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Figure 21: Examples of colliding galaxies.

Figure 22: The Milky Way and the Magellanic stream.

2.3 Peculiar galaxies

Lecture 4 : Cosmic Perspective 21.2, 21.3

In this section, we will talk about unnusual, but nevertheless very important objects : colliding galax-ies, starburst galaxies, and active galactic nuclei.

Colliding galaxies. Earlier we stated that irregular galaxies are generally small objects that aretruly unstructured. However, many galaxies originally classified as “irregular” turned out to be largegalaxies in the process of colliding and possibly even merging. Unlike stars, galaxies naturally getclose together, so interaction is likely. In Fig. 21 you can see how “tails” of material are pulledtowards neighbouring galaxies. Over time, some galaxies can orbit around each other repeatedly andgradually merge. If by chance one galaxy heads straight towards the middle of another, you can getbeautiful structures known as ring galaxies. (You will see more pictures in the lectures)

Mergers in the Milky Way . In an earlier section, we mentioned that the Magellanic clouds aresatellites of the Milky Way. The truth is a little more dramatic. Fig. 22 shows an all-sky picturein which white-blue represents the distribution of stars, and pink traces the distribution of hydrogengas. You can see that the Magellanic clouds are actually being torn apart by the Milky Way, leavinga stream of material. Over time the Magellanic clouds will be completely swallowed by the MilkyWay. Recent stellar surveys show several streams of stars in the Milky Way, which are the remains ofdwarf galaxies swallowed in the past.

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Figure 23: Three stages of star formation. In the Orion nebula (left) the stars are still completely buried. Inthe Eagle nebula (middle) they are just starting to burn out. In the Pleiades (right) there is just a little gas anddust left hanging around the new star cluster.

What Happens in a collision ? Individual stars almost never collide. The typical separation betweenstars is many orders of magnitude larger than the size of a star, so a chance collision is extremelyunlikely. By contrast, the typical separation between galaxies (Mpc) is only about 20 times biggerthan the diameter of a large galaxy, so every so often they get quite close to each other. But galaxiesare made of stars, which as we have seen above, essentially never collide with each other. So whengalaxies collide don’t they just slide straight through each other ? Actually the effects of a collision arequite dramatic. This is because what is going on is not a simple physical collision, but a gravitationalinteraction.

First, think about a single star inside an isolated galaxy. It feels the summed-up gravitational forcedue to the mass of all the other stars in the galaxy, and this force determines the motion of the star. Inother words, each star responds to the galaxy overall. Because isolated galaxies are nice symmetricalthings, that net force is usually towards the centre of the galaxy, and the star does a neat orbit aroundthe centre. Now imagine bringing another galaxy close. Our single star can then feel a force due toboth galaxies. The net result is quite complicated, and whats more keeps changing with time, as thegalaxies move closer. Rather than moving in a nice simple orbit, the stars do quite complex things.Its actually quite hard to predict the detailed behaviour with pencil and paper theory, so astronomersresort to simulating the behaviour on large computers. Sometimes these get made into very attractivemovies. Some beatiful examples are on John Dubinski’s galaxy dynamics web page.

What happens depends on the distance of closest approach (the impact parameter). If galaxies justskirt by each other they will perhaps pull out tidal tails from each other and then carry on. If they headclose towards each other, the material is likely to orbit around in a complex manner until eventuallythey completely merge.

Star formation in galaxies. We now move to an apparently separate subject which actually willlink up later... star formation. In the Milky Way and similar spirals, new stars are being formed atan average rate of about one solar mass per year. Those new stars are formed with a large range ofmasses. The massive stars are hot and blue and very luminous, but last a relatively short time - themost massive ones only last for a few million years. Low mass stars are cooler and redder and muchless luminous, and last a long time - billions of years.

In elliptical galaxies, it seems that all the stars formed a long time ago, and there is no gas to makenew stars. The massive hot blue stars have all gone, and so the galaxy looks red.

In spirals, the disc has lots of gas, and new stars keep forming, so there are plenty of luminous hotstars around, and the galaxy looks blue.

Hidden star formation. In the Milky Way, new stars are formed inside gas and dust clouds, like theOrion nebula. At first the new stars are completely hidden. Their radiation heats up the dust cloud,which then shines in the infrared. After a while, the stars start to “burn out” of their parent clouds, as

Discovering Astronomy : Galaxies and Cosmology 19

Figure 25: The radiated energy distribution of a typical AGN over a large range of wavelengths.

seen in the famous HST Eagle nebula picture. In the Pleiades, the parent cloud is almost gone, butthere is still some gas and dust hanging around. So... the energy from the very newest star formationcomes out in the infrared; from young regions, in blue light; and from old regions in red light.

