SIRTF a Window on Cosmic Birth

8/7/2019 SIRTF a Window on Cosmic Birth

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Cower PhotoThe Pleiades Nebula0 alifornia Institute of Technology

Cover Illustrations

Asteroids Collide ~-Stellar Birth

Quasar Telescope inSpace


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SIRTFSpace Infrared Telescope Facility

A VMndowon Cossmic Birth

Written by:Dr. Nelson J. lrvineDr. Cynthia E. lrvineDr. Donald GoldsmithDr. Michael W. Werner

National Aeronautlcs andSpdce AdministrailonAmes Research CenterMoffett Field Calliornla 94035


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Introductionach window on the universe -each type of light - provides aunique view which leads to new

understanding and furtherquestions.

During the pa st 40 years, as hum ans have learned new ways to lookat the universe, we ha ve discovered secrets which were previouslyconcealed from our view like the pa ges of an unopened book. Onlyvisible light and radio waves can pa ss totally unimpeded through theatmosphere to observatories on the ground. Before the dawn of thespace age, only brief and tantalizing glimpses of the unseen pa ges wereobtained with instruments carried above the atmosphere by rockets,balloons, and aircraft. These brief glimpses showed the universe to berich in objects and pheno mena extending far beyond the s tars, planets,and nebulae visible to the human eye and know n to early astronomers.

During the p ast 15 years, an array of orbiting satellites has allowed usto observe what could never be seen before - a universe full of gammarays and X-rays, of ultraviolet and infrared light. The orbiting satelliteshave shown how each pa rt of the spectrum of light- rom radio waves togamma rays - eveals unique aspects of the universe, and that e ach isesse ntial for our understanding of the cosmos . The m ost recent of thesepioneering achievements - he preliminary examination of the universeusing infrared light - concluded with the final observations m ade by theInfrared Astronomical S atellite (IRAS) on November 21, 1983.

In the future, long-lived orbiting observatories will allow astronomicalobse rvations in all portions of the spe ctrum of light. For the scientificallycrucial investigations to be ca rried out with infrared light, the future is theSpace Infrared Telescope Facility (SIRTF).

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The dark lanes seen in the visible photo of the Milky Way (top) are clouds of dust thatobscure our view of the numerous stars behind them. The real shape of our galaxy isrevealed in the infrared image (below) obtained by IRAS. Infrared light penetrates thedust clouds and shows that the galaxy appears as a thin disk, just like the edge-on spiralgalaxies we see throughout the cosmos.

INFRARED 1 Radio WavesGamma Rays X-rays Ultraviolet I1 0

C

en Complete Atmospheric2 AbsorptionEPmC+

n" VisibleModern astronomy uses all regions of the electromagnetic spectrum. The Earth's atmosphere absorbs most infrared light, as well as gamma rays,X-rays, arid ultraviolet light. To observe this radiation we must go above our atmosphere.

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The H orsehead Nebula.A wide variety of molecules are synthesized in the cool, protected interiors of dark clouds such as this.

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Infrared light or infrared radiation, sometimes called h eat radiation,was first identified tw o centuries aao bv Sir William Herschel. a Britishhat is Infrared - -astronomer best known for his discovery of the p lanet Uranus. Herschelsent bea ms of sunlight through a prism, which dispersed or separatedthe ligh t into the rainbow of visible colors: violet, indigo, blue, green ,yellow, orange, and red. He used a thermometer to measure the eneravRadiation? 1.carried by each color o f light. But the tempera ture rose even if thethermom eter was placed in the part of the spectrum beyond the red.Something was reaching the thermometer - but what?

Infrared radiation, althoughand information in the same wayto the carries energythat visible light does. Hersche l realized that som ething like visible light, but invisible tohuman eyes, must have forme d part of the beam of sun light. He calledthis infrared, radiation beyond the red . Today we recognize thatinfrared radiation has longer wavelengths and lower frequ encies (fewervibrations per second) than red visible light does. H uman eyes are blindto the infrared, but we have made de tectors sensitive to infraredradiation, often composed of crystals of german ium or silicon which aredoped with small quantities of other elements. Infrared radiationshining on these crystals produces electrical signals which allow us todetermine how much infrared is reaching them. An array of thousands ofsuch detectors provid es a sensitive camera for imaging the universe inthe infrared.

