* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download 13.1 Galaxy Evolution: Introduction
Perseus (constellation) wikipedia , lookup
Outer space wikipedia , lookup
Dark energy wikipedia , lookup
Dark matter wikipedia , lookup
Space Interferometry Mission wikipedia , lookup
Non-standard cosmology wikipedia , lookup
Physical cosmology wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
Gamma-ray burst wikipedia , lookup
Stellar kinematics wikipedia , lookup
Malmquist bias wikipedia , lookup
Modified Newtonian dynamics wikipedia , lookup
Corvus (constellation) wikipedia , lookup
Hubble's law wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Cosmic distance ladder wikipedia , lookup
Observable universe wikipedia , lookup
Lambda-CDM model wikipedia , lookup
High-velocity cloud wikipedia , lookup
Structure formation wikipedia , lookup
H II region wikipedia , lookup
Star formation wikipedia , lookup
13.1GalaxyEvolution:Introduction [slide1]Wenowturnourattentiontothesubjectofgalaxyevolution,whichisatthecoreof mostofmoderncosmologyandextragalacticastronomy. [slide 2] First of all, galaxies must evolve for two reasons. They're made out of stellar populationsandstarsevolve.Also,galaxiesmerge,assemblingeverlargerobjectsthrougha hierarchical structure formation. So, therefore, there are two aspects to galaxy formation. One is the assembly of the mass, and the other one is conversion of gas into stars and energy.Theformerisrelativelywell-understood-thisissomethingwecanmodelverywell innumericalsimulations-andit'sdrivenbythedarkmatter.Thelatterismuchmoredifficult becauseitinvolvesmessydissipativeprocesses,includingstarformationitself,thefeedback of stellar populations, supernovae active nuclei, and so on. And, there we can model processesuptosomelimit,usingmodernhydrosimulationsbecausetheanalytictheoriesof starformationdonotexist. So, even though dark matter dominates the process of galaxy assembly because it's the dominantmasscomponent,that'snotwhatwesee.Whatweseeisthelight.And,fromthat wehavetoinferabouttheassemblyofthemassaswell. [slide3]First,let'stakealookattherelevanttimescalesforgalaxyevolution.Onetimescale willbetypicalstarburstscalewhichwenowthinkisoftheorderof10to100millionyears. That also happens to be the timescale of the life of the very massive stars, which are energeticallythedominantonesandwhichalsoprovidemostofthephotonsthationizethe interstellarmedium. Italsoturnsoutthatthisistheestimatedtimescaleofactivegalacticnucleusepisodes,and it's not obvious whether this is coincidence or not. The internal timescales within galaxies are commensurate with their free-fall timescales, which would be a few hundred million years.Andthetimescalesneededformergingandassemblyaretypicallyoftheorderofa billionyears.And,finally,thereisHubbletimeoftheorderof10billionyears,throughwhich galaxiesevolve. Therearedifferentobservationalapproachestothisproblem.Firstofall,wecanlookatour own Milky Way and try to deduce from its constituents and properties how it may have formed.Next,wecanlookatnearbygalaxiesandstudythesystematicsoftheirproperties, as we discussed earlier through Hubble sequence, through use of scaling relations, and so on. And, finally, we can observe it directly by looking deep, since light travels at a finite speed;thefurtheroutwelook,thedeeperinthepastwelook.Andso,wecanseegalaxy evolutionunfoldingonourpastlightcone. [slide4]Todothis,weneedagoodcombinationoftheoreticaltoolsandobservationaltools. On theory side, we can use numerical simulations of structure formation to tell us about massassembly.Wecanalsopredictbehaviorofthestellarpopulationsbecauseweknowa lotaboutstellarevolution,andsomakingsomeassumptionsaboutinitialmassfunctionof stars and the actual history of star-formation rate in galaxies, we can predict what their spectrawouldlooklikeasacompositeoftheseevolvingstellarpopulations.And,thereare alsohybridsemi-analyticalschemesthatcombinethese. [slide5]Ontheobservationalside,thereagainarethreeapproaches.First,wecantakedeep images, the deeper the better, if you want to look in the past over a full range of wavelengths.And,fromthosewecaninferalotaboutstar-forminghistoriesorgalaxiesthat maybemergingaswell.Moreimportantapproachisthroughspectroscopy,becausethisis whatrevealsnotjustredshiftsofgalaxies,butphysicalpropertiesthathappeninthem,such asstar-formationrates,presenceofanactivenucleus,ifthereisone,etc,etc.And,finally, wecanlookatthediffusebackgrounds,thecollectiveemissionofallgalaxiesever.Now,that has a drawback of not knowing which source is where until you actually resolve the background.But,itbypassesselectioneffects,becauseyougeteverything,whetherornot individualgalaxiesaredetectableornot. [slide 6] A very important thing is to be aware of the selection effects. And, they come in multipleguises.Themostobviousoneisthefluxlimit.Anygivenastronomicalobservation runsintothesignal-to-noiseproblematsomefaintlevel,andthat'showdeepyoucango. But,moresubtly,thereisasurfacebrightnessselectionlimitthatforextendedobjects,it's thenumberofpixelsaboveacertainthreshold.Thatis,notjusthowmuchlightthereis,but howdiffuseit'sdistributed.Themorecompactgalaxieswouldbemucheasiertodetectthan thediffuseones.Andso,weknowthat,forsure,thedeeperwelook,wearegettinganever moreluminousorhighersurfacebrightnessobjects,andwe'remissingtheintrisicallyfaint onesortheintrinsicallydiffuseones.Ifwedon'tknowwhatwearemissing,thenitwillbe veryhardtocorrect.But,wecanmakereasonableextrapolationsfromnearerpartsofthe universe,andseehowthatworks. [slide 7] Let's refresh our memory of the dynamical evolution – galaxy merging. We've discussedthisearlier,andseenhowhierarchalassemblyofgalaxiesisaprocessthatkeeps goingonfromtheearliestdaysoftheuniverse,andweseeiteventodayinmajormergersof galaxies.Thisobviouslyarrangesthemass,butthankstothedissipationandinfallofgasto thecenters,itcanalsotriggeraburstofstarformationaswellasfeedingofactivenuclei. Thus, stellar population evolution of galaxies and their dyamical evolution can be very deeplyinterconnected. [slide8]Animportantphysicalprocesshereisdynamicalfriction,anditworksasfollows.If youhaveamassivebody–inthiscase,say,agalaxy–movingthroughaseaofmasspoints– stars of another galaxy, say – it will accelerate them. And, as it keeps moving, you'll be encounteringmoreandmorestarstowhichitwillexertitsacceleration.Thosestarsacquire acertainvelocity,andthatkineticenergyhastocomeattheexpenseoftheperturber.So, thestellarpopulation,orpopulationoftestparticlesinwhichthisperturberplunges,gains kinetic energy, usually in the form of random motions, whereas the perturber loses its orbitalkineticenergy.Andthiswhygalaxiesthatareinitiallyinparabolicorbitscanmerge. Incidentally, a good local example is the Magellanic Clouds. They are being torn apart by tidalinteractionwiththeMilkyWay,andinafewbillionyearsthey'llmergewiththeMilky Way.Thishasalreadyhappenedtonumerousdwarfgalaxiesinthepast,andweseefossil evidenceofthatinMilkyWay'shalo. [slide9]Thetimescalefordynamicalfrictioncanbeexpressedasfollows.Youstartwiththe velocity – relative velocity of the perturber – divided by the rate in which that velocity is beingdiminished–decelerationduetotheenergyloss.Aftersometheoreticalcomputation, you obtain a formula like this. It has several interesting features. The denser the medium, whichhasbeenperturbed,andthehigherthemassoftheperturber,theshortertimescale, which is intuitively clear. But also, the timescale gets to be longer if the velocity of the perturberishigh.And,thereasonforthisisthatperturbersthatzipbyveryfastsimplydon't have enough time to deposit enough kinetic energy in the perturbee galaxy, so it'll take manydifferentpassesforthedynamicalfrictiontoactuallydoitsthing.Thisiswhythetime goesup. [slide10]We'veseennumericalsimulationsofmerginggalaxiesearlier,andthisisasetof snapshots from one of those showing what dark matter particles do. In these simulations, always,eventually,themergerproductlookssomethinglikeanellipticalgalaxy. [slide 11] But, if you follow the evolution of the gas, you find out that the gas is reacting muchmoretotheencounter.Andit'slosingkineticenergyveryquicklyandveryeffectively, meaningithastosinktothebottomofthepotentialwell,whichalsomeansitwillachieve highdensities.Thisisexactlythekindofconditionsthatcanleadtoburstofstarformation, ortheycanalsofeedagiantblackholeifthereisonethere. [slide11]Next,wewilltalkaboutthemodelsofthestellarpopulationevolution. 13.2StellarPopulationSynthesis [slide 1] Evolution of stellar populations in galaxies is one of the key ingredients of our understandingofgalaxyevolutioningeneral. [slide 2] If we look at colors of galaxies nearby, we can immediately see that there is a bimodaldistribution.Thereisafairlynarrowredpeak,whichisduetotheellipticalgalaxies and bulges, and there is a fairly broad blue distribution that corresponds to the disks of spirals.Inmoremodernrenderings,youcanplotcolorsasafunctionofintrinsicluminosity, and you still see those same two blobs, but they're a little tilted. In the case of elliptical galaxies,thatridgelineisreallymass-metalicityrelation.Themoreluminousellipticalsretain more of their chemical evolution products. They're more metal-rich. Metals absorb more lightintheultraviolet–galaxylooksredder.So,thisislargelyamass-metallicitysequence. For spirals, there isn't a very simple relation like that. Their colors are a mixture of stellar ages,starformationrates,aswellasextinctionbydust. [slide 3] So, we can produce predicted theoretical spectra of evolving galaxies in the followingfashion.Wehavetomakeanassumptionofwhattheirstar-formationhistoryis. Thisispurelypre-parameteringthemodel,butwecanmakereasonableguesses.Weneed to know what is the initial mass function - the distribution of stars by mass when they're formed-becausestarsofdifferentmassescanevolveatverydifferentpace.Then,foreach stellarmassandandageweneedaspectrum.So,weneedlibrariesofstellarspectrathat can be associated with all components of the stellar population at any given age. This is actuallynotaneasythingtodo,becausewecanobservealotnearusbut,forexample,we have no spectra of very metal-poor very massive stars because those have burnt out long timeago.And,againfromtheorytestedbyobservationsofstarclustersandsoon,weneed stellarpopulationevolutiontracks:howdoesdistributionofstarsandcolormagnitudespace –theHRDiagram–changesasafunctionoftimeforgivenamountofchemicalenrichment, andsoon. Youtakeaquantityofgasandturnitallintostarsinstantaneously–adeltafunctionstarformation rate – distributed according to your initial mass function, and then follow its evolutionintime.Thisiswhat'sknownasasimplestellarpopulation.Thereisnosuchthing inreality,althoughglobularclusterscomeclose.Andthen,youcanrepresentnettotalstarformationhistoryofagalaxyasacollectionofthese,andfollowthemap.Onepopularway ofexpressingstar-formationhistoriesisasanexponentiallydecliningrate,whichisprobably not a bad overall assumption, averaged over many galaxies. In this case, for early-type galaxies,thereisalotofstarformationearlyon-theexponentialverysteep.Forthisgalaxy, it'sveryshallow.Forirregulargalaxiesmaybeessentiallyflatorevenrising. [slide4]Stellarevolutionisoneofthebestunderstoodsegmentsofastrophysics.Wereally do know how stars work and evolve, and that's been confirmed again and again through many decades of observations. There are certainly details that still need to be ironed out, but,atthelevelthatwecareabouthere,wereallydounderstandthat.Animportantthing torememberisthatmoremassivestarsevolvemuchfaster.Theyaremoreluminous.They arealsohotter,sothey'reenergeticallymoreimportant,butnotforlong. Thus, spectrum of an evolving galaxy will change more rapidly in the blue parts of the spectrum,thankstotheshortlifetimesofthesemassivestars,andwillbechangingrelatively slowlyintheredderpartsofthespectrum,wherethemostofthelightmaybecomingfrom, well,allslowlyevolvingpopulationofredgiants.There'remanystellarpopulationsynthesis methods out there, and they sometimes disagree on some details, but, by and large, the agreement's pretty good. One popular set is called GISSEL, which is a library of the galaxy evolutionspectrabyBruzualandCharlot.Weknowhowstarsevolve.Weevenhavespectra of lots of different kinds of stars. And, we have some idea of the initial mass function, at least we measure it locally in the Milky Way, and then we have to make assumptions whether it's the same in all galaxies at all times. We can test some of those assumptions. But, then we have to assume star-formation rate, which again is a completely free parameterinthisexercise. [slide 5] This is what stellar evolution tracks look like. They're computed from stellar evolution models. And, as a star evolves, it moves in the color magnitude space. How and where–dependsverymuch,oralmostentirely,onitsmass,withminordependenceonits metallicity. [slide 6] So, we get these stellar evolutionary tracks from theory - our understanding of stellarstructureandevolutionthat'sprettysolid.Then,weneedtohavetheirspectra.And, forkindsofstarsthatwecanobservenearus,that'saneasythingtoacquire.Forkindsof starsthatarenolongerwithus–say,veryyoung,metal-poor,veryhigh-massstars–this willrequiresometheoreticalmodelingandextrapolation. Wemakeanassumptionaboutinitialmassfunction,andagain,weunderstandthatinthe localMilkyWayconditions,butitwouldbeverydifferentintheearlyuniversewhere,say, starsweremadeoutofhydrogen,heliumandhardlyanythingelse.Infact,webelievethat wasthecase,thattheinitialmassfunctionofprimordialstarswasverydifferent.And,then again,wehavetoassumestar-formationhistory.Inreality,weassumesometypesofstarformationhistories,computetheconsequences,compareittotheobservations,anditerate until we have a model that seems to fit observations, and that's telling us what the likely star-formationhistoryofthesegalaxieswas. [slide7]So,starsevolve,theblueoneskeepdisappearingfaster.Galaxieswillbechanging theircolors,andhereincolor-colorspaceyoucanfollowthetheoreticalbehaviorofevolving stellar populations. You can also see where the galaxies are. And, indeed, they form somethingofasequencethatcorrespondstotheyounger,hotterstarsonlateHubbletypes, older,redderstarsfortheearlyHubbletypes. [slide 8] Here is a comparison of predictions of three different types of stellar populations andsynthesismodelsbydifferentgroups.And,itshowspredictedcolorsandmass-to-light ratios for model galaxies. By and large, they agree very well. The minor disagreements usually are at very young ages, and there is still some debate actually what is the correct thingtoassume.But,qualitativelyatleast,weunderstandverywellhowthingsareworking. [slide 9] And so, this is what evolving spectra of stellar populations look like. This is for a simple stellar population. Remember, this is a scoop of stars made all at once and let go. Note, these are logarithmic plots of log flux versus log wavelength, so they really hide the strongcontrast.And,ifyoustareatcurves,whicharelabeledbytheirage–thosenearthe toptendtobeyoungestbecauseyoungerstarsaremoreluminous–youfindoutthatinthe ultraviolet,bluepartofthespectrum,thefluxwillplummetveryquickly,whereasitwould be changing in the red part of the spectrum, but much slower, which is exactly what we expect. [slide 10] Here is a comparison of predicted model spectra of different ages, assuming differentlibrariesofstellarpopulationevolutiontracksandspectra.And,again,youseethat different models seem to be in an excellent mutual agreement, which is giving us some confidencethatweactuallydounderstandhowthisworks. [slide 11] Another kind of models that they are now more popular are so-called semianalytical or hybrid models, where one can use stellar population synthesis models, associate them with galaxies that they're being assembled through hierarchical structure formation either from a numerical simulation or from some statistical description thereof, andmakepredictionsofhowwilltheircolorschange,andsoon.Thereare,unfortunately, way too many two-number parameters in these models. And so, they have some modest success,buttheyillustratethecomplexityofalldifferentthingsthathavetobetakeninto account,andalotofassumptionsthathavetobemade. [slide12]Nexttime,wewillturntotheactualobservationsofgalaxyevolution. 13.3:ObservingGalaxyEvolution [slide1]Nowlet'sseewhattheobservationssayaboutgalaxyevolution. [slide 2] The simplest thing to do is take images of different wavelength that is observationally much cheaper than taking spectra, which require longer integration times. And, this can be used to do galaxy counts as a function of their brightness, color etc. But, that can only go so far, and to really understand what's going on, redshifts really are necessary. So, it was only in the recent years that we have obtained sufficiently large samples of galaxies in deep fields to really reach a good observational understanding of galaxyevolution. Thereisoneimportantdichotomy.Youcanthinkofstarformationascomingintwoflavors: thesimple,unobscuredstarformation,whereyouseethelightfromstellarphotospheresas theyare,orlightabsorbedandre-radiatedbyinterstellardust,whichnowmovesallofthe energyintofar-infraredorsubmillimeterregime.So,therearetworegimesinwhichwecan observegalaxyevolutionandstarformationinthem,andeachofthemhasitsowntoolsand selectioneffectsandlimitations. [slide3]Youmayrecall,whenwefirstmentionedsourcecountsasapotentialcosmological test,thattheevolutionaryeffectsreallymessthingsup.And,theyalwaysworkinthesense of making counts higher than they would be in the absence of the evolution. Even the galaxies that are evolving in brightness, say, due to the fading of stellar populations that weremoreluminousinthepast,andthereforetheywouldbeatthehighermagnitudelevel. But,therewouldbemanymoreofthem,thus,they'dbemovingtotheleftandproducinga less declining curve. Likewise, if galaxies are assembled from smaller pieces, there were more smaller pieces in the past than exactly the same observational effect appears. To disentanglethese,weneedredshiftsurveys. [slide4]Still,hereatthedeepgalaxycountsfromtheHubbleDeepFields,andthesedayswe do this down to about 29th magnitude or thereabouts, which is really spectacular, and by extrapolationovertheentiresky,theremaybeacoupleofhundredbilliongalaxieswithin theobservableuniverse. [slide 5] You may also recall that evolution is expected to appear stronger in bluer wavelengths,andlesssoinredder,andthat's,indeed,exactlywhat'sseenhere.Now,thisis now infrared galaxy counts, and they show roughly minor discrepancy, compared to the muchstrongeronesthatareobservableinthebluerpartsofthespectrum. [slide 6] But, those galaxies evolve as their stellar populations evolve. Their colors evolve too,generallygoingfrombluertoredder.However,that'salsocomplicatedbytheredshift. Thewholespectralenergydistributionismovingfrombluerfilterstotheredderfilters.So, anygiventimeyoucanlookatcolor-colorspaceandseewhatgalaxieswilldointhere.Now, you can make use of their complex trajectories, and use those to evaluate redshifts from colors alone. Those are so-called photometric redshifts. You can think of those as a really low-resolutionspectroscopy. [slide7]And,theyseemtoworkremarkablywell,typicallyinatleastfourorfivedifferent filters,andhereareexamplesofsomeofthemeasurements.Thosearedotswitherrorbars with models of stellar populations drawn through them. This actually looks too good, and there could be many different models that can fit the same set of photometric measurements,butthatcanbealsoevaluatedstatistically. [slide 8] So, here is a typical plot. Usually, there is a really excellent agreement between spectroscopicredshifts,doneasacontrol,andpredictedphotometricredshifts.Thestateof the art is that maybe down within a few percent. However, there are always outliers, galaxiesforwhichgrosserrorismade,andthat'susuallyduetosomethinglikepresenceof anactivenucleus,orsomeotherpeculiarhappeninglikethat. [slide9]Aparticularformofphotometricredshiftsreliesonthepresenceofdeepjumpsin the spectrum of the galaxy. There are a couple of those. There is the limit of the Balmer seriesofhydrogen,whichthenresultsinastepofmagnitudearound3,600angstromsinthe rest frame. So, you can use that by measuring flux in filters blueward of the jump and redward of the jump. An even stronger effect occurs at the limit of the Lyman series, and thoseareextremelyusefultofindgalaxiesatveryhighredshifts.Moreover,forthereasons we'll be discussing later, intergalactic medium absorbs light blueward of Lyman-alpha line. So, it's really the wavelength of the Lyman-alpha line that serves as an interesting jump point. This has been used to great effect, in particular by Steidel and collaborators, who discover large numbers of galaxies of high redshifts, and then study their evolution and properties. [slide10]Butagain,colorsandmagnitudeshavetheirlimitations,andredshiftsareneeded. So,thattheadventofeight-,ten-meterclasstelescopeslikeBLTinChile,orKecktelescopes in Hawaii, in Subaru, and so on, it became possible to actually do this in an effective way. Also, there was the development of multifibre spectrographs, which you may recall also revolutionizedtheredshiftsurveysatlowredshift.And,nowadays,thousandsandhundreds ofthousandsoffaintgalaxyredshiftshavebeenobtained. [slide 11] A good winning strategy is to obtain really deep images from space where you don'thavetoworryabouttheeffectsofEarth'satmosphere.ImagestakenfromHubblecan gomuchdeeperthanthosetakenfromthegroundwithabetterresolution.Andso,thereis asetofselectedfieldsinthesky,whereverydeepobservationshavebeenobtained.Hubble DeepFieldwasthefirstone,followedbytheUltra-DeepField,andChandraDeepField,and eXtremeDeepField.So,thesearethedeepestwindowsintheuniverseweobtainedsofar. Once you have these images in a number of filters, you can deploy large telescopes to measure redshifts of as many galaxies as you possible can, and that's still very much an ongoingenterpriseinobservationalcosmology. [slide 12] Here is the first one of those, the Hubble Deep Field with its characteristic B2 bomber shape, and the histogram of redshifts obtained with the Keck telescope. So, even thosewerethedeepestobservations.Untilthen,thebulkofthesegalaxiesisactuallynotat theveryhighredshifts,aboutredshift½.Theygobeyondredhiftof1,butnotbyalot. [slide13]Insubsequentwork,pushingeverdeeper,galaxieswerefoundatredshiftsof5or evenalmost6,butstillthebulkofthese,evenatthelimitofthepresentdayobservations with eight- and ten-meter class telescopes, is of the order of unity or less. So, we do not actuallyprobeevolutionofgalaxiesverydeepthroughdirectmeasurements.Smallnumbers wedosee,butthenonehastobeawareofselectioneffects.Thecompleteunderstandingis reallyatredshiftslessthan1. [slide 14] This was done now by numerous groups, both in north and south, and tens of thousands of galaxy redshifts have been obtained. The results are usually in a really good mutualagreement.Thisoneisfromapencilbeamsurveyinso-calledGOODSField,whichis whereHubbleDeepFieldwas,plusotherobservationssurroundingit. [slide15]Andso,hereareacoupleinterestingdiagrams.Ontheleft,youseetheredshift histogram.Andit'sveryspiky.It'snotspikybecauseit'snoisy,it'sspikybecauseofthelargescale structure, that the line of sight intersects filaments, or even clusters and voids, and that'swhatproducestheobserveddistribution.Ontheright,youseeabsolutemagnitudesin therestframeofgalaxiesplottedversusredshift.And,youseethereisasharpcutoffthat correspondstomagnitudelimit.Peoplewhodothesesurveysdecidethattheycanonlygo down to some magnitude level, say, 24 magnitude, and that maps into different absolute magnitudesatdifferentredshifts.Sothisisabuilt-inbutwell-understoodselectionoffacts. Nevertheless,onehastobeawareofit.Naively,ifyoulookedatthis,youwouldconclude that galaxies of higher redshifts are more luminous. No, you simply only see the luminous oneswithhighredshifts,youarenotseeingthefainterones. [slide16]So,thiswasdonefordeepfields,andtheresultisasfollows.Individualgalaxies cannot be really compared, you need to look at the whole population, and the simplest description of the entire population is the luminosity function – distribution of galaxy luminosities.So,ifyoulookatthatindifferentredshiftshells,youfindoutthattheobserved galaxy luminosity function is very similar to the one we've seen near us. It evolves very slowly,butyoubegintoseeacoupleofinterestingeffects.Byaboutaredshiftof1/2orso, thefaintendsteepens.Weseemoreevolutioninintrinsicallyfaintergalaxies.And,second thingisthatasyoupushdeepenough,youstarttoseebrighteningatthebrightend,which is what you expect from fading of stellar populations. The steepening was a little bit of a surprise.Itwasthoughtbeforethatgalaxiesresponsiblefortheexcesscountsintheskies,in theimaging,areevolvinggalaxiesatlargerredshifts.Thebulkofthemturnouttobedwarf galaxies at modest redshifts. And, the evolution of galaxies depends very much on their intrinsicluminosity.Thisisknownasthedownsizing.Atfacevalue,it'sexactlyoppositewhat youexpectfromahierarchicalstructureformation,whereyouslowlybuilduplargegalaxies, youexpectthelargergalaxiestobeevolvingfastest,butit'sexactlytheopposite. [slide 17] Moreover, the evolution of the lumosity function depends on the galaxy morphology. And, if you can split them in morphological type – say, early and late-type spiralsandellipticals–youfindoutthatthelater-typegalaxies,thosefurthertotherightin Hubble sequence – star-forming disks and irregulars – have the strongest evolution. GalaxiesontheearlierparttoHubblesequenceprettymuchdonetheirevolvingbyabouta redshift of 1. They do evolve since then, but bulk of the change appears in the faint populationandalsothelateHubbletypes. [slide18]Itisonlywhenwereachredshiftsoftheorderoftwoandbeyond,thatwestartto see clear effects of strong evolutional stellar populations at the very bright end. So, the genericconclusionthesedaysisthatmostoftheHubblesequencewasprettymuchinplace byaboutaredshiftof1.And,thingshavebeenevolvingatrelativelymodestpacesincethen. We'llseesomeotherapproachestothisalittlelater. [slide 19] Next, we will talk more about some of the observed results, as well as the evolutioninclusters,asopposedtofield,whichiswhatwejusttalkedaboutnow. 13.4GalaxyEvolution:SomeResults;GalaxyEvolutioninClusters [slide1]So,wehavestartedtalkingabouttheresultsongalaxyevolutionstudiesingeneral field.First,wefoundouthowtheluminosityfunctionseemstobeevolvingslightly.Butnow, let'sseesomeoftheotherresultsthathavebeenobtained. [slide2]Asimplethingtodoistolookatcolorsofgalaxiesasafunctionofredshift.And,if we then predict what galaxies of different Hubble types would look like - if there was no evolution, just a K-correction, which you may recall from way back in Hubble diagrams - what would measurements look like, given the redshifted spectrum? This is what it's like. The points are the actual measurements and the three lines labeled with different Hubble types correspond to the colors that those particular Hubble types would have at a given redshift.And,asyoucansee,theobservationsprettymuchfollowtheband.So,thereisn't very much, in terms of color evolution, for the Hubble-type sequence galaxies all the way afterredshiftofunity. [slide3]Hubblespacetelescopehasalsoaffordedusanotherpossiblemeasurement,which is to look at galaxy sizes, say, looking at their half-light radii or some other form of objectively defined radius. And so, this is the result of what the radii, average radii of galaxies,doasafunctionofredshift,namely,galaxiesgrowintime.Iftheradiiwerefixedin propercoordinates-iftherewasnoevolutionatall-thensolidlineshowswhatfwouldlook like.But,insteadofthat,thepointsshowthatthereisasubstantialgrowthofgalaxies.This isconsistentwithourunderstandingofhowdiskgalaxiesseemtoformfromtheinsideout. Asyou'llrecall,thebulgeistheoldestpartinthemiddle.Then,youhavestellardisk.Then, hydrogen extending beyond it. That hydrogen gas has to turn into stars. And, here we'll probablyseethecollectiveeffectofthat. [slide 4] Modern spectra of high-redshift galaxies can be used to measure not just the redshifts,butalsovelocitybroadening,inotherwords,getavelocitydispersion.Withradii measured from Hubble space telescope, we can infer dynamical masses, or from fitting of thespectralenergydistribution,wecaninfertheirstellarmasses.So,thatisshownhere.The sizesofsymbolscorrespondtothemagnitudes.Wecanseethatthemostmassivegalaxies seem to be already in place at redshift 1 or 2 – this is sort of the upper envelope of this distribution–whereasthelowermassesseemkepttobeevolving–thisisknownasgalaxy downsizing. Naively, you would expect a hierarchical formation scenario, that you make smallonesfirstandthenyougraduallybuildthebigones.But,thatseemstobetheopposite to what's observed. The solution to this is probably due to biasing effect we talked about earlier. [slide5]Wecannotwaitlongenoughtoseegalaxiesmerge,butwecanassumethatsome numberofcloseprojectedpairswilleventuallymerge.So,bydoingstatisticsofclosepairsof galaxies,allowingforprojectioneffectsandthingslikethat,wecaninferthelikelymerger rateasafunctionofredshift.Andhereitis.It'sapowerof1+z,andit'smoreorlessexactly what'sexpectedfromthemodernmodelsofhierarchicalstructureformation. [slide 6] You will recall that when we talked about scaling relations, like the fundamental plane,Imentionedthattheycanbeusedasasharpprobesofgalaxyevolution.Luminosity function is a very broad distribution, but these correlations are, by construction, the sharpest we can have. And so, if we can follow, say, the evolution of their intercept or maybe slope as a function of redshift, we can gain new understanding in the evolution of galaxies-inthiscase,ellipticals.What'sshownhere,intheopenpointsthroughwhichthe lineisfitted,istheessentiallyzeroredshift,thefundamentalplaneedge-on.Thesoliddots areellipticalsinoneofthemostdistantclusternowknown.Therearetoofewpoints,but youcanseethatthey'reconsistentwithbeingonashiftedversionofthefundamentalplane. [slide7]Numerousstudieshavebeennowdone,bothforfieldandclusterellipticals.And,it was always seen that ellipticals at higher redshifts show a shift in the intercept to the fundamentalplane–thesurfacebrightness–thatwouldcorrespondtothefadingofstellar populationintime,asexpectedfromevolutionarystellarpopulations.And,wecanevensay somethingaboutthestar-formationhistories. [slide8]Wecanfitdifferentevolutionmodels,andfindoutwhichonesseemtodescribethe datathebest.And,theansweristhatmodelsinwhichellipticalgalaxiesformrelativelyearly onanddon'tevolveverymuchsincethen,justpassivelyfadeawayseemtofitthedatafairly well.Thatisshownintheplotontheleft.Theplotontherightshowsfadingor,ifyoulook with redshift, brightening of surface brightness, which, remember, is related to luminosity density of stellar populations as a function of redshit. And, you can see that it is systematicallyincreasinginredshiftforbothfieldandclusterellipticals. [slide 9] So, these studies are consistent with what we've seen earlier, that as far Hubble sequencegalaxiesareconcerned,thereseemtobeprettymuchinplacebyaboutredshitof unity.Theellipticalsareevolvinginawaywewillexpectfromearlyburstofstarformation, andrelativelymodeststar-formationhistoryafterthat.And,wealsobegintoseechanging thetiltoffundamentalplane,whichreallymeansthatgalaxiesofdifferentmassesevolveat slightly different pace. And, it goes in the sense that those at the lowest mass end evolve fastest,whichis,again,anotherexampleofthegalaxydownsizing. That's at first fundamental plane's concern. What about Tully-Fisher? Well, that has been doneaswell,butitturnsouttobemuchmoredifficultandcomplextodoitforspiralsat highredshifts,andtheseresultsarestillnot100%clear,butareatleastbroadlyconsistent withthispicture. [slide10]Sofarwe'velookedattheevolutionofgalaxiesinthefield.Whataboutclusters? Thisisadenseenvironment-youexpectgalaxyinteractionstoplaysomerole.Thefirsthint that something interesting is going on in clusters was so-called Butcher-Oemler effect, established very early on. These astronomers found out that clusters at larger redshifts seemstocontainlargerproportionofbluergalaxies,forwhateverreason.Generically,you expect the galaxy evolution leads from bluer galaxies to redder galaxies, so, at least qualitatively,thisseemedaboutright. [slide 11] Remember that in clusters some merges will occur, but vast majority of interactions will not lead to merging. However it will disrupt galaxies, may remove tidally some of their gas or stars and dark matter, and that's the process called the galaxy harassment.There'llbealotofcumulativesmallencountersthatwouldmayberemovegas from galaxies little by little. This would tend to transform late-type spirals and dwarf irregularsintoS0sandellipticalsintime. [slide 12] With the Hubble space telescope it became possible to look directly at a morphologyofgalaxiesindistantclusters,andhereisoneofthem,wheredifferentsymbols correspond to galaxies of different morphological types. With that and spectroscopic measurementswecantrytodisentanglewhat'sgoingon. [slide13]Aninterestingfindingwasmade.Thereisanoveltypeofgalaxiesfoundinthese evolving cluster populations, so-called post-starburst galaxies, galaxies that are bluer than theyshouldbeatthezeroredshift-therewasnothinghappening-andhavespectralenergy distributions consistent with having undergone a burst of star formation maybe up to a billion years earlier. This could have been caused by some interactions, and these galaxies willthenpresumablyfadeintoredderHubbletypes. [slide 14] And so, the summary of these results is that, as far as we can tell, there is a conversionoflate-typespiralsintoS0sandellipticalsintheclusters.Thisisrelatedtodensity of clusters as well – the morphology-density relation – and that can account for the observedButcher-OemlereffectandpresenceofthingslikeE+Aorpost-starburstgalaxies. [slide15]Soonepossiblescenarioisthatspiralsfromfieldarefallingintoclusterpotential well. As they do that, they encounter dense intracluster gas, the extra gas. Some of their owngasgetsstrippedaway-thatquenchesthestarformationinthem.Theymayundergoa burstofstarformation,triggeredbysomeoftheinteractions,buteventuallythatfadestoo. Andintheendyoumayhaveanolddisk,likeS0galaxy.So,thisisaplausiblescenario.Thisis not necessarily the only way to make 0s, but it can account for the observed effects in clusters. [slide 16] Next, we will talk about the obscured component of star formation in history in galaxies. 13.5DustObscuredGalaxies [slide1]Sofar,welookedattheobservationsinvisiblelight,whichalsocorrespondstothe redshiftedrest-frameultravioletlightingalaxies.Inotherwords,whatyoujustseedirectly instellarsurfaces.However,wedoknowthatgalaxiescontaindust.And,likelytheydidso fromveryearlyon.Thedustabsorbsultravioletandvisibleradiation,andre-emitsitinfarinfrared or submilimeter. Thus, there has to be a obscured star-formation component involvedingalaxyevolution. [slide2]And,indeed,asweaquiredabilitytoobservedeepuniverseonthesewavelengths, itwasdiscoveredthattherearenewsourcesappearingintheskythataresimplynotvisible at all in visual light. Here we see a composite in the upper left, visible light image from Hubbleandthemid-infraredimagefromtheSpitzerSpaceTelescope.Asyoucanseeinthe upper right, there isn't a trace of that red source. In the lower left, you can see it is just beginningtoshowup,zerotonear-infrared.And,inthelowerright,it'spuremid-infrared image,andsourceisveryobvious.So,skywouldlookdifferentonthesewavelengths,and therecouldwellbeapreviouslyunaccountedpopulationofsources. [slide 3] And this is, indeed, the case. The first observations that uncovered such a population of sources were done at James Clerk Maxwell telescope in Hawaii. It is a submillimeter telescope. It is equipped with a bolometer array measuring submillimeter radiationfromsourcesinthesky.Theseareverydifficultobservationstomake;thisiswhyit tooksolong.And,theylookedatacoupleofthedeepfields–theHubbleDeepFieldand another one called Lockman Hole – and found that there are sources there, faint sources, whose nature at the time was unknown but were of surely obscured star-forming galaxies faraway.Thereasonwhytheylooksobloppyisthepoorresolutionofthesetelescopes.You mayknowthattheangularresolutionoftelescopeisproportionaltoitsdiameterdividedby thewavelength.So,foroptical,thisisaveryhighnumber.Therearemanywavelengthsof photons stretched over, say, the mirror of the Keck telescope. Not so in radial or submillimeter – there you have very low resolution. Nowadays, we have new interferometers, like ALMA in Chile, that will actually produce optical light resolution in thesewavelengths.Butbackthen,thiswasn'tthecase. [slide 4] Now, here is a really nearby example of what we might expect. This is the galaxy M82, which is a nearby starburst galaxy. The picture shown here combines regular visible light and the purple is ionized hydrogen emission. What we see here is an intensely starforming disk galaxy obscured in the middle, but the supernovae exploding pushed the gas out.Theydriveagalacticwind,expellingitintointergalacticspace.Now,ifwetakeabroadbandspectrumofM82,weseethattherearetwobumps.Thereisone,theoptical–which correspondstoasortofquasi-blackbody,sumofallstellarphotospheres–andthebigger oneinfar-infrared–whichisthermalemissionfromdustthatwasheatedupbytheseyoung stars. In this particular galaxy, there is actually more energy emerging in the form of submillimeterorfar-infraredradiationthanvisiblelight.But,onaverage,inthispartofthe universe, there is roughly equal amount of obscured and unobscured star formation. The sameturnsouttobetrueathigherredshifts. [slide 5] You may recall the concept of K-corrections, which means that as you observe in some stationary instrument on planet Earth, you're looking at different parts of the restframespectrumforsourcesatdifferentredshifts.Now,byandlarge,forgalaxies,thismakes themdimmer,becausegalaxyspectratendtoberedder,moreenergyintheredpartofthe spectra. It turns out that for submillimeter sources it's exactly the opposite. You are redshiftingintothePlanckcurve-you'reclimbingfromtheWienside.Andso,eventhough sourcesgetfurtherawayandthereforeshouldbedimmer,you'resamplingabrighterpartof theirintrinsicspectrum.Andso,someofthoseK-correctionsactuallybecomenegative.The upshot is that for many of these sources the apparent brightness almost does not change withredshift.And,therefore,ifyoucanreachthatfluxlevel,youcanseeveryfaraway.This iswhatSCUBAsourcesturnouttobe. [slide6]...whichalsomeansthatwecandofairlydeepsourcecountsinthesewavelengths, and here are some examples of it. They can be used then to constrain directly the contributionofthesesourcestotheoverallstar-formationhistoryintheuniverse,andsay somethingabouttheirevolution. [slide7]Thereisonedifficulty,however.Thepoorangularresolutionofthesesubmillimeter observations means that optical counterparts, which we need in order to measure the redshifts, are hard to guess. In some cases, there is an obvious counterpart, but, in many cases,therearemanyfaintgalaxiesinsidethiserrorcircleofasubmillimetersource.It'snot clearwhichone,ifanyofthem,istheactualcounterpart.So,ittookawhiletogetredshifts, measured for these objects. The trick that was used is that the radio measurements can detect some of them, and radio measurements have precision that is perfectly good to matchtoopticalobservations. [slide 7] Then the spectra were taken of those, and, in fact, may turn out to be strong emission-line sources, where that line emission could be powered by star formation or, possibly, by an active nucleus in those galaxies. Possibly, a little bit of both is happening. And,thepredictionsfromfittingthesourcecountswerethatmanyofthesesourceswillbe aroundredshifts2or3,andthatexactlyturnouttobethecase. [slide 8] So, now that we've seen how we can detect both obscured and unobscured component of evolving galaxies, we'll look into the overall star-formation history of the universe. 13.6:TheStarFormationHistoryoftheUniverseandtheChemicalEvolution ofGalaxies [slide 1] So, now that we've seen all these measurements of evolving galaxy populations, let'sseeifwecantieittogetherintheoverallstar-formationhistoryoftheuniverse. [slide2]Hereisaplotthat'scalledaMadaudiagram,afterthefirstauthorofthepaperthat introducedhim–PierreMadau.And,itshowstheinferredproductionofmetals,ordensity of star-formation rate, as a function of redshift, as inferred from Hubble Deep Field measurements.Thethreesetsofpointsandcurvescorrespondtothreedifferentfilters,and you can see that they're in reasonable agreement. The actual curves are particular galaxy evolutionmodelsthatarefittothedata.And,theinterestingthinghereisthatyoucansee that there is a peak. There is a time in the history of universe, roughly around redshift of unity,whereitseemsthatco-movingstar-formationrateoverallgalaxieswasatitshighest. And,itdeclinedsincethenasitapproachedredshiftofzero.Italsoseemedtohavedeclined athigherredshifts,whichyoumightexepectwouldbethecaseaswefirstbuildupgalaxies, they get brighter and brighter and then they fade off. Turns out, a lot of this apparent decreaseofhigherredshiftsisactaullyduetotheobscuration.But,let'shavealook. [slide3]Now,wecanmeasurereddeningsofgalaxies,andwecancorrectfortheapparent absorptionbydust.And,ifyoudothat,youplotthesamediagram-thiswouldbediagram ontheupperrighthere,duetoSteidelandcollaborators.Youfindoutthat,trueenough,as yougofromheretotheredshiftof1,theco-movingstarformationdensityoverallgalaxies increasesrapidly,butthen,itkindofstaysflatallthewayouttoredshiftof4,andmaybe even higher. Now, plotting against the redshift is maybe slightly misleading, because you really want to plot as a function of the lookback time – time history of the universe. And, whenyoudothis,thenthisdeclineafterredshiftofunityisactuallymuchmoregentle,but, nevertheless,thereisadecline. [slide4]Thatwasallfortheunobscuredstarformation,orjustmildlyobscured.Now,ifwe addcomponentfromdusty,obscuredsources,liketheSCUBAsources,thenthatboostsup thetotalevenhigher.Thecurvesshownherearemodelsthatdon'tseemtofitterriblywell, but they do imply that there'll be substantial component of hidden star formation contributingtotheoverallhistory. [slide 5] Subsequently, measurements have