimeter, Epistemos, Fredgoat, Hqb, Jdubs409, John Belushi,JohnBlackburne, JorisvS, Jschulman555, Petersam, Pickom, R9tgokunks, RHB, Roberto Mura, Sakurambo, SeanMD80, Smithbrenon, The Mad Genius, Utanapishti, Variable, WilliamKF, Zotel,凌雲, 16 anonymous edits

Protogalaxy  Source: http://en.wikipedia.org/w/index.php?oldid=396062968  Contributors: A2Kafir, Andres, Argo Navis, Autoplayer, Calton, CharlotteWebb, Cyde, DW40, Dori, Dr.Submillimeter, Grendelkhan, Imaninjapirate, Karol Langner, Kikuyu3, Mani1, Neelix, Rentier, Roberto Mura, ScienceApologist, WilliamKF, XJamRastafire, Zzzzzzzzzzz, 5 anonymous edits

Quasar  Source: http://en.wikipedia.org/w/index.php?oldid=393863317  Contributors: (jarbarf), 1to0to-1, AStext, Aalejandrino, Abrech, Academic Challenger, AdjustShift, Ageekgal,AgentFade2Black, Agge1000, Ahoerstemeier, Alain r, Alansohn, AlexiusHoratius, AlphaPikachu578, Anarchy Cave, Anchoress, AndersMNelson, Andre Engels, Andrew Hampe, Andromachi,AndyWestside, Animum, Anoko moonlight, Anonymous Dissident, Anotherwikifan, Antandrus, Antelan, Apetre, Aragorn2, Archiesteel, Arlen22, ArnoLagrange, Art Carlson, AstroNomer,AstroPaul, Ataleh, Atlant, Avenue, Awakened crowe, AxelBoldt, BatteryIncluded, Bcz, Beefcalf, BenRG, Bobo192, Bongwarrior, Bryan Derksen, Bushytails, C0N6R355, Calcobrena, Can'tsleep, clown will eat me, CanadianLinuxUser, Chaos, Chaos0mega, CharlesC, Chetvorno, Chickenfeed9, Chimesmonster, Chlaub, Chrisbolt, Christopher Thomas, Closedmouth, Cmapm, CoderDan, Conchobhair II, Coneslayer, Conversion script, Corpx, Crazycomputers, Cremepuff222, Crusty007, Css, Curps, Cyde, Cyp, DVD R W, Dangeruss79, DataWraith, DaveGorman, DavidGerard, Db099221, Dbfirs, DeadEyeArrow, Deathphoenix, Defender of torch, Delldot, DerHexer, Deskana, DinDraithou, Distantbody, Diverman, DivineAlpha, Donarreiskoffer, DoubleBlue,Doug Bell, Dr. Submillimeter, Dragana666, Drrebellious, Dukeofalba, Długosz, EWS23, EddEdmondson, Edwinhubbel, Eilthireach, Ekilfeather, El C, Eleo87, Elliotontheradio, Enviroboy,Essjay, Etacar11, Eteq, EvilSupahFly, Extra999, Ezzeloharr, Faradayplank, Fireburnme, Fivemack, Flehmen, Fox, Freddyd945, Freedomlinux, Fyyer, Gaius Cornelius, Gene Nygaard, Giftlite,Gilliam, Gogo Dodo, Goodant, GregorB, Gurch, Gökhan, Hadal, Hagerman, Haham hanuka, Hairy Dude, HappyCamper, Happywaffle, Harp, Headbomb, Henning Makholm, Hobartimus,Horsten, II MusLiM HyBRiD II, Iain99, Iantresman, Icairns, Ilke71, Inuit7, Irbisgreif, Isomorphic, Isoptera, Ispy1981, Ixfd64, JForget, JLaTondre, JRSpriggs, JYolkowski, Ja 62, James919,Jaysweet, Jbeans, Jeandré du Toit, Jeffrey O. Gustafson, Jerry-va, Jhbdel, John Darrow, John0101ddd, Joke137, Jose piratilla, Josh Grosse, Joshua777, Jovianeye, Juliancolton, Jyril, Jérôme,Kashk1, Khamosh, Khukri, Killiondude, Kirx, Knakts, Kostya30, Kozuch, L337 kybldmstr, Laurascudder, Lee J Haywood, Lengis, Lesmothian, LiDaobing, LiamE, Liberal Classic, Lithiumcyanide, Loopygrumpkins, Looxix, Lordofhyperspace, Lottamiata, MER-C, MPF, Mac Davis, Madhero88, Maelor, Maggosh, Magioladitis, MaizeAndBlue86, Makeyev, Malamockq, Marcello,Markjdb, Marqueed, Marshallsumter, Martarius, Martin451, MartinElvis, Maurice Carbonaro, Maximaximax, Maximillion Pegasus, Maximus Rex, McSly, Megaton, Melsaran, Mhardcastle,Mhking, Mike Peel, Minesweeper, Mintleaf, Mirv, Moogenmeister, Mozzerati, Mrwuggs, Myrrhlin, N328KF, NSH001, Naddy, NatureA16, NawlinWiki, NewEnglandYankee, Newone, Nivix,Nneonneo, Noclevername, NotAnonymous0, NuclearWarfare, Numbo3, Objectivist, Octahedron80, Orionus, OverlordQ, Pagw, Palica, Paranoid, Parejkoj, ParticleMan, Patbert, Patespi,Patrick1982, Paulley, Pbn, Pc13, Person6789, PeterJeremy, Phil Boswell, Philip Trueman, PhySusie, Picus viridis, Pklolkyle, Plautus satire, Poor Yorick, Pqn0308, Prickus, Prolog, Prototime,Puchiko, Punarbhava, Pwhitwor, Quidproquo2004, Quintote, Quizmaster1, Qxz, RJHall, Random astronomer, RandomGuy42, Raul654, RexNL, Richard Taylor, Rje, Rnt20, Roadrunner,RobertMfromLI, Robinh, Robma, Robogun, Roflbater, Rory096, Runningonbrains, Ruslik0, S.Bowen, SaRiisRipples, Salamurai, Schneelocke, ScienceApologist, ShakataGaNai, Shii, Silsor,Smartech, Songjin, Sonicology, Spark Moon, SpookyMulder, Sportachris, Srborlongan, Srleffler, Stephenb, SteveVer256, Sverdrup, TUF-KAT, Tango, Tavilis, Tempodivalse, Texas.veggie, ThatGuy, From That Show!, The Thing That Should Not Be, Thegoodlocust, Thunderbrand, Tide rolls, Titoxd, Toby Bartels, Todder1995, Tohd8BohaithuGh1, Tonyfaull, Trusilver, Unbeatable0,UnitedStatesian, Unschool, Uranometria, Useight, Uxh, Vanished user 03, Variable, VoxLuna, Vsmith, W4rg, Waggers, WereSpielChequers, Whisky drinker, Wiki alf, WikiLaurent,WikiMarshall, Wikkidd, WilliamKF, WilliamThweatt, Wimt, Wired2narnia, Wwagner, Wwheaton, Wysprgr2005, XJamRastafire, Zeptomoon, Zzzzzzzzzzz, 699 ,دمحأ anonymous edits

