DavidBurstein1,GeorgeBlumenthal2
Received
–2–ABSTRACT
Mostspiralgalaxiesarefoundingalaxygroupswithlowvelocitydispersions;mostE/S0galaxiesarefoundingalaxygroupswithrelativelyhighvelocitydis-persions.ThemassofthehotgaswecanobserveintheE/S0groupsviatheirthermalX-rayemissionis,onaverage,asmuchasthebaryonicmassofthegalax-iesinthesegroups.Bycomparison,galaxyclustershaveasmuchormorehotgasthanstellarmass.HotgasinS-richgroups,however,isoflowenoughtem-peratureforitsX-rayemissiontosufferheavyabsorptionduetoGalacticHIandrelatedobservationaleffects,andhenceishardtodetect.Wepostulatethatsuchlowertemperaturehotgasdoesexistinlowvelocitydispersion,S-richgroups,andexploretheconsequencesofthisassumption.Forawiderangeofmetallicityanddensity,hotgasinS-richgroupscancoolinfarlessthanaHubbletime.Ifsuchgasexistsandcancool,especiallywheninteractingwithHIinexistinggalaxies,thenitcanhelplinktogetheranumberofdisparateobservations,bothGalacticandextragalactic,thatareotherwisedifficulttounderstand.Subjectheadings:galaxies:formation—galaxies:evolution—intergalacticmedium
1.Introduction
BoththeNearbyGalaxiesCatalog(Tully1987)andthediscoveryofX-rayemittinghotgasingalaxygroups(Mulchaey,etal.1993;Heldson&Ponman2000;Mulchaey2000,hereafterJSM00)haveprovidedstrongevidencethatmostgalaxiesresideingalaxygroups,notunlikeourownLocalGroup(e.g.,allof∼2500galaxiesintheNearbyGalaxiesCatalogarewithin1Mpcofagalaxygrouporcluster).Indeed,theevidencesuggeststhatupto
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90%ofgalaxiesresideingalaxygroups,ratherthaninthebetter–studiedgalaxyclusters(cf.Tully1987;Nolthenius1993).
Galaxygroupscanbedividedintotwotypesbythemorphologyoftheirgiantgalaxies(Mulchaey,etal.1996a;Burstein,etal.1997).ThedividinglinechosenbyMulchaey,etal.(1996b)isthatanygroupthathasmorethan1/3ofitsgiantgalaxiesoftypesgEorS0iscalledE/S0-rich;allothersarecalledS-rich.Two-thirdsoftheNolthenius(1993)nearbysampleofgalaxygroupsareS-richbythisdefinition;1/3areE/S0-rich(Burstein,etal.1997).ThemeanobservedvelocitydispersioninthegalaxygroupsclassifiedbyNoltheniusdecreasesbyafactoroftwofromE/S0-richtoS-richgroups(Mulchaey,etal.1996b).Subsequently,Zabludoff&Mulchaey(1998)andMulchaey&Zabludoff(1998)foundthatwhileE/S0-richgroupscancontain30ormoregalaxiesmoreluminousthanMB∼−16,typicalS-richgroups,suchasourownLocalGroup,containonly4-8galaxiesthisluminous.
Mulchaey,etal.(1996b)pointoutthatthehighervelocitydispersionofE/S0-richgroupscorrespondstoahotgastemperatureof∼0.9keV,whichiseasilyseenwithROSAT.Incontrast,forS-richgroups,thepredictedtemperaturebasedontheirlowervelocitydispersionaverages1/4ofthisvalue,makinganyX-rayemissionfromhotgassubjecttosevereextinctionbymetalsassociatedwithHIgasinourownGalaxy(Raymond&Smith1977).
Herewemakethereasonableassumptionthatlowvelocitydispersion,S-richgalaxygroupsdocontainhotgas(insomeS-richgroupssuchhotgashasbeenfound;cf.JSM00;Xu&Wu,2000).InSection2weshowthat,forarangeoftemperature,metallicityanddensity,muchofthehotgasingalaxygroupscancoolinfarlessthanaHubbletime.InSection3,webrieflydiscusstheconsequencesoftheinterrelationshipthatmustexistamongthedarkmatterdistributionandhotgasingalaxygroups,andtheconsequencesthatrelationshipholdsfortheformationandevolutionofgalaxiesinthegroups.Section4
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discussessomeofthevariedobservationsofgalaxiesandgalaxygroupsthattheassumptionsofthisLettercanhelpexplainorpredict.