been obtained out redshift of 6, where we actually have spectroscopic measurements of galaxies, and nowadays they push them to redshiftoftheorderof10,butthoseareallbasedonphotometricredshifts.Andyes,there is a decline that sets in roughly past redshift of 4 or 5, and that's the picture that we expected.Atfirstthereisnothing,thenyoubuildupgalaxies,yougetmoreandmorestarformationrategoingon.Thereisaverybroadmaximumofthataroundredshift2+/-factor of2,andthenitdeclinessincethen.So,wearenowinthephaseofhistoryoftheuniverse where it's gradually fading away. Most of the action happened when universe was a few billionyearsold. [slide 6] We can convert these measurements into the actual buildup of the present observedstellarmassingalaxies.And,hereiswhatitlookslike.Itstartsathighredshiftsasa verysmallfraction.Byaboutredshiftofunity,mostofthestellarmassingalaxiesisalready assembled, and then stays roughly constant. This is very much consistent with everything elsewe'veseenbefore. [slide 7] The more modern observations push this further out to redshift of 6, now even beyond,andthetrendcontinues.So,westartwithnogalaxieswhatsoeverinourstars,and buildthemupgradually.Therateofthebuildupslowsdramaticallyaroundredshiftofunity. And,thereisstillsomebuild-upbecausethereisstillsomestarformation,butwenowcan actuallyseegalaxiesbeingassembled,starsbeinggenerated,andgalaxiesovercosmictime. [slide8]So,thiswasthedirectapproachtolookingatgalaxyevolution.Welookatindividual sources,measuretheirdistances,brightness,andsoon.Analternativewayistoobservesky and integrate all of the energy that we get from it, and actually obtain spectrum of that overallintegratedcosmicbackground.Note,thisisnotthecosmicmicrowavebackground. Thisistheradiationduetogalaxies,starsandgalaxies,andalso,maybe,activenuclei. Thisisaverydifficultmeasurementtodo,becauseit'shardtogetzerocomparison.You're lookingatthenormalpatchoflightandcomparingitto–what?So,thisiswhyittookso long to do it. But, nevertheless, it was done in both optical and near-infrared. And, the upshotisthatthetotalintegrateddensityofopticalandinfraredbackgrounds,almostallof whichisduetostarformation,isabout100nanowattspermetersquaredpersecond.Ifyou have telescope of certain collecting area, and integrate some time, that will give you the amountofpowercollected.Thisturnsouttobeonlyfewpercentoftheenergydensityof cosmicmicrowavebackground.And,ofitonlyfewpercentisactuallycontributedtoactive galacticnuclei. [slide 9] So, here is the broadband picture. This is what happens when you integrate the spectrumoftheuniverse,inopticalandinfrared.There'redifferentmeasurements;there're upper limits. They come from ground, from space, a variety of sources. If you recall the broadbandspectrumofStarburstGalaxyM82,ithadtwohumps.Therewasblack-bodyish radiationfromunobscuredstars,inthevisiblelight,andtherewasblack-bodyishradiation fromheateddustinfar-infrared.Well,thesamenowappliestotheoverallspectrumofthe universe,ifyouwill.And,what'splottedhereisaquantitythat'sreallyproportionaltothe energy per unit logarithmic interval. And, the fact that a two peaks are roughly the same heightmeansthatapproximatelyequalamountofallstarformationeverwasinunobscured andobscuredsystems. [slide 10] Let us now turn to the question of chemical evolution. As stars are made, they explode, they release chemical elements and interstellar medium, new stars are formed. Someofthoseareexpelledinintergalacticgas,freshgascomesin.Allofthatcontributesto theoverallchemicalevolutionofgalaxies.How'stheirmetallicitychangingasafunctionof time? Hereisaroughschematicdiagram.Youbegin,ofcourse,withhydrogen,heliumandnothing else. So, stars cook up heavy elements. Some of those recycle into new stars, some are expelled out, fresh material comes in, and so on. Often times, these extremely complex processesaresimplified.Forexample,galaxiesareputallinabox,andthereisnoinflowor outflow. Or, it's assumed that the moment stars explode, that material is immediately recycledinnewstars.Thosearecrudeapproximations,buttheygiveusatleastsomeinsight ofwhat'sgoingon. [slide11]Anotherschematicdiagramwhichconveysmoreorlessthesamestoryisshown here. But, you may want to look at it and find it a little more informative as to all the differentconnectionsbetweendifferentprocessesandcomponentsare. [slide12]Now,starbusts,likethatoneinM82,candrivegalacticwinds,thatexpelenriched materialjustproducedbysupernovae,thankstothekineticenergyofsupernovaejectaout intheintergalacticspace.Thiscanbemodeledandisalsoobserved.Whathappenstothat enrichedgasisthatitcontributestotheoverallchemicalevolutionofintergalacticmedium. And,athighredshiftsitwillbeobservableintheformofmetalabsorption-lineclouds,which we'lladdressinthenextchapter. [slide 13] So, as you make stars, you make metals. The star-formation history of the universe, and the chemical enrichment history of the universe are tightly coupled, and, qualitatively, should look the same. So, here is, essentially, a Madau plot, that shows the productionofmetalsasafunctionofredshift. [slide 14] And, with modern measurement we can look at the dependence of galaxy metallicity and mass. You'd expect that more massive galaxies would achieve higher metallicities because they retain more of the supernova ejecta and recycle them more effectively.And,thereis,indeed,amass-metallicityrelation.It'saverynoisyone.It'sshown inthisplotasthegreyareathat'sfor,essentially,redshiftof0.And,now,thereisasetof measurements for distant galaxies, now put in bins to make it more obvious. And, we see thatthatkindofarelationshipexistsalreadyearlyon,whichiswhatyouexpect.Regardless oftheredshift,moremassivegalaxieswillretainmoreoftheirprocessedmaterial.But,the overall curve is shifted down, which is again what you expect – that you have lower metallicities and they grow. They grow in galaxies of all different masses and at every redshiftthereisthisdependencethatisgenerallyexpected. [slide 15] ...which will lead us into the next chapter – evolution and structure of the intergalacticmedium.