The upper panel shows an ordinary optical photograph of a small region of a dust cloudlocated near the belt of Orion. An infrared camera using a detector array provided theinfrared image of the same field shown in the lower panel. This reveals many stars unseenin the ordinary photo, and illustrates the ability of infrared observations to study objectshidden within dark clouds. Much more powerful imaging capabilities will be provided bythe infrared detector arrays to be used on SIRTF.

0 National Optical Astronomy Observatories

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1. Infrared observations can examine the u niverse as it was when itwas on ly a fraction of its present age. Because of the e xpansion of theuniverse, such a look back through time means that we observe objectsmoving away from us at high velocities. As a resu lt their light is stretchedin wavelength or red-shifted from visible or ultraviolet wavelengths intothe infrared region of the spectrum.

InfraredAstronomy2. The dust clouds which are found in most astrophysicalbservations of the universe withinfrared radiation are uniquelyimportant for the followingreasons: environments absorb starlight very efficiently and reradiate the energyin the infrared. As a result, much of the rad iant energy in the universe isfound at infrared wavelengths. Galaxies have bee n identified whichproduce hundreds of times m ore infrared than visible light energy. Somegenerate more than on e thousand times the energy output of our e ntiregalaxy and are among the most energetic objects in the universe.3. Cool objects radiate primarily at infrared wavelengths. Com pleteunde rstanding of the planets, of stars cooler than the Sun, and o f cloudsof gas and dust in interstellar space requires infrared observations,which can also lead to the discovery of new types of cool objects notseen in any other way.4. Infrared observations allow us to peer into dusty reg ions -t h ecores of the galaxies and star-forming clouds - to study phenomenahidden from view in visible light.5. Atoms an d m olecules absorb and radiate particular frequencies ofinfrared radiation, m aking fingerprints in the radiation we observe.From these fingerprints we can identify and study atoms and moleculesin p lanetary atmospheres, interstellar clouds, and distant galaxies.

Energy

Ultra- Visible Near Farviolet Infrared Infrared

Any object warmer than absolute zero emits electrom agnetic radiation at allwavelengths, with a characteristic spectral shape determined by itstemperature . Hot objects, like the Sun, emit most of their radiation in thevisible-light reg ion. Cooler objects, in space or on the Earth, radiate primarilyin the near and far infrared.6


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Infrared astronomy from the Earth's surface is severe ly ham pered bythe life-giving water vapor that pervades our atmosphere. To avoid theabsorption produced by water vapor, astronomers have gone to greatlengths - and great heights. They have es tablished infraredobservatories on mou ntains so high that altitude sickness can b e aproblem . They have taken aircraft IO miles above the Earth's surface,and they have launched balloons to still higher altitudes. These effortshave yielded improved knowledge of the cosm os, but the full rang e ofinfrared observations can b e realized only from space.

The unavoidable infrared brightness of the atmosphere and a warmtelescope drown weak cosm ic infrared sources in a se a of localradiation. As a result, infrared observations from within the atmosphereare as inefficient as optical observations in broad daylight. In space , freeof the atmosphere, we can cool the telescope to just a few degreesabove absolute zero, so that its own infrared radiation is very weak. Thesupercooled telescope in space thus becomes 1,000 to 10,000 timesmore sensitive to infrared radiation than the be st telescopes on or n earthe Earth's surface. IRAS pioneered this advanced technology forinfrared astronomy; SlRTF will bring it to m aturity.

Astronomers working from within the atmosphere have made infrared observations using instruments mounted on telescopes at mountaintopobservatories, flown in aircraft, and suspended from balloons.

0 rnithsonian Astrophysical Observatory

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In 1983, the great veil of the cosm os was b riefly lifted when theInfrared Astronomical Satellite (IRAS) scanned the skies for 10 months.IRAS, a joint effort of the United States, the U nited Kingdom , and theNetherlands, was sensitive to four different frequency regions within theinfrared portion of the spec trum. Helium in its extremely co ld superfluidform was used to cool the telescope and its detectors to 4 degreesabove absolute zero (-454F). Freed from Earth's interferingatmosphere, IRAS surveyed almost the e ntire sky twice between itslaunch on January 25, 1983, and Novem ber 21, 1983. On that date,three-quarters of the way through a third sky su rvey, the supply of heliumcoolant was depleted, the IRAS telescope warmed up, and its infrareddetectors ceased to function.

IRAS provided an infrared survey a thousand times more sensitivethan any previously available. In its brief, astoundingly successfullifespan , IRAS observed a quarter of a million individua l sources, only afew thousand of which had previously been observed in the infrared.The catalogs of infrared sources com piled from the IRAS observationscontain a myriad of objects and phenome na unknown to astronomersbefore 1983.

IRAS's infrared imageof Comet IRAS-Araki-Alcock revealed an immense tail of dust,invisible in ordinary light. This comet came within 3 million miles of Earth, the closestcometary approach to our planet in more than two centuries.

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Among IRASs many discoveries, the m ost exciting were our firstglimpses of what may be solar systems in formation. IRAS found d isks ofparticles surrounding the nearby stars Vega, Fomalhaut, and BetaPictoris. The disks consist of bits of dust, ice, and rocky m aterial, andthey may signa l the presence of planet-sized objects around these stars.

WhatIRAS FoundMuch closer to home, IRA S revealed hitherto undetec ted matter in thesolar system. Six new comets, each faintly glowing in infrared emission,

revealed themselves to IRASs detectors. Mysterious trails of dust werefound to accompany other come ts in their orbits around the Sun. IRA Salso found, strewn among the asteroids between Mars and Jupiter,enormous bands of dust particles, thought to have been created incountless collisions between asteroids.IRAS found numerous protostars - stars still forming - shrouded incocoons of dust. From these dus t-laden stellar nurseries, stars like ourown Sun will shine forth to join the hundred s of billions of other stars inthe galaxy.IRAS also discovered thin w isps of infrared emission from regionsabove and below the p lane of the M ilky Way. C alled the infrared cirrus

because it resembles terrestrial cirrus clouds, this emission is thought tobe produced in part by large, complex m olecules, which must thereforeabound in the interstellar medium.

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An artists conception depicts the cloud of dust, ice, and rocks in orbit around the starVega.0 Seth Shostak. K apteyn Laboratorium (Netherlands)


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This image of the constellat ion Orion as seen byIRAS uses false colors to show the infraredradiation. Within Orion lies a great mass of dustand gas, from which the young stars in Orionhave formed within the past few million years.The band of blue at the top traces the locationofwarmer foreground dust within the solarsystem.

IRAS found bands of dust in orbit around the Sun, between the orbits of Mars and Jupiter,tilted with respect to the orbits of the planets. This dust is thought to be debris fromcollisions between asteroids like that shown in the artists conception.

Galaxies are the building bloc ks of the universe. IRA S provided thefirst assessment of their total energy outpu t by detecting infraredradiation from some 20,000 galaxies and showing that most galaxieslike our own shine as brightly in the infrared as in visible light. The IRAScatalog also contains another class of galaxies, which radiate hundredsof times more infrared than visible light. These luminous infraredgalaxies radiate as prodigiously as do the distant and mysteriousquasars. Together with IRASs discovery of a new population of infraredquasars, this has raised the intriguing possibility of an evolutionaryconnection between infrared galaxies and quasars.

IRASs spectacular achievement was a beginning, not an end. Wecan think of IRAS as a quick, blurry first look through a new window onthe universe. IRAS was primarily intended to survey the infrared sky, notto m ake detailed studies of the objects it discovered. A more powerfulinfrared observatory in space is needed to answer the questions raisedby IRASs discoveries and to continue the exp loration of the universethrough the infrared window. The S pace Infrared Telescope Facility(SIRTF), to be launched by NASA in the 199Os,will do this.

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SIRTFWill Be a TrueAstmnomikalObservatory

SlRTF will be placed into orbit by the S pace Shuttle and then bo ostedto a n orb ital altitude of 900 kilometers by N ASAs Orbital ManeuveringVehicle. SlRTF will be ope rated as an international a cility, withobservations conducted from a ground-based control center andobserving time awarded to the b est research proposals submitted byastronom ers from a ll countries .SlRTF will orbit aboard a platform-type spacecraft, possibly onedeveloped as part of N ASA s Space Station program. The spacecraftwill prov ide the bas ic func tions - powe r, pointing, and thecommunications link to E arth -tha t are needed to support the SlRTFmission. An essential function of the spacecraft is to keep SIRTFstelescope oriented so that the Suns rays never reach the co ld interiorsurfaces of the telescope.SIRT Fs telescope, and its infrared instruments, will reside within acylindrical cryogen tank. The hollow walls of the tank will co ntain thesuperfluid helium that cools the telescope to its operating temperature, afew degrees above absolute zero.SlRTF will carry three versatile instruments to analyze the radiation it