2.CoolingTimesforHotGasinGalaxyGroups
ThemeasuredtemperaturesofX-rayemittinggasingalaxygroupsrangefromT∼0.3−1.0keV,withastrongcorrelationbetweenthevelocitydispersionofthegroupsandtheirhotgastemperatures(e.g.,JSM00).Thislowertemperaturelimitarisesnotfromtheabsenceofgroupswithlowvelocitydispersions(predominantlyS-rich),butratherfromtheHeandmetalnucleiabsorption(associatedwithGalacticHI)ofX-rayemissionfromhotgaswithT≤0.3keV(T≤3.5×106K)(e.g.,Raymond&Smith1977).Thefactthatlowerhotgastemperaturemeanslowerluminosity,andthattheX-raybackgroundishigherforT<0.5keV,alsocontributetothelackofX-raydetectedlowvelocitydispersiongroups.
Itiswell-knownthatX-rayemittinggaswithT∼106−107Kcancoolefficientlyunderthe“right”circumstancesofgasdensity,metallicityandtemperature(Nulsen,Stewart&Fabian1984).InFigure1weplotthegascoolingtimesasafunctionoftemperatureforarangeoftemperatureandmetallicityrelevantforgasingalaxygroups,usingtheresultsgivenbySutherland&Dopita(1993).Coolingtimesareinverselyproportionaltothedensityofgas;thedensity(nH=10−3cm−3)usedforFigure1correspondstothetypicalgasdensitiesseenintheinteriorsoftheE/S0-richgalaxygroups(cf.Mulchaey,etal.1996a).JSM00findsthatmostgalaxygroupshavehotgaswithmetallicitiesz>0.1.EvenforgasdensitiesafactoroftenlessthanusedforFigure1,coolingtimesarestilllessthanaHubbletimeforz>0.1,especiallyforthegastemperatures(T<3.5×106K)weexpecttofindinS-richgroups.
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3.
HotGasandGalaxyEvolution
TheexistingdataisconsistentwiththemassofhotX-rayemittinggasbeingcomparabletothevisiblemassofgalaxies,starsandgas(cf.discussioninJSM00).Thefactthathotintragroup/intraclustergascontainsmetalsimpliesthatanyprimordialgasthatwasoriginallytherewassubsequentlyenrichedfromstarformation.Consequently,duringthehistoryofthesegalaxies,theirstarformationprocessesmusthaveproducedacomparableamountofmassinhotgasreleasedintotheintragroupmediumasthereismassinthestarsthemselvestoday.ThecorrespondingproductionrateforhotgasinstellarevolutionimpliesanaveragestarformationratenotwildlydifferentfromwhatweseeinourGalaxytoday(cf.Bachiller1996).
Bycreatingthishotgasviastellarevolution,galaxiesareputtingasubstantialamountoftheirmassinto“hotstorage”,wheresubsequentcoolingandinteractionswithgalaxiesmaysignificantlyaffectthegalaxies’structureandevolution.Thisisyetanotherreason(cf.thedensity-morphologyrelationforgalaxies,Dressler1980)toviewtheformationandevolutionofgalaxiesnotinindividualterms,butratherinthecontextofthegalaxygroupsandclusterstowhichtheybelong.
Themass-to-lightratiosforgalaxygroups,derivedfromtheX-rayobservations,aresimilartothoseforgalaxyclusters,100-120insolarunits,Bmag(cf.JSM00),implyingdarkmatterdomination.ThemeanvelocitydispersionforS-richgroupsis1/2thatofE/S0-richgroups(cf.above),andE/S0-richgroupsalsohaveeffectiveradiithatare50%largerthanS-richgroups(cf.datainBurstein,etal.1997).Onaverage,E/S0-richgroupsaremoremassive(6×)anddenser(1.7×)thanS-richgroups.Becauseofitstightcorrelationwithvelocitydispersion,thