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2. SC IENCE WITH T HE LA RGE SPACE TE L ESCOPE The LST will have two astronomical capabilities that are unique: ultraviolet study of faint objects and high-resolution, diffraction-limited imagery. In addition, Infrared and submillimeter detectors can cover a further three decades of wavelength, from I!Jm to I Jllln. Emhuslasm for the LST is wideÂ· spread among astronomers, who see how these unique cupablllties can be utilized to solve a variety of problems at the frontier of astronomy. Of special interest are the capabilities of the LST for study of the evoluÂ· tlon of galaxies and the universe. Despite the great advances in understanding of the blrlh, evolution, and death of stars on both the observational and theoretical fronts since the early 1950s, there has been very llnle increase in our baoic undersllnding of the physics and evolution of galaxies. This applies even to the under1tanding of our own galaxy-e.g., of Its chemical evolution and of the nature and origin of the activity in its cenual rCJion or nucleus. We still do not know precisely the ratio of abundances of the two most irnÂ· porlant elements, hydrogen and bellum, in the oldest stars of our galaxy. If we can determine this ratio, we will have important Information about the origin and early history of the universe. It should be possible to achieve a thorough understanding of the ways in which stars have fonned in our galaxy over its lifetime of 101 a years, as well as an understanding of the history of the enrichment of our galaxy in eleÂ· ments heavier than hydrogen that resulted as stan exploded and spewed newly formed elements Into interstellar space. By the time the sun condensed 5 X 109 years ago, enough heavy elements had been fonned to provide the solid materitlthat eonstitutes the solar system, including planet Earth, with Its potentiJIIity for life. How is this, and the presumed formation of other similar planetary systems elsewhere, integrated in the evolution of our galaxy ua whole? In the following sections we shall outline some ways in whleh the LST will advance our knowledge on this broad front. We shall fust discuss the problems of detenninlng the distance seale of lhe univer1e, from nearby objects In our galaxy, through similar objects in the nearest neighboring galaxies, to those distance indicators that lhe LST will enable us to study out to 10 times the distance attainable today. We !hen outline ways in which the imaging and uv eapabWties of the LST ean be applied to achieve understanding of the evoluÂ· 2
tionary history of our nurest ut"'liletle neiaJ>bon. In the next~tetlon we disc:USlthe tluskal cosmoiCJCical problem of the upanslon of the uni.. rae, its history and future dnelopmeot, and its origin. We then tllm to a ciJocus. liion of the results obtainable from the im.Jcin& and opectrOKOpy of distant plaxlu. F!nally, we twn to th- plaxoes that hove attlvfry In their nuclear rqioos vastly exceed~ the mode1t aetmry in our own, and to the Wlla- IWna problem of the quason. Here we discuss 10m0 problems of current Interest quasan, blade bolet, and lnterplactlc mauer. In this lut field the relationship of optkal utrooomy with radio astronomy and hiah-entrJY astrophyslca, partlculally x-ray astronomy, reachetlts area test importance. Throu&l>out tl\ls brief discussion or cxcihn& problems, ll>e necessity of fu ll- scale support and backup observations using ground-based teltscopet will be obvious. Distance Scale of the Universe Knowled&e of the dillance sca1t of ll>e universe is fundamental to the study of ext"Plactk evolution and cosmology. This distance determination de- pends upon, and Is only as aood as, its startln& point, the distances to nearby standard llan found from the parallax Induced by the Earth's motion about the sun. The lumin<llltiu of theoe nearby stars can then be calibrated for use in determUW. distanc:a to farther objects via the inverse-oquare Jaw. Step by step,tl\e Jumlnotltlts of rarer and more lwninous objecll, a~ch u Ctpheld variable stan, must be calibrated foe use In deducin& dlstanc:a to other p!axies. The hlahÂ·resolutlon capabUity, point in& stabiUty, and lone lifetime (at least 10 yr) of the LST are upected to pve a tenfold lncr-ln the accu- racy with whic:h relall.. s111r positions and 1\ence parallues (and proper motions) can be measured. With the LST , the distance to the llyadesstar cluster, about whleh there Is still controveny, can be measured accurately, and 1\ence the luminosities of its unevoived (main-sequence) stan found. These will form the basis for estimates of the distances to Population I a tars (metal-rich, youna stars), includina the Cepheid variables. EnouJh distances to local unevolvcd stars of Extreme Population II (metal-poor, old stan) can be obtained to aet up an aocurate Population II main aequence, to which main aequences of aJobular clusten (the oldest stellar agreptes ln our p!axy) can be Otted. The faintlimitJna ""&J'itude of the LST, based upon the blcJt contrut with the niabt sky, will allow study of the hiably lwninous nan (Cepbelds, entire aJobular dust en, and unstable stars with lumlnooilles 400,000 lima that of the sun) far beyond the present distance Umlts. For example, Ctphelcls wiD be observed in the V'qo cluster, and the mOitlwni- nousstars may be studied out u far 11 the Coma cluster. Thus, the comblnaÂ· 3
lion or precision astrometry and faint limiting magnitude will permit a decisive advance in the accurate determination or distance to other galaxies. Evolution in Nurby Galuies The s-taxles that are members of our local group provide the best opportunity for studying the evolution and history of star formation and chemical enrich- ment outlide our plaxy. The tenfold increase in spatial resolution of the LST will enable these studies to be carried to much fainter stars in richer star fields than is possible from the ground. For example, study of main-sequence or un- evolved stars in our nearest neighbors, the Magellanic Clouds, wUI be possible, allowing insight into the history or star formation there. Did this star forma- tion occur In bursts, as has recently been suggested, rather than at a steady, continuous rate? If so, why? What has been the chemical evolution in the Magellanlc Clouds, and how has it differed from that in our galaxy? What is the underlying stratum or the oldest stars like? How do they relate to the oldest stars In our plaxy? Detailed comparison will be possible between the globular clusters in the Magellanic Clouds and those in our galaxy. In our own globular clusters, we shall be able to observe the while dwarfs that should be pretent In very large numbers if current stellar evolution theory is correct. In other nearby 8l'laxles, such as the Draco system, we shall be able to study in some detail the highly evolved stars, as weU as stars only just ap- proaching the red pnt stage after leaving the main sequence . The Andromeda Nebula (M31) hu many limilarities to our galaxy, but some important differ- ences as well. It is impor!Jlnt to understand the differences observed In inte- grated light between the globular clusters in M31 and the metal-poor ones in our plaxy. Normal stars of both Populations I and II will be observable in the spiral arms and disk of M3 1 and also in the spiral galaxy M33. The LST wiil enable us to tackle the history of star formation and chemical evolution in these galaxies. aasslcal Cosmology Extension of the absolute distance s.:ale 10 times farther out than is presently aualnable means that we shall be able to attack with new precision the clu- lical cosmological problem, first set out by Hubble, or defining and triCking back in time the expansion or the universe. The high resolution of the LST will provide thls extension to greater distJtnces. We can address such questions as: How fast is the expansion decelerating7 Is this deceleration uniform in space and time? What is the mean density of maner in the universe that pro- duces th.is deceleration? How is the matter distributed? What will be the fate of the universe- expansion forever or an eventual coUapsc into another 4
"primordial fireball" followed by rebirth and eternal rocydina? Or we may tum to W>Orlhodox questions: It !he "Universal expiiWon" isotropic on both small and luse scale, or nrilher? Is our pretent view of !he unlvene too limpl...ru..ded,limited as it Is by !he aoope of our math.. matlcalabWty, vision, and unapnatlon and, above all, by our limited poundÂ· baaed flew of thole im""'112ly diUant pbxlts that form the raw material for <comolosY? The LST may throw newliaftt on thete queotions. Chanica I Evolution â¢ nd the Morpllolosv of DiJunt Golui., Because the most abundant elements have resonance lines in the uv, the uv capability of the LST for faint-object spectroscopy wUI permit obstrvltlons of the most obundanl heavy elements over the whole range from nearby 10 extremely distant plaxJes. We therefore hope to be able to trace the origin o( the elements back throu&h time as we reach out to ever more distant pwdeL Furthermore, !he hi&flÂ·r..olulion capability of the LST wUi be menlial for study ina !he fonns of clistant plaxl., as they appeared lona ~&o. Nearby plaxl.,lnclude spirals lilte ow own (contailiJna sun and unconden2d matter arran,..S predomirwuly In spin! form, out of which new 11111 form), e1Uptlcal1 (sy11ems contailiJna only sws, with no unoondented maltrlalltfi) and lrrtSulalt (stars plus a considerable amcunt of unoondenJed matter in irfftulat form). We cannot teD what forms the moot diSiant detectable piÂ· ~have, nor how th<K forms may evolve from one type to another. The hiJh retolutlon of the LST wUI permit a hundred time1 u many piC1ure tl<Â· menta in aaaJ.uy lmap as can be obuined with a JIOUnd-baJed teletcope, enablina spirals to be dlstlnsulshed from eWptkals at JJtal disttnceL The colors of the dJJTerent types, including uv and Infrared wavelengths, can then be related to the redshifl and hence to the evolutionary 1111e. By making such mcasurementa on galaxies with redshifls as Iorge as z"' O 11 which point !he .S, agel are only half the present local value, ond comparlna these meaaurements with thoac or nearby galaxle1, one may be able to ace dllllnctlvedlrTerenc:es in color Ihat would be deciJive proof of the evolution of the unlverae and that would also &lve â¢ quantilltive handle on the rste or evolution or sal.ules. Active Nude! of Gol.ules and Quasan In !he poll few years there has been an enonnouslncrease in lllterCSI In !he p<oblem of the explosive octlv!ty Ill the nuclei of some pl.ules. Much tJme on pound-based telescopes has been devoted to studylna tho quasan, thooe IllÂ· trillllnsand puwins objeets whose Jar&â¢ redshifia may make them the moot s
distant objecu In the universe. In spite of this aucntion, the nature or the quasars Is sUI I as mysterious as ever. They are fain t and dlfncult to study in detail. AlthouaJl plausible theories have been proposed for the source of their energy. including vavitational collapse into huge black holes, explosions of mal5ivo stars, and acceleration of particles by large number> of pulsars, these theories are difncuh to test because of current limitations of waveltnJth coverage and angular rooulion. The Ltrge redshifis of quasars have made it possible in many eases to study the far uv spectra from the ground: for quasar> with z"' 2,the lyman" line appears at 3648 A; for z: 3, the Lyman limit is easily visible. tlowevcr, we are unable to put t<>sether the whole spectrum for any object, and we must do th.ls to obtJin a coherent picture of the similarities and differences beÂ· tween highÂ· ond lowÂ·redshlft objects, pOS$ible differences In their chemical composition, and the LJ](e. The broad spectral coverage of the LST will make this possible. We would also be able to study with the LST the He land He ll resonance lines, and the He I ioni.zation edge, in the highestÂ·Z quasars (z = 3 to 3.5), thus addressing the question of the origin of helium In the universe. A particularly Interesting problem concerns highÂ·z quasars; they often have absorption lines in their spectra appearing at many differ<nt redshlfts. These are probably produced by the ejection of blobs and lilamonts of gas from the central object at up to half the speed of light. Does this also happen in low-t objects? We shall know this only when we can lool: at the uv spectra of lowÂ·z objects from above the earth's atmosphere. How does this ejected matter in- teract with uncondtn.std inlergalactk. matter? Or have the quasars themselves been ejected from galaxle1, os some statisti- cal studies SUa&est? What is the relationship between quosars and &alaxle$? Is there a real cutoff in red shifts of quasars, or do there exist large numbcr1 of faint quasars that we could locate with the LST by uv and blue high-resolution imaging? Study of the faint fun of llght occasionally visible around quasars has been possible from the ground in two cases of low-redshift quasars, and It has been found that this light comes from hot gas. If we could observe in the uv, we could sec whether these objects, like the high-redshlft quasars, arc ejecting gas at high speeds. There Is a deflnlte similarity between the active nuclei of some galaxies, 1 and the activity in quasars. The high-resolution spectroscopic capability of the LST will enable us to see what is happening, IS regards both moss motions and the interaction between radiation and matter near the smallacthoe central â¢ nuclei of Seyfert and N-type galaxies. In all these objects we are dealing with phenome.u whose ofiajn Ues in the r<alm of hlgh-entrl)' ISlropbysics. At least one quasar is a.n x-ray source, u are the active nuclei of some galaxie.s, both quasars and active nuclei are radio 6
SOUrc:tS, 1nd ortcn IOUJtel of I great deal o( in(rartd CMfl)'. The ultimate C1'1CIJY JOUret seems likely to be conne<:ted with lfl>'llatlonaleneriY release, and benet with pulsorund perhaps with black holes. Conclutlon This outllne o( probkms that ooe can presently env~sa&e tockllq with the LST tS necesardy inadequate, because we cannot foresee 'ltiaac new turns utrophy<k:s may toke in the 8 yean "'mlininc before bunch. Furthermo.,, we cannot foresee what new discoveries will be mode "'hen th< first data 11e pthered with the LST. Astronomy and astrophyoics have for several dceld"' been the mo" cltallen!Jng branch of pllyoical science, jull because discovery 1uts outstripped theory. lfthls has happened with our limited, shuttered view of the universe from th~ cloud-bound earth~s surf~ee, wh11 may happen when we flm llart uslnc the LST? Perhaps our view of the universe wUl be dras- tically chanaed apln. 7