Radio galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=373408585  Contributors: Bobblewik, Brroga, Casliber, ClarketheK, Da Joe, Dr. Submillimeter, Edwinhubbel, Epbr123, Epolk,Fig wright, FillyfromPhilly, Fxmastermind, Iridescent, Jonverve, Katieh5584, Krash, Looxix, Mattisse, Mhardcastle, Mnmngb, Naturehead, PaddyLeahy, Parejkoj, Ph0kin, Privong,Quantumobserver, Reyk, ScienceApologist, Slicky, Stepa, Trik The Atheist, Variable, WilliamKF, X-shaped, Zzzzzzzzzzz, 24 anonymous edits

Article Sources and Contributors 169

Ring galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=394890168  Contributors: A2Kafir, Alfio, Captmondo, Cyde, DA3N, DIUZOMA, Da Joe, Dr. Submillimeter, George100,Icairns, Jkelly, JorisvS, Kalki, MER-C, NatureA16, Petersam, Roberto Mura, Romanc19s, Rparle, SiegeLord, Tarnum, XJamRastafire, Zzzzzzzzzzz, 9 anonymous edits

Seyfert galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=366107695  Contributors: Ahpook, Alfio, Argo Navis, Arpingstone, Astroal1947, Bronger, Coccoinomane, Conti, CrypticC62, Da Joe, Deschain785, Dr. Submillimeter, GraL, Habj, Hairy Dude, Icairns, Ingolfson, Jason.grossman, Joseph Dwayne, KGyST, KnightRider, Kurtan, Looxix, MPF, MartinCZ, Megan1967,Mike18xx, Mnmngb, Mylon, Naddy, Paranoidzachandroid, Pie4all88, Pol098, RJHall, Rdb, Rentier, Rich Farmbrough, Rjwilmsi, Serguei S. Dukachev, StuartCarter, Sweetmoose6, Tetracube,Tsiaojian lee, Vicki Rosenzweig, Wikiborg, Zzzzzzzzzzz, 17 anonymous edits

Spiral galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=396144136  Contributors: (jarbarf), AEVanVogt, Acroterion, Aeon1006, Agathoclea, Ajr, Alansohn, Aldaron, Alfio,Amwilson2000, Anton Markov, Arakunem, Argo Navis, ArnoldReinhold, Arpingstone, Art LaPella, Arthena, Astrotwitch, Attilios, Babomb, Ben Standeven, BlueMoonlet, Bogdangiusca,Brighterorange, Caco de vidro, Cam, Canuck100, Captmondo, Confession0791, Cool Blue, Coolieboy84, Cosmo0, Courcelles, Curps, Cyrillic, DARTH SIDIOUS 2, Da Joe, DarkAudit,Dbenbenn, DerHexer, Dicklyon, DivineAlpha, Dr. Submillimeter, Dragons flight, Duemellon, Edgerck, Ekespe, El C, Elmoro, Epbr123, Etacar11, Evil Monkey, Exphysus, FKmailliW, Foobaz,Fvw, Gangsta124, GuyQuest, Hairy Dude, HexaChord, Hi IM Bi, Hongkongresident, Hurricane Devon, Hydrogen Iodide, I dream of horses, Iantresman, Icairns, Ikiroid, JNW, Jkelly, Jmencisom,John D. Croft, Johnuniq, Joseph Dwayne, Jruderman, Junglecat, Jyril, KGyST, Kalsermar, Keraunos, Kitty the Random, Kooolioa, Kubigula, Lars Lindberg Christensen, Leia, Lightmouse,Looxix, Lpgeffen, MC10, Macinapp, Magnus Manske, McSly, Mentifisto, Michael Devore, Mike s, Mintleaf, Mysid, N328KF, NHRHS2010, NatureA16, NellieBly, NewEnglandYankee,Numbo3, NyyDave, Oleg Alexandrov, Onebravemonkey, Oskar71, Pagw, Pepper, Persian Poet Gal, Pika ten10, Polylepsis, Quaeler, RJHall, RQG, RainbowOfLight, Random astronomer,Razimantv, Reconsider the static, Resident of arkham, Reyk, Rich Farmbrough, Robertgreer, Rrburke, Schneelocke, ScienceApologist, Sciurinæ, Serendipodous, Shadowjams, SimonKagstrom,SimonP, Space girl 9, Spiritia, Squash, Ste4k, Stuart Morrow, Submitter to Truth, SuyoungL, Sverdrup, Sławomir Biały, Tarnum, Tide rolls, Titanium Dragon, Tommy2010, Tonicthebrown,Tony Fox, Tsiaojian lee, TutterMouse, Tv316, Ularevalo98, Vreejack, Wavelength, WilliamKF, Wimt, Wisdom89, Ykemper, Yuckfoo, Zbayz, Zmcdargh, Zzzzzzzzzzz, 259 anonymous edits

Starburst galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=390054892  Contributors: Ageekgal, Alansohn, AndrewHowse, Argo Navis, Axeman89, Bobblewik, Bobo192, Ceyockey,Chris the speller, Cmdrjameson, ConradPino, Corpx, Discospinster, Dorftrottel, Dr. Submillimeter, El C, Emc2, Epolk, Etacar11, Eugene van der Pijll, Fatal!ty, Fernando Estel, Fournax,Hellothere17, Icairns, Jehochman, JesseW, KGyST, Kurt Shaped Box, KyNephi, Lars Lindberg Christensen, Lights, Lzz, Marasama, Marcelo-Silva, Matthewhayes, MeganKA, Pathoschild,Pilchard, Platyfish625, Richard Nowell, Rjwilmsi, Roberto Mura, Sam Hocevar, Scog, Seth Ilys, Shyam, SimonP, Toby Douglass, Tom Lougheed, Tryphiodorus, Variable, WilliamKF, Wnt,Ynilp, Zzzzzzzzzzz, 63 anonymous edits

Type-cD galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=394500235  Contributors: Alpha Quadrant, Edrowland, Emmykm, Ilmari Karonen, IncognitoErgoSum, LilHelpa,NHRHS2010, Rich Farmbrough, Skysmith, 39 anonymous edits

Unbarred lenticular galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=395648075  Contributors: Dr. Submillimeter, Fotaun, OlEnglish, Rich Farmbrough, WilliamKF, Zzzzzzzzzzz, 2anonymous edits

Unbarred spiral galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=395647563  Contributors: Dr. Submillimeter, Fotaun, George100, John Belushi, Martarius, Numbo3, OlEnglish,Poulpy, WilliamKF, Yupik, Zzzzzzzzzzz, 19 anonymous edits

Brightest cluster galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=357785283  Contributors: Argo Navis, Auntof6, ClarketheK, Colonies Chris, Dominic Renshaw, UoD 2006,Domren111, Dr. Submillimeter, Edwinhubbel, El C, EugeneForrester, Hadrianheugh, Jyril, Mel Etitis, Mike Peel, Quaristice, Rjwilmsi, Roberto Mura, Twinsday, Vegasbri, Wikiborg,WolfmanSF, Zzzzzzzzzzz, 4 anonymous edits

Galaxy color-magnitude diagram  Source: http://en.wikipedia.org/w/index.php?oldid=369414282  Contributors: Mike s, ScienceApologist, Scorpion0422, StaticGull, Tjic, 6 anonymous edits

List of galaxies  Source: http://en.wikipedia.org/w/index.php?oldid=392192901  Contributors: 1234r00t, Aka042, Alexander110, Amerias, Anton Gutsunaev, Art LaPella, Blankfaze, BrianY,Bunnyhop11, CHG, CWitte, CanadianLinuxUser, Canis Lupus, Captain-n00dle, CaptainMike, Count Iblis, Curps, Czj, Davecrosby uk, Dispenser, Dr. Submillimeter, Elijya, Evil Monkey,Excirial, Falcon8765, Fatal!ty, FillyfromPhilly, Frankie816, Gaius Cornelius, Goobergunch, Gtrmp, Hewholooks, Iam on andromeda, Icairns, Icemaja, Iridescent, JMK, Ja 62, JamesHoadley,John Vandenberg, JohnLynch, Johnuniq, Joseph Dwayne, Karol Langner, Kheider, Kungfuadam, Ladsgroup, Latitude0116, Leuko, MER-C, Maccoat, Maxis ftw, Nergaal, Neurophyre, Nono64,Noodle snacks, Optim, Pika ten10, Polylepsis, RC Master, Ragesoss, RainbowOfLight, Rholton, Rich Farmbrough, Rmrfstar, SJP, Seth Ilys, Signalhead, SkE, Starcluster, SuperHamster, Texture,Tommy2010, Uber nemo, Vegasbri, WIKIKNIGHTX, Wienerline, Wikipelli, WilliamKF, XJamRastafire, Xezbeth, 371 anonymous edits

Fossil group  Source: http://en.wikipedia.org/w/index.php?oldid=380113920  Contributors: Radagast83, Zzzzzzzzzzz, 13 anonymous edits

Image Sources, Licenses and Contributors 170

Image Sources, Licenses and ContributorsImage:NGC 4414 (NASA-med).jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_4414_(NASA-med).jpg  License: unknown  Contributors: NASA Headquarters - GreatestImages of NASA (NASA-HQ-GRIN)Image:Milky Way Galaxy and a meteor.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Milky_Way_Galaxy_and_a_meteor.jpg  License: unknown  Contributors: Mila ZinkovaImage:Herschel-Galaxy.png  Source: http://en.wikipedia.org/w/index.php?title=File:Herschel-Galaxy.png  License: Public Domain  Contributors: BRUTE, FredA, H.Seldon, 5 anonymous editsImage:M51Sketch.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M51Sketch.jpg  License: Public Domain  Contributors: Juiced lemon, Szdori, WiniarImage:Pic iroberts1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pic_iroberts1.jpg  License: Public Domain  Contributors: Isaac Roberts (d. 1904)Image:GalacticRotation2.svg  Source: http://en.wikipedia.org/w/index.php?title=File:GalacticRotation2.svg  License: Creative Commons Attribution-Sharealike 2.0  Contributors:User:PhilHibbsImage:UDFy-38135539.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:UDFy-38135539.jpg  License: unknown  Contributors: NASA, ESA, G. Illingworth (UCO/Lick Observatoryand University of California, Santa Cruz) and the HUDF09 Team.Image:Hubble sequence photo.png  Source: http://en.wikipedia.org/w/index.php?title=File:Hubble_sequence_photo.png  License: GNU Free Documentation License  Contributors: Harp,KGyST, Mdd, Wikiborg, 1 anonymous editsFile:Messier51 sRGB.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Messier51_sRGB.jpg  License: unknown  Contributors: NASA and European Space AgencyImage:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg  License: unknown Contributors: NASA, ESA, and The Hubble Heritage Team STScI/AURA)Image:Hoag's object.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Hoag's_object.jpg  License: Public Domain  Contributors: NASAFile:File-Ngc5866 hst big.png  Source: http://en.wikipedia.org/w/index.php?title=File:File-Ngc5866_hst_big.png  License: unknown  Contributors: NASA, ESA, and The Hubble Heritage Team(STScI/AURA)Image:Antennae galaxies xl.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Antennae_galaxies_xl.jpg  License: unknown  Contributors: NASA, ESA, and the Hubble HeritageTeam (STScI/AURA)-ESA/Hubble CollaborationImage:M82 HST ACS 2006-14-a-large web.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M82_HST_ACS_2006-14-a-large_web.jpg  License: unknown  Contributors: NASA,ESA, and The Hubble Heritage Team (STScI/AURA)Image:M87 jet.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M87_jet.jpg  License: unknown  Contributors: Original uploader was Dan Gardner at en.wikipedia Later versions wereuploaded by Joseph Dwayne, Ylai at en.wikipedia.File:Young_Galaxy_Accreting_Material.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Young_Galaxy_Accreting_Material.jpg  License: Creative Commons Attribution 3.0 Contributors: ESO/L. CalçadaImage:Hubble - infant galaxy.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Hubble_-_infant_galaxy.jpg  License: unknown  Contributors: NASA, ESA, Y. Izotov (MainAstronomical Observatory, Kyiv, UA) and T. Thuan (University of Virginia)Image:Seyfert Sextet full.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Seyfert_Sextet_full.jpg  License: unknown  Contributors: NASAImage:NGC891.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC891.jpg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: Martin BaessgenImage:HubbleTuningFork.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:HubbleTuningFork.jpg  License: unknown  Contributors: Original uploader was Cosmo0 at en.wikipedia(Original text : None given)Image:M101 hires STScI-PRC2006-10a.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M101_hires_STScI-PRC2006-10a.jpg  License: unknown  Contributors: Andersmusician,Fabian RRRR, Gorgo, Juiced lemon, KGyST, Lars Lindberg Christensen, Papa November, Takabeg, Tryphon, Winiar, Yann, 3 anonymous editsImage:warped galaxy.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Warped_galaxy.jpg  License: unknown  Contributors: NASA and The Hubble Heritage Team (STScI/AURA)Image:Abell S740, cropped to ESO 325-G004.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Abell_S740,_cropped_to_ESO_325-G004.jpg  License: Public Domain  Contributors:J. Blakeslee (Washington State University)Image:NGC4676.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC4676.jpg  License: unknown  Contributors: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin(STScI), G. Hartig (STScI), the ACS Science Team, and ESAImage:Artist's Concept Illustrating Bulge & No Bulge Spiral Galaxies.jpg  Source:http://en.wikipedia.org/w/index.php?title=File:Artist's_Concept_Illustrating_Bulge_&_No_Bulge_Spiral_Galaxies.jpg  License: Public Domain  Contributors: NASAImage:Galaxy morphology.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Galaxy_morphology.jpg  License: GNU Free Documentation License  Contributors: Dr. T.H. Jarrett(Caltech)Image:NGC 6782 I HST2002.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_6782_I_HST2002.jpg  License: unknown  Contributors: Original uploader was Clh288 aten.wikipediaImage:NGC 7793SpitzerFull.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_7793SpitzerFull.jpg  License: Public Domain  Contributors: NASA/JPL-Caltech/R. Kennicutt(University of Arizona) and the SINGS TeamImage:Large.mc.arp.750pix.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Large.mc.arp.750pix.jpg  License: Public Domain  Contributors: CWitte, Friendlystar, Juiced lemon,Julo, KGyST, TlusťaImage:Ngc5866 hst big rotated.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Ngc5866_hst_big_rotated.jpg  License: unknown  Contributors: HSTImage:large.mc.arp.750pix.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Large.mc.arp.750pix.jpg  License: Public Domain  Contributors: CWitte, Friendlystar, Juiced lemon,Julo, KGyST, TlusťaImage:Dark matter halo.png  Source: http://en.wikipedia.org/w/index.php?title=File:Dark_matter_halo.png  License: Public Domain  Contributors: http://en.wikipedia.org/wiki/User:Cosmo0File:Rotation curve (Milky Way).JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Rotation_curve_(Milky_Way).JPG  License: Creative Commons Attribution-Sharealike 3.0 Contributors: User:Brews ohareImage:Messier 81 HST.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Messier_81_HST.jpg  License: unknown  Contributors: NASA, ESA and the Hubble Heritage Team(STScI/AURA)Image:M63.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M63.jpg  License: unknown  Contributors: Abestrobi, Cäsium137, Emijrp, Ilmari Karonen, Juiced lemon, KGyST, MartinH., Ruslik0, Spacebirdy, Winiar, 2 anonymous editsImage:NGC 4314HST1998-21-b-full.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_4314HST1998-21-b-full.jpg  License: unknown  Contributors: G. Fritz Benedict, AndrewHowell, Inger Jorgensen, David Chapell (University of Texas), Jeffery Kenney (Yale University), and Beverly J. Smith (CASA, University of Colorado), and NASAImage:M104 ngc4594 sombrero galaxy hi-res.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M104_ngc4594_sombrero_galaxy_hi-res.jpg  License: unknown  Contributors:NASA/ESA and The Hubble Heritage Team (STScI/AURA)Image:Galaxies AGN Inner-Structure-of.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Galaxies_AGN_Inner-Structure-of.jpg  License: GNU Free Documentation License Contributors: MrbrakImage:NGC_5128.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_5128.jpg  License: Public Domain  Contributors: Boivie, Eleferen, Jarekt, KGyST, Mo-Slimy, Nordelch,Ruslik0, Twincinema, 1 anonymous editsFile:Onde-radioM87.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Onde-radioM87.jpg  License: Public Domain  Contributors: Glenn, KGyST, Loveless, RamaFile:HST-3C66B-jet-O5BQ06010.gif  Source: http://en.wikipedia.org/w/index.php?title=File:HST-3C66B-jet-O5BQ06010.gif  License: Public Domain  Contributors: Hubble Legacy ArchiveImage:rxj1242 comp.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Rxj1242_comp.jpg  License: Public Domain  Contributors: Medium69Image:Supermassiveblackhole nasajpl.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Supermassiveblackhole_nasajpl.jpg  License: Public Domain  Contributors: WilyDImage:galaxy.group.hickson.arp.500pix.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Galaxy.group.hickson.arp.500pix.jpg  License: unknown  Contributors: Original uploaderwas Arpingstone at en.wikipediaImage:ACO 3341.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:ACO_3341.jpg  License: unknown  Contributors: ESO

Image Sources, Licenses and Contributors 171

Image:Nearsc.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Nearsc.gif  License: Creative Commons Attribution-Sharealike 2.5  Contributors: Richard PowellImage:Local galaxy filaments 2.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Local_galaxy_filaments_2.gif  License: Creative Commons Attribution-Sharealike 2.5  Contributors:Klaus DolagImage:Superclusters atlasoftheuniverse.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Superclusters_atlasoftheuniverse.gif  License: Creative Commons Attribution-Sharealike 2.5 Contributors: Richard PowellImage:2dfdtfe.gif  Source: http://en.wikipedia.org/w/index.php?title=File:2dfdtfe.gif  License: GNU Free Documentation License  Contributors: Willem SchaapImage:2MASS LSS chart-NEW Nasa.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:2MASS_LSS_chart-NEW_Nasa.jpg  License: Public Domain  Contributors: w:InfraredProcessing and Analysis CenterIPAC/Caltech, by Thomas JarrettImage:Galaxies AGN Jet Properties-with-LoS.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Galaxies_AGN_Jet_Properties-with-LoS.jpg  License: unknown  Contributors: RonKollgaard ()Image:commons-logo.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Commons-logo.svg  License: logo  Contributors: User:3247, User:GruntImage:NGC 2787.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_2787.jpg  License: unknown  Contributors: NASA and The Hubble Heritage Team (STScI/AURA)Image:Ngc253 2mass barred spiral.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Ngc253_2mass_barred_spiral.jpg  License: unknown  Contributors: CWitte, Common Good,Eleferen, Ruslik0, WilliamKF, WiniarImage:NGC 4921 by HST.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_4921_by_HST.jpg  License: unknown  Contributors: NASA, ESA and K. Cook (LawrenceLivermore National Laboratory, USA)Image:Messier object 095.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Messier_object_095.jpg  License: unknown  Contributors: Emijrp, Friendlystar, RimshotImage:NGC3953HunterWIlson.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC3953HunterWIlson.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:User:HewholooksImage:N1073lipscomb.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:N1073lipscomb.jpg  License: unknown  Contributors: Clh288Image:Messier108.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Messier108.jpg  License: unknown  Contributors: CWitte, Friendlystar, Juiced lemon, KGyST, Malo, SanbecImage:NGC 2903 GALEX.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_2903_GALEX.jpg  License: Public Domain  Contributors: NASA/GALEX/WikiSkyImage:NGC 5398SST.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_5398SST.jpg  License: Public Domain  Contributors: NASA/JPL-Caltech/K. Gordon (Space TelescopeScience Institute) and SINGS TeamImage:Phot-14a-09-fullres.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Phot-14a-09-fullres.jpg  License: Creative Commons Attribution 3.0  Contributors: ESOImage:Galaxies AGN Jet Line-of-Sight.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Galaxies_AGN_Jet_Line-of-Sight.jpg  License: unknown  Contributors: MrbrakImage:NGC 1705.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_1705.jpg  License: unknown  Contributors: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)Image:M110 Lanoue.png  Source: http://en.wikipedia.org/w/index.php?title=File:M110_Lanoue.png  License: Public Domain  Contributors: Original uploader was Tomruen at en.wikipediaImage:NGC147.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC147.jpg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: Ole NielsenImage:Ngc4414 hst.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Ngc4414_hst.jpg  License: Public Domain  Contributors: Original uploader was Med at fr.wikipediaImage:NGC 2841 Hubble WikiSky.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_2841_Hubble_WikiSky.jpg  License: Public Domain  Contributors: , ,Image:ESO-Spiral-Galaxy-phot-14b-09-fullres 2.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:ESO-Spiral-Galaxy-phot-14b-09-fullres_2.jpg  License: Creative CommonsAttribution 3.0  Contributors: ESOImage:Ssc2003-06c.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Ssc2003-06c.jpg  License: Public Domain  Contributors: NASA/JPL-Caltech/S. Willner (Harvard-SmithsonianCenter for Astrophysics)Image:Whirlpool (M51).jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Whirlpool_(M51).jpg  License: Public Domain  Contributors: R. KennicuttImage:M61.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M61.jpg  License: unknown  Contributors: Uber nemo, WilyDImage:Messier object 065.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Messier_object_065.jpg  License: unknown  Contributors: Emijrp, Friendlystar, Juiced lemon, KGyST,Marcin Suwalczan, RimshotImage:NGC 4725.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_4725.jpg  License: Public Domain  Contributors: Clh288, KGyST, Tano4595, WiniarImage:Phot-33c-03-fullres.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Phot-33c-03-fullres.jpg  License: unknown  Contributors: ESOImage:NGC 4258GALEX.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_4258GALEX.jpg  License: Public Domain  Contributors: NASA/JPL-CaltechImage:NGC2403 3.6 8.0 24 microns spitzer.png  Source: http://en.wikipedia.org/w/index.php?title=File:NGC2403_3.6_8.0_24_microns_spitzer.png  License: Creative Commons Attribution2.5  Contributors: Clh288, Juiced lemon, Med, 1 anonymous editsImage:NGC 4625 I FUV g2006.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_4625_I_FUV_g2006.jpg  License: Public Domain  Contributors: Original uploader was Clh288at en.wikipediaImage:Magellanic Clouds ― Irregular Dwarf Galaxies .jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Magellanic_Clouds_―_Irregular_Dwarf_Galaxies_.jpg  License: unknown Contributors: ESO/S. BrunierImage:Irregular_galaxy_NGC_1427A_(captured_by_the_Hubble_Space_Telescope).jpg  Source:http://en.wikipedia.org/w/index.php?title=File:Irregular_galaxy_NGC_1427A_(captured_by_the_Hubble_Space_Telescope).jpg  License: unknown  Contributors: NASA, ESA, and The HubbleHeritage Team (STScI/AURA)Image:NGC 1553 Hubble.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_1553_Hubble.jpg  License: unknown  Contributors: Original uploader was Friendlystar aten.wikipediaImage:IRAS 19297-0406.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:IRAS_19297-0406.jpg  License: unknown  Contributors: El CImage:Lyman Alpha Blob.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Lyman_Alpha_Blob.jpg  License: Public Domain  Contributors: Left panel: D.Alexander et al. S.Chapmanet al. T.Hayashino et al. J.Geach et al. Right Illustration: M.WeissFile:Cardamone Peas.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Cardamone_Peas.jpg  License: GNU Free Documentation License  Contributors: Richard Nowell. Originaluploader was Richard Nowell at en.wikipediaFile:Hubble Peas Ed.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Hubble_Peas_Ed.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Carolin CardamoneFile:Pea Star Formation.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pea_Star_Formation.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: CarolinCardamoneFile:PeaAGN Graph ed.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:PeaAGN_Graph_ed.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: CarolinCardamoneFile:Peas Equiv Width ed.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Peas_Equiv_Width_ed.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: CarolinCardamoneFile:GANDALF 587724241767825591 ed.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:GANDALF_587724241767825591_ed.jpg  License: Creative CommonsAttribution-Sharealike 3.0  Contributors: Carolin CardamoneFile:Peas Reddening ed.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Peas_Reddening_ed.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: CarolinCardamoneFile:Peas_Amorin.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Peas_Amorin.jpg  License: GNU Free Documentation License  Contributors: Ricardo O. Amorín, E.Pérez-Montero, J.M. VílchezFile:Wiki Peas Montage.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Wiki_Peas_Montage.jpg  License: Creative Commons Attribution 2.0  Contributors: Richard NowellFile:Colour Split 2.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Colour_Split_2.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Carolin CardamoneImage:NGC 4650A I HST2002.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_4650A_I_HST2002.jpg  License: unknown  Contributors: The Hubble Heritage Team(AURA/STScI/NASA)Image:NGC660.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC660.jpg  License: Creative Commons Attribution 3.0  Contributors: User:Jschulman555

Image Sources, Licenses and Contributors 172

File:Black hole quasar NASA.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Black_hole_quasar_NASA.jpg  License: Public Domain  Contributors: NASA Original uploader wasMilk's Favorite Cookie at en.wikipediaFile:QuasarStarburst.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:QuasarStarburst.jpg  License: Public Domain  Contributors: Jodo, Mattes, SchimmelreiterImage:7107.tnl.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:7107.tnl.jpg  License: Public Domain  Contributors: Author: Dave Dooling Curator: Linda Porter NASA Official: M.Frank Rose, Dr. John M. Horack, Director of Science CommunicationsImage:QSO 0836+710.gif  Source: http://en.wikipedia.org/w/index.php?title=File:QSO_0836+710.gif  License: Public Domain  Contributors: Author: Dave Dooling Curator: Linda PorterNASA Official: M. Frank Rose, Dr. John M. Horack, Director of Science CommunicationsImage:PKS 1127-145 X-rays.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:PKS_1127-145_X-rays.jpg  License: Public Domain  Contributors:NASA/CXC/A.Siemiginowska(CfA)/J.Bechtold(U.Arizona)Image:Centauros a-spc.png  Source: http://en.wikipedia.org/w/index.php?title=File:Centauros_a-spc.png  License: GNU Free Documentation License  Contributors: Martin HardcastleImage:3C98.png  Source: http://en.wikipedia.org/w/index.php?title=File:3C98.png  License: GNU Free Documentation License  Contributors: MhardcastleImage:3C31.png  Source: http://en.wikipedia.org/w/index.php?title=File:3C31.png  License: GNU Free Documentation License  Contributors: MhardcastleImage:circinus.galaxy.750pix.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Circinus.galaxy.750pix.jpg  License: Public Domain  Contributors: Juiced lemon, KGyST, Kauczuk,Stan Shebs, 2 anonymous editsFile:HAWK-I NGC 1300.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:HAWK-I_NGC_1300.jpg  License: Creative Commons Attribution 3.0  Contributors: ESO/P. GrosbølImage:spiral galaxy arms diagram.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Spiral_galaxy_arms_diagram.svg  License: GNU Free Documentation License  Contributors:User:Dbenbenn, User:MysidImage: NGC 3810 (captured by the Hubble Space Telescope).jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_3810_(captured_by_the_Hubble_Space_Telescope).jpg License: unknown  Contributors: ESA/Hubble and NASAImage:NGC 1569.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_1569.jpg  License: unknown  Contributors: ESA, NASA and P. Anders (Göttingen University GalaxyEvolution Group, GermanyImage:Ssc2008-12a small.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Ssc2008-12a_small.jpg  License: Public Domain  Contributors: NASA/JPL-Caltech/P. Capak (SpitzerScience Center) Telescopes: Hubble, Spitzer, Chandra, Galex, Keck, CFHT, Subaru, UKIRT, JCMT, VLA, and the IRAM 30m.File:AM 0644-741.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:AM_0644-741.jpg  License: unknown  Contributors: NASA, ESA, and The Hubble Heritage Team (AURA/STScI)Image:Messier51 sRGB.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Messier51_sRGB.jpg  License: unknown  Contributors: NASA and European Space AgencyImage:Ngc3593.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Ngc3593.jpg  License: unknown  Contributors: Torsten Boeker, Space Telescope Science Institute, and NASA/ESAImage:NGC 3169.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_3169.jpg  License: unknown  Contributors: , ,Image:M88HunterWilson.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M88HunterWilson.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:User:HewholooksImage:NGC 3949.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_3949.jpg  License: unknown  Contributors: NASA, ESA and The Hubble Heritage Team (STScI/AURA)Image:M33.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M33.jpg  License: unknown  Contributors: Emijrp, Friendlystar, Haade, Juiced lemon, Makary, Shaqspeare, Vesta, WiniarImage:N300.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:N300.jpg  License: Public Domain  Contributors: Original uploader was Hurricane Devon at en.wikipediaImage:NGC 45 GALEX WikiSky.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_45_GALEX_WikiSky.jpg  License: Public Domain  Contributors: ,Image:Ngc4395.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Ngc4395.jpg  License: Public Domain  Contributors: Original uploader was Clh288 at en.wikipediaImage:Abell S740.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Abell_S740.jpg  License: Public Domain  Contributors: J. Blakeslee (Washington State University)Image:Galaxy color-magnitude diagram.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Galaxy_color-magnitude_diagram.jpg  License: GNU Free Documentation License Contributors: Joshua SchroederImage:Hubble ultra deep field high rez edit1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Hubble_ultra_deep_field_high_rez_edit1.jpg  License: unknown  Contributors:User:Noodle snacks

License 173

LicenseCreative Commons Attribution-Share Alike 3.0 Unportedhttp:/ / creativecommons. org/ licenses/ by-sa/ 3. 0/