collects. The M ultiband Im aging Photometer will m easure the intensity ofradiation at all infrared wavelengths. The pho tometer plus the InfraredArray Camera will provide infrared images tremendously improve d indetail, allowing the identification and study o f objects discovered byIRAS and carrying out deeper infrared surve ys as well. The InfraredSpectrograph will obtain infrared spectra, dividing the infrared radiationinto its constituent wavelengths and providing unique new informationon the composition, temperature, and mo tions of both nearby and verydistan t objects - ncluding objects discove red in SIRTFs own surveys.SIRTFs long lifetime - 5 years or more -w il l permit astronom ers of alldisciplines to use the facility to carry out a wide variety of astrophysicalprograms. It will provide ongoing coverage of v ariable objects, such as

quasa rs, as well as the capability to study rare and transient events suchas com ets and supernovae. SIRTFs long lifetime will also allow it todistinguish nearby ob jects by detecting their gradual mo tions relative tothe more distant background stars.

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SlR TF will be a s ophisticated astrophysical facility for the study of thesuccess in its use of a superbly instrumented, supercooled telescope inorbit above the atmo sphere. SlRTF w ill investigate in detail thephenomena disco vered by IRAS. In addition, SIRTFs pow erfulobservational capabilities will allow it to study scientific problemsuntouched even by IRAS.

Theo b s e ~ a t o ~osmic Sources that em it infrared radiation. SlRT F will build on IRASs

The Cone Nebula in the constellation Monoceros contains huge amounts of dust thatabsorb visible light. Infrared light from within the nebula can escape with relative ease,revealing newly formed stars that remain hidden from optical investigations.0 Anglo-Australian Observatory

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SlRTF will have a mirror 0.85 meter in diameter, with twice the area of IRAS's mirror. In addition, SlRTF willincorporate the following major advances:Spectral Coverage

Sensitivity

Imaging Capability

Spectral ResolutionSatellite Lifetime

SIRTF's detectors will cover infrared wavelengths from 2 to 700micrometers (25micrometers equalsone-thousandthof an inch) - a region six times broader than IRAS covered. SIRTF's spectrum ofobservations will be fully 200 times wider than the visible-light spectrum that our eyes can detect.SIRTFs detectors will reach the fundamental sensitivity limits set by the faint infrared backgroundsin space. Taking into account as well the larger size of SIRTF's mirror and the longer observationtimes available, SIRTF will be able to detect infrared sources thousands of times fainter than thoseseen by IRAS.SIRTFs larger, more perfect mirror and more stable pointing will allow it to achieve an angularresolution of one arcsecond and thus to form images hundreds of times sharper than those providedby IRAS. The large detector arrays to be used on SIRTF can image an entire planet, comet, or galaxywith this level of detail in a single exposure.SIRTFs spectrographs will be able to discriminate among neighboring spectral wavelengths with aresolution of better than one part in one thousand-a scale 100times finer than that achieved by IRAS.The use of on-orbit cryogen resupply will permit SlRTF to achieve a lifetime in excess of 5 years,six times IRAS's 10 months of active life.

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DiscoveryPrograms:TheSolarSystem

xcept for the p lane ts themse lves, most of the objects in our solarsystem - planeta ry sate llites, asteroids, and the frozen nuclei of distantcomets - are so small that they can be im aged in detail only b y deepspace probes. But these objects may contain the history of the solarsystem, history which can be read from the Earth by infraredobservations.Comets in particular are the oldest of all objects in the solar system,primordial lumps of rock and ice that accumulated m ore than 4.6 billionyears ago, as the Su n and its family of orbiting debris conde nsed from arotating pancake of interstellar gas an d dust. Ma ny of the smallersatellites of the g iant plane ts are likewise thought to con sist of primordialice and rock, perhaps even to be comets captured into their presen torbits billions of years ago. SlRTF will allow us to measure the chem icalcomposition and the temperature of the se m ost primitive objects in thesolar system, and thus to study what the solar system was like when itformed. SIRTFs measuremen ts of the changing app earance of cometsas they approach an d recede from the Sun will be particularly useful fordeterminations of the structure and com position of com etary nuclei. Forexample, SlRTF can study com ets like Comet Halley, not only duringtheir rapid and infrequent passages through the inner solar system, but

also when they are as far from the Sun a s the planet Uranus. Charon,the m ysterious moon of Pluto -t he largest satellite in proportion to itsplanets size -will yield some of its secrets to SIRTFs spectrograp hs,possibly to reveal a history uniqu e among a ll solar-system objects.

Infrared image of Saturn at a wavelength chosen so that sunlight is reflected by the ringsbut absorbed by methane in Saturns atmosphere.0 JPL, California Institute of Technology

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IRAS found disks of matter around several nearby stars that suggestthe formation of planetary systems. By applying SIRTF s high resolutionand sensitivity to the study of circumstellar ma terial around nearby stars,we can learn how commonly such disks occur, as well as theirdimensions, structure, a nd ch em ical composition. By solar-systemstandards, the rings have immense sizes. For example, the ring a roundVega ha s a radius 100 times the distance between the Earth and theSun. If smaller rings of warmer matter analogous to the Suns owninterplanetary dust cloud occur around nearby stars, SIRTF will b e ableto observe them as well.

Planetary*stemsInfrared observations extend thehunt for planets beyond theboundaries of our solar system.

Even m ore exciting is the possibility that S lRTF will e nable us for thefirst time to observe directly a planet around another star. SIRTF coulddetect Jupiter-sized objects around the nearest stars, and could detectbrown dwarfs - arger planets, with masses a few dozen tim es Jupitersmass -aroun d stars within a few hundred light-years of the Sun. TheSlRTF data, complemented by other observations, will provide us w ithan exce llent picture of the nature of planetary systems that areassociated with stars in the vicinity of the S un. Such studies will he lp usto understand whether the circumstances which gave rise to life on theEarth occur commonly or rarely.

Superimposed on this artists conception of the dust cloud around Veg a is a scale thatshows the size of the cloud (1 AU = Earth - Sun distance = 93 million miles: J and P showthe sizes of the orbits of Jupiter and Pluto, respectively) and a box that represents a SIRTFpicture cell or p ixel, he basic size unit of SIRTF observations. SIRTF will be able toobserve de tails as small as the pixel size.

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BirthandDeath of StarsSlRTF is an ideal instrument forthe study of stars and planetarysystems in formation.

Stars continually form and die within our galaxy. Indeed, the galaxywas more than half its present age before the Sun began to form. SlRTFis ideally suited for the study of star formation. Stars are bo rn within thickclouds of interstellar gas and dust. These dust clouds absorb any visiblelight from stars which are forming within them, bu t infrared radiation fromthese stars in formation can escape from even the densest cloud. SlR TFwill provide images and spectra of star-forming regions in theneighborhood of the Sun. These images will greatly increase ourunderstanding of the chemical, structural, and dynamical evolution ofstars in the ea rliest stages of their lives. Wh ile allowing us to study theformation of stars like the Sun , SlRTF may also permit us to examine theearly evolution of planetary systems like our own.Dust shrouds the dea th as well as the birth of stars. In their deaththroes, many stars lie concealed within dense shells of dust and gas.SIRTFs intruments will be able to penetrate these dust man tles toexplore the final ph ase s of a stars life.

This artists conception of a newborn star, still hidden in visible light by the dust cloudswithin which it formed, shows matter in orbit around the rotating star. Such leftover debrismay ev entually form cornets, planets, satellites, and asteroids. Mate rial sq ueez ed out bythe formation process is thought to be ejected along the star s rotation axis in relativelynarrow, high-velocity streams of matter.

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Specfroscopyand msmicChemistrySIRTFs spectrographs will greatlyenhance our knowledge of thechemical and physical conditionsin space.

One of the principal tools of the m odern astronomer is thespectrograph, which separates the radiation from a celestia l object intoits constituent wavelengths or colors and the n records this spectrumelectronically. Su ch a spectrum is a kin d of cosmic fingerprint, a patternof greater and lesser amounts of radiation at different wavelengths. Thedetails of such a pattern depend bo th on the specific types of atoms ormolecu les in the object and on the na ture of their environment. Thesespectral features reveal much about the conditions in the regions givingrise to the infrared radiation we observe. These conditions include notonly the chemical composition, temperature, and density, but also thepressure, rotational velocity, and magnetic field strength. SIRT Fsand molecules, greatly enhancing our knowledge of the chemical andphysical conditions in planets and stars, i n the interstellar medium, andin distant galaxies.