Starburst galaxies. In a typical spiral galaxy, the rate of forming new stars is about one solar massper year. Ninety percent of the luminosity comes out in visible light, and ten percent in IR, partlyfrom those buried new stars, and partly from general heating of the interstellar medium. In rareobjects called starburst galaxies stars are being made at a rate of hundreds of solar masses per year.They are a hundred times more luminous than normal galaxies - however, you wouldn’t spot thisfrom a normal visible light picture, because most of that energy comes out in the IR. Visually, theseobjects often look like late-stage mergers, so somehow a galaxy collision has triggered the massiveburst of star formation.

[They are called starbursts because this phenomenon can’t last long; at a rate of hundreds of stars per year, itwill use up all the available gas in much less than the lifetime of the galaxy ]

Galactic Winds.

Figure 24: The starburst galaxyM82.

In starbursts, most of those short-lived new massive stars endup as supernovae. The outflows from all those supernovaemerge into a bubble of hot gas, which expands outwards, driv-ing a wind. Fig. 24 shows the starburst galaxy M82. Thewhitish light is the galaxy starlight; the red light shows dis-turbed gas being blown out of the galaxy. These galactic out-flows are sometimes also seen in X-ray images, as the bub-ble of gas is very hot. Some astronomers argue that this pro-cess is how ellipticals are made. Two spirals collide, a star-burst ensues, then the remaining gas is blown out, and a gas-less elliptical is left behind. We will return to this issue later.

Active Galactic Nuclei (AGN). A small fraction of galaxies showa bright starlike nucleus, with strange properties which we will discuss below. This “activity” showsitself fairly obviously in about 1% of local galaxies, known as Seyfert galaxies after Karl Seyfert whofirst studied them in the 1940s. However, we now know that at a much lower level, something similaris going on in perhaps ⇠ 20% of galaxies. At the other extreme, some of the rarest examples haveactive nuclei which are so powerful that they can completely outshine their host galaxy. For distantobjects, where the host galaxy appears small on the sky, that host can be hard to see underneath thepowerful nucleus, and so what we see just looks like a point of light. This is known as a quasar.

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Figure 26: The visible light spectrum of a typical AGN, showing broad emission lines from fast moving gas.

[Historically, quasars caused much confusion. The name is short for “quasi-stellar radio source” because thefirst objects found were radio sources but just looked like stars. Cutting a lot of history short, we now knowthat quasars are just very luminous distant examples of AGN. However, the fact that there are so many of themis of great interest, as we will see shortly.]

AGN properties : power. AGN, and especially quasars, can radiate a huge luminosity - up to a 100times more than all the starlight from a normal galaxy. That power is radiated over a wide range ofwavelengths from radio waves to X-rays (see Fig. 25, which is very different from normal galaxieswhich radiate most of their energy in visible light, with some IR. The brightest part of AGN radiationhowever comes in the ultraviolet, indicating that the main power source has a temperature of around100,00K.

AGN properties : fast gas. If we look at the visible and UV light spectrum of an AGN in moredetail, we see that as well as continuous light, we see bright emission lines at particular wavelengths.(See Fig. 26.) This shows the presence of ionised gas. We also such emission lines in star formationregions in the Milky Way - the hot stars ionise surrounding gas. In the AGN case, the bright UVsource must be ionising local gas. However, in AGN, unlike in star formation regions, the emissionlines are very broad. This is due to our old friend the Doppler effect - the gas must be moving aroundas well as being ionised. If we measure the width �� of a line at wavelength � we can calculate thetypical gas velocity from v = c ·��/�. The result is that typical AGN gas is moving at ⇠ 10,000 kms�1 - several percent of the speed of light !!

AGN properties : variability. The radiation from AGN changes on short timescales - in someobjects, days or even less. An object cannot change by a substantial amount faster than it can com-municate across itself, which happens at a speed no faster than the speed of light. In other words,from an objects of size R we shouldn’t see variations faster than

�tmin = R/c

So �t ⇠ 1 day means that R < 2.6⇥ 10

10 km. You can compare this to the distance to Pluto, whichis ⇠ 10

10 km. We find therefore that a typical AGN must be similar in size to the solar system - oreven smaller - despite being more powerful than a whole galaxy.

AGN properties : radio lobes. About 10% of galaxies containing an AGN also have giant radiolobes - large regions of radio emitting plasma, a huge distance either side of the galaxy. Such objects

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Figure 27: Radio galaxies. On the left is a map of the radio emission from the source Cygnus A showing largelobes separated by hundreds of kpc, with a bright central core, and thin jet-like features. The box indicateswhere the host galaxy is. In the middle is the visible light image of the host galaxy for a similar radio source,M87. On the right is the central region of M87, showing a jet-like feature continuing right down into the core.

are known as radio galaxies. Radio emission is caused by extremely high energy electrons spirallingin magnetic fields, a process known as synchrotron radiation. Some very energetic process has tocause those electrons to be accelerated up to such high energies. The nucleus is often linked to thelobes by thin streaks of emission called jets, terminating in hot spots. (See Fig. 27). This stronglysuggests an outward ejection - so although the AGN phenomenon has its origin in a tiny spot in thecentre of the galaxy, it can have an influence on huge regions much bigger than the galaxy itself.

