Constraints on Galaxy Evolution and the Cosmologic

arXiv:astro-ph/9703076v111Mar1997ConstraintsonGalaxyEvolutionandtheCosmologicalConstantFromDampedLyαAbsorbersEricWoods1andAbrahamLoeb2AstronomyDepartment,HarvardUniversity,60GardenSt.,Cambridge,MA02138ABSTRACTWeusetheexistingcatalogofDampedLyman–Alpha(DLA)systemstoplaceconstraintsontheamountofevolutioninthebaryoniccontentofgalaxiesandonthevalueofthecosmologicalconstant.Thedensityofcoldgasatredshiftsz=3±1isobtainedfromthemeanHIcolumndensityofDLAspercosmologicalpathlength.Thispathlengthperunitredshiftisinturnasensitivefunctionofthevacuumdensityparameter,ΩΛ.WecomparethetotalinferredmassofcoldgasathighredshiftstothatobservedinstarstodayforcosmologieswithΩm+ΩΛ=1,whereΩmisthematterdensityparameter.Wedefineηtobenetfractionofthebaryoniccontentoflocalgalaxieswhichwasexpelledsincez=3,anduseBayesianinferencetoderiveconfidenceregionsinthe(η,ΩΛ)plane.Inallcosmologieswefindthatη0.4withatleast95%confidenceif25%ofthecurrentstellarpopulationformedbeforez=3.Themostlikelyvalueofηisnegative,implyinganetincreasebyseveraltensofpercentinthebaryonicmassofgalaxiessincez=3±1.Ontheotherhand,recentobservationsofhighmetalabundancesintheintraclustermediumofrichclusters(Loewenstein&Mushotzky1996)requirethatmetal–richgasbeexpelledfromgalaxiesinanamountapproximatelyequaltothecurrentmassinstars.BasedonourresultsandthelowmetallicityobservedinDLAsatz∼2,weinferthatmorethanhalfofthebaryonicmassprocessedthroughgalaxiesmusthavebeenassembledandpartlyexpelledfromgalaxiesafterz=2.WeexpectourconstraintstoimproveconsiderablyasthesizeoftheDLAsamplewillincreasewiththeforthcomingSloanDigitalSkySurvey.Subjectheadings:cosmology:theory–quasars:absorptionlines–galaxies:formationSubmittedtoTheAstrophysicalJournal,March19971email:[e-mail]2email:[e-mail]–2–1.IntroductionTodate,some∼80DampedLyman-Alpha(DLA)absorptionsystemshavebeenidentifiedinthespectraofhigh-redshiftQSOs.Theobservedabsorptiontroughsindicateconcentrationsofneutralgas,withlargeHIcolumndensities,N∼1020cm−2.Thereisconsiderableevidencethattheseobjectsareassociatedwiththeprogenitorsofpresent–daygalaxies.ThishasbeenconfirmedinseveralcasesbydirectimagingofQSOfields(Steideletal.1994,1995,1996;Djorgovskietal.1996;LeBrunetal.1996).TherearevariousindicationsthatDLAsareassociatedwithyounggalaxies.TheabundancesofmetalsatlowionizationstagesinDLAsarecomparabletothosefoundindiskgalaxies(Wolfe1995).Themetalabsorptionlineprofilesshowaleading–edgeasymmetry(Wolfe1995;Prochaska&Wolfe1997)andareshiftedrelativetotheLyαemissionredshift(Lu,Sargent,&Barlow1997),asexpectedfromarotatingthickdiskwithcircularvelocitiescomparabletothoseseeninspiralgalaxies.Furthermore,observationsofredshifted21-cmabsorptionandemissionfromDLAsindicatedisk-likestructuresofgalacticdimensions(Briggsetal.1989;Wolfeetal.1992).However,recentHSTimages(LeBrunetal.1996)haverevealedthatDLAgalaxiesspanawiderangeofmorphologicaltypes.Theexistenceofasubstantialgalaxypopulationatredshiftsz∼2isconsistentwithmostCDMmodels,whichpredictthatgalaxiesshouldformbyz≈2−3(Frenketal.1996,andreferencestherein).TheHIcolumndensitiesinDLAscanbederivedfromtheequivalentwidthsoftheobservedabsorptionfeatures.ThemeanHIcolumndensityalongarandomlineofsightmaythenbesummedupanddividedbytheabsorptionpathlengthprobedtoobtainthecomovingspatialHImassdensityρHIingalaxiesatagivenredshift.SinceDLAsdominatetheHImassintheuniverse(Lanzettaetal.1995,andreferencestherein),theirstatisticscanbeusedtoinfertheevolutionofthecomovingHIdensityfromhighredshiftsuptothepresentepoch.Recentwork(Lanzettaetal.1995;Wolfeetal.1995;Storrie-Lombardi,McMahon,&Irwin1996)hasshownthat,foruniverseswithazerocosmologicalconstant(Λ=0),theinferredcomovingtotalgasdensityρg=1.3ρHI(includingHIandHe)ataredshiftz≈3.5iscomparabletothestellarmassdensityobservedinpresent–daygalaxies.Thiswasseentobeconsistentwithasimple“closed–box”pictureinwhichgalaxieshadformedbyz=3.5,andtheneutralgaswassubsequentlyconvertedintostars,whilethetotalbaryonicmass(gas+stars)isconserved.Notethattheclosed-boxmodeldoesnotassumethatthebaryonicmassesofindividualgalaxiesareconservedbutratherthatthetotalbaryonicmassofallgalaxiesisnotevolvingwithtime.Thismodelallowsformergerswhichconservethetotalmassofallofthegalaxiesinvolved.Evolutionarymodelsmaybegeneralizedtoincludeanetinfallofgasfromtheintergalacticmedium,oranetoutflowofgasfromDLAgalaxies.Theclosed-boxassumptionrepresentsthesimplestevolutionarymodel,butingeneralwedonotexpectthetotalbaryonicmassingalaxiestobeconserved.Inparticular,theintraclustermedium(ICM)inrichclustersofgalaxiescontainsanamountofironwhichisapproximatelyequaltotheamountlockedinstarsintheseclusters(Renzinietal.1993;Loewenstein&Mushotzky1996).ThenaturalexplanationisthatthemetalsobservedintheICMwereproducedinsideofgalaxiesandsubsequentlyexpelledintotheICMby–3–supernova–drivenwinds.Inthispaper,weexaminewhethertheDLAdataareconsistentwiththesubstantialoutflowneededtoaccountforthisobservation.Inthesubsequentdiscussion,wealsoallowΛ6=0,andexplorethefamilyofmodelswithΩm+ΩΛ=1,whereΩmandΩΛarethemeanpresent–daycosmicmassdensitie