spectrographs wi!! record ?he nfrared slgnl,!ures n! nlrmernus a!tgms

In recent years, astronomers using infrared and microw avespectrographs have discovered a rich and unexpected variety ofcomplex molecu les in the space be tween the stars. Chains of carbonatoms w ith associated clusters of hydrogen, oxygen, and nitrogenabound in dark interstellar clouds where they are shielded fromultraviolet radiation. More durable carbon-ring molecules, complexversions of familiar substances such as benzene , apparently existthroughou t interstellar space. S IRTFs spectrographs, studying theun ique spectral fingerprints of each type of m olecule, will iden tify newmolecular species and provide detailed information on the chemicalprocesses which supply this fascinating cosmic chemical storehouse.These studies could have unexpected implications for other areas ofscience; the types of molecules found in interstellar space are thosethought to have been abundant when primitive life formed in Earthsoceans.

I1 I7I I 1 I 1WAVELENGTH +Composite infrared spectrum of a dust-embedded protostellar source, showingabsorptions due to icy and mineral grains in the interstellar medium. Proposedidentifications for the absorbing species: 1 and 5-water; 2-methyl-isocyanide;3-carbon monoxide; 4-carbonyl sulfide; 6-methyl a lcohol; 7-silicate m inerals.

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Shrouded in veils of dust, invisible in ordinary light, the dense centerof our galaxy lies some 30,000 light-years from Earth. Infrared radiationcan penetrate the dust to allow detailed investigations of the core of thegalaxy. SIRTF's unpreceden ted sensitivity and improved resolution willprovide new details of the structure and mo tions at the galactic center.These observations may provide further evidence for the mass ive blackhole suspected to exist at the very heart of the galaxy .

The motions of stars and gas within a few light-years of the center of the galaxy indicatethat a massive black hole with a million times the Sun's mass may lie at the galactic core.Matter streaming toward the black hole would be heated by high-velocity collisions andemit radiation before falling completely in.

The concentration of matter and luminosity toward the center of the galaxy is clearly seenin IRAS's image of the inner few thousand light-years of the galactic plane.

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Infrared Galaxies Seen in the in frared, away from the Milky Way, the n ight sky isdomina ted by distant galaxies, each containing billions of stars. IRASprovided our first in frared glimpse of the realm of the galaxies; SIRTFwill provide the first deta iled pictures.Interacting and colliding galaxies are unusually com mon amongstrong infrared galaxies. How ca n a collision betwee n two galaxiespower an infrared source which is 10 to 100 times brighter than the

starlight of an entire galaxy? Two compe ting theories exist: hot stars andblack holes. The infrared emission from these infrared galaxies probablycomes from clouds of warm interste llar dust, which m ay be hea ted byyoung, short-lived, hot stars which have formed w ithin them. Thecollision of two galax ies may trigge r intense bursts of star formation,which in turn produce large amounts of infrared radiation.The second theory assigns the heating of the dust to an outburst fromthe vicinity of a du st-shrouded, supermassive black hole in the center ofthe galaxy. Such an outburst in the core of the galaxy c ould be triggeredby a collision with ano ther galaxy, or even by a near miss that markedlyincreases the rate at which matter falls into the ga laxys centra l blackhole. SIRTF s detailed imaging and spectroscopic studies of infrared

galaxies should p rove crucial in resolving the nature of these ob jects.

The gravitational forces between these two interacting galaxies have produced the tail-likeshapes that have led to the galaxies familiar name among astronomers: The Mice.0 National Optical Astronomy Observatories

A computer simulation of colliding galax iesshows matter drawn into long tailsbygravitational interaction.21


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QuasarsSince their discovery 25 yearsago, quasars have been one ofthe major enigmas and attractionsof astronomy.