AGN properties : nuclear jets. Measurements in the central regions of radio galaxies show that suchjets can continue to be seen down to very tiny scales. Repeated measurements on the very smallestscales show that structure within those jets is indeed moving outwards, and that the measured velocityis close to the speed of light.

Black holes and gravity power. So somehow we need to do the following :

• generate huge amounts of power

• do this in a tiny region

• eject high-speed jets

The only way we know how to do this is to have a supermassive black hole at the centre of the galaxy,and to have material fall onto it. Matter falling onto any gravitating object will gain energy, but ablack hole is so compact it can generate much more energy per unit accreted mass than anything else.The energy you can gain from a lump of matter �m starting far away and then falling onto a massM and radius R is

E =

GM�m

R

So a for a given M the trick is to make R as small as possible. The smallest possible size is that of ablack hole, the size of which can be taken as the radius of the event horizon or Schwarzschild radius,which is given by RSchw = 2GM/c2. (We won’t prove that in this course !) Nothing can escape fromthe event horizon, including radiation. So even if our mass M were to shrink below this size, anyenergy gained by matter falling past this point won’t get out. So the absolute maximum amount ofenergy our mass �m could liberate would be given by formula above if we substitute R = 2GM/c2

E = GM�m · c2

2GM=

1

2

�mc2

So it looks like we could extract half the Einstein rest mass energy of �m. Can we get that much inpractice ?

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Figure 28: Evidence for nearby supermassive black holes. The left hand picture shows HST data from M87showing the the H↵ emission line from hydrogen gas. At locations either side of the nucleus the emission isdisplaced in wavelength, indicating rotation. The right hand picture shows the orbits of stars in the nucleus ofthe Milky Way, measured by groups in Germany and California. An animated version can be seen here

Black hole accretion discs. If the available gas simply fell radially downwards towards the blackhole, the energy it would gain would be kinetic energy, and it wouldn’t give much radiation; it wouldjust disappear down the black hole. However, if, as is very likely, the gas is rotating around the blackhole, it will actually spiral in slowly. This is called an accretion disc.

The energy is gained more gradually. As each layer of the disc slips downwards, half of the gainedgravitational energy goes towards speeding up the rotation; the other half goes into heating up thegas, so finally we get radiation. The temperature we expect is about 105K, just right for explainingthe ultraviolet radiation we see. Detailed calculations show that at three times the event horizon, gasorbits become unstable and after that the gas plunges into the black hole. So the true inner radius forgenerating energy is ⇠ 6GM/c2, and the maximum output we can get is ⇠ �mc2/12.

The rotation can also cause twisted magnetic field lines. Charged particles then get flung out alongthese field lines to form the jet.

The broad picture of accretion onto black holes explaining AGN seems quite convincing, but thedetails are still controversial - this is a very active area of research (pardon the pun).

Evidence for massive black holes. Are there really massive dark objects at the centres of galaxies?Most nearby galaxies that seem to be “passive” i.e. not showing obvious current signs of activity,nonetheless show stellar motions that seem to be faster than you can explain with those stars alone,i.e. there are signs of a dark central mass in essentially all galaxies. There is particularly strongevidence in two very nearby galactic nuclei - M87 in the Virgo cluster, and in the centre of our veryown Milky Way.

In M87 we can see gas that on one side of the nucleus is receding from us, and on the other side isapproaching us, indicating rotation at a speed of ⇠ 800 km s�1. Because we know the distance, wecan calculate the central mass from Newton’s laws, just like we use the orbital velocity of the Earthto calculate the mass of the Sun. The result is a mass of ⇠200 billion solar masses. However, thestarlight we can see in this central region represents much less than this - so its not the stars causingthis - its a dark mass.

At the centre of our own Galaxy, there are some luminous stars which we can spot individually. Overa period of years these stars move, so we can track their orbits. These orbits indicate an unseen objectwith a mass of about two million solar masses. The size of this object must be well inside all thosestellar orbits, which makes it quite small, and hard to be anything other than a black hole.

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Evolution of AGN. Luminous quasars are seen at very large distances, with large redshifts. As weshall discuss more carefully later, objects at large distances are seen as they were in the past. Thenumber of quasars we see at such distances, compared to the AGN we see locally, indicates that suchluminous AGN we much more common in the past. This is a topic we will return to in a later section.