Quasars are apparently the m ost distant and the m ost pow erfulsources o f radiation yet detected. In the most conse rvative theories, theirtremendous luminosity comes from vast amounts of matter heated toenormously high temperatures just be fore being swallowed by asuperm assive black hole at the center of the quasar. Much more exoticprocesses have also been proposed.The nearby infrared galaxies discovered by IRAS are more powerfuland m ore numerous than are the quasars in the same volume of space.If a dust-shrouded black hole is the central powerhouse of such aninfrared galaxy, its radiation could burn through the dust shell. Theregion around the black hole itself would then s hine forth to appear as aquasar, overw helming the starlight from the surrounding galaxy. IRAShas given further tantalizing hints that this may occu r by disco veringinfrared quasars, which may be the m issing link in the evolution ofluminous infrared galaxies into optically bright quasars. SlRTF canexplore this po ssible evolutionary connection to great depths by detailedstudies of individual objects and by d etermining how the num bers ofinfrared galaxies and infrared quasars have varied as the universe hasevolved.The most distant known optically visible quasar is reced ing at 92% ofthe speed of ligh t. It is so distant that the ligh t we receive from it has bee ntraveling through sp ace for approximately 10 billion years. If infraredquasa rs similar to those discovered by IRAS are present at greaterdistances, SlRTF can observe them and thereby probe still deeper intospace and time.

An artist s conception shows a quasar as a tremendous outburst in the core of a far-distantspiral galaxy.

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A portion of SIRTFs observing time will be devoted to examiningThe sun/ey small areas of the sky at the greatest achievable sensitivity. The resultsof this more sensitive survey should be as e xciting as those of IRAS. TheDeep Survey may provide our first look at primordial galaxies, younggalaxies, seen as they w ere billions of years ago, in the most distantobservable reaches of space. It should also prov ide us with our firstobservations of nearby, but weakly radiating, objects such a s b rowndwarfs. Finally, as the results from IRAS show, such a survey atunprecedented sens itivity levels will lead to the discovery of new andunexpected phenomena in the universe.

S/RTF w ill see much deeper intospace than did IRAS.

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Brown m a r i band theMissing MassMost of the mass of our galaxy hasnot yet been properly identified.

Astronomers have deduced the total ma ss of the Milky Way from themotions of the stars within our galaxy. These m otions depe nd on thecomb ined gravitational force from all the ob jects in the galaxy, an dreveal the presence of far mo re gravitational force than the directlyobserved dust, gas, and stars can provide. Furthermore, other galaxiesare thought to possess extensive, invisible halos that con tain most oftheir ma ss. This missing mass, both in our galaxy and in other g alaxiesmay take the form of brow n dwarfs. Brown dw arfs are failed, w ould-bestars, spheres of gas with too little mass to ignite the fusion reactions thatpowe r true stars. The heat energy released as a brown dwarf slowlycools should appear primarily as infrared light. SIRTF may b e ab le tomeasure the accumu lated infrared radiation of a halo of brow n dwarfsaround a nearby galaxy. Even if such objects are no t sufficientlynumerous to supply the missing mass, a numb er of them most likely arein the near vicinity of the Sun. SIR TF s deep survey should furnish ourfirst detailed inform ation on the a bundance a nd ph ysical characteristicsof these po orly studied constituents of the universe.

An artists conception of a group of faint brown dwarfs silhouetted against the backdrop of the immensely rich star fields of the Milky wa y.Brown dwarfs may prove to contain most of the mass of our galaxy and of similar galaxies .

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Beyond the quasars lies a region so re mote and so far in the pas t thatwe expect to find galaxies forming out of the origina l matter created inthe b ig bang ?hatbegan the expansion of the un iverse. As a result of theexpansion, the radiation from these events may be so red-shifted as tobe.detectable only from SIRTF. SIRTF offers us the exciting possibility ofwitnessing the birth of galaxies near the beginn ing of time,approximately 15 billion years ago, beyond the limits of the presen tlyobservable universe.About 15 billion years ago, theuniverse began its expansion in aprimordial, all-inclusive "bigbang. '

SlRTF will allow us to observe cosm ic birth and evo lution at work.SlRT F can observe enormously distant, and hence tremendously red-shifted, primordial galaxies. SlRT F and other telescopes can studysimilar objects at intermediate distances and nearby examples a s well.SlR TF will therefore allow us to describe the evolution with time of thephysical and chemical properties of galaxies. This will help guide ourunderstanding of how ga laxies develop from massive clouds ofhydrogen and helium into highly organ ized systems of s tars, gas, anddust that contain a rich variety of chemical elements - a variety requiredto produce and sustain what we call life.

This "false color" image shows a quasar (brighter object) interacting with a galaxy (fainterobject) that has apparently been distorted in shape by the gravitational force from thequasar. Such interacting systems often prove to be strong sources of infrared emission.

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SlRTF will provide us with unique opportunities o examine theprocesses of cosmic birth o ver an enorm ous range of physical scalesand environments.SlRTF can study sm all bodies in the outer solar sys tem, bodies intowhich the history of the formation of the solar system may literally befrozen.from the doorsteD of the solarsystem to the fringes of the

universe, SlRTF will reveal objectsand events never before observed. SIRTF can revea l the presenceof planetary systems, both alreadyformed and in the process of formation, that may exist around nearbystars. Such observations will provide further insights into the birth of ourown solar system.

All along the spiral arms of our Milky Way galaxy, new generations ofstars are being born, hidden from the v iew of optical telescopes bycocoons of interstellar dust. SlRTF will allow u s to probe our galaxy'sstellar nurseries and provide us with a new u nderstanding of the birth ofstars.Through observation of d istant primord ial galaxies, SlRTF will allow usto see the birth of galaxies, and thus to understand how the Milky Way

itself came into existence.SIRTF will provide the detailed data that we need to begin tounderstand the birth processes of planetary system s, of stars, and ofgalaxies. By revealing the details of how these objects are born andevolve, SIRTF will add to our know ledge of how the universe is anunderstanding of how the universe came to

A computer-generated picture modeling the infrared radiation from distant galaxies.Images like this will be obtained by SIRTF's infrared detector arrays in studies of theevolution of galaxies over billions of years.

0 . L. Wright, UCLA

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EpilogueWith the evaporation of IRASs coolan t in Novem ber 1983, astronomyonce ag ain became nearly blind in much of the infrared. IRASsremarkable results dramatize both the scientific promise an d the urgencyof NA SAs next step in infrared astronomy: SIRTF.SIRTF is one of the fam ily of Great Obse rvatories scheduled tobecom e operational in space during the 1990s. Other family m embers

observations), he G amma Ray Observatory, and the A dvanced X-RayAstrophysics Facility. Working together, this family of observatories willcover all of the electromagne tic spectrum from gamma rays through thefar infrared. A decade from now, for the first time in our history, we shouldhave continuous cove rage of the spectrum of celes tial radiation.

ai2 Hl;b& Space Telescope (for vi&qe-!igh? 2nd u!!:a;#i$e!

The new family of orbiting observatories will unseal the hidden pagesof the book of cosmic electromagnetic radiation. Once we can read allthe pages of that book , we c an expect to find a profoundly exciting story,one ev en more fascinating than the partially told tale we have alreadyuncovered.

H. G. Wells wrote:. . . the past is but the beginning of the beginning, and all thatis and has been is but the twilight of the dawn . . . .A day willcome when beings who are now latent in our?houghts shallstand upon the earth as one stands upon a footstool, and shalllaugh and reach out their hands amid the stars.

SIRTF, which w ill open the infrared frontier, represents a key step inhum anitys effort to know the cosmos - and to b ring this vision to reality.

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SIRTF Science WorkingGroupDr. Giovanni Fazio ....................................................................... Principal Investigator, Infrared Array Camera

Principal Investigator, Infrared SpectrographDr. Michael Jura.. ................................................................................................... Interdisciplinary ScientistDr. Frank Low .................................................................................................................... Facility ScientistDr. George Rieke ............................................................... Principal Investigator, Mu ltiband Imaging PhotometerDr. Edward Wright ................................................................................................. Interdisciplinary ScientistDr. M ichael Werner ............................................................................................................ Project Scientist

Smithsonian Astrophysical O bservatoryCornell UniversityUniversity of California, Los AngelesUnive rsity of ArizonaUniversity of ArizonaUniversity of California, Los A ngelesNASA Ames Research Center

Dr. James Houck .........................................................................

Participating InstitutionsNASA HeadquartersNASA Ames Resea rch CenterNASA Goddard S pace Flight CenterCalifornia Institute of TechnologyCornell Un iversityJet Propulsion Laboratory

Pennsylvania State Un iversitySmithsonian A strophysical ObservatorySteward Observato ry, University of A rizonaUniversity of Californ ia, BerkeleyUniversity of California, Los AngelesUniversity of Rochester

All p hotographs are NASA photograph s unless credited otherwise.The SlRTF Program is managed by Ames Research Center for the Astrop hysics Division, office Of space Sciencesand Applications, National Aeronautics andSpaceAdministration.

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SIRTF a Window on Cosmic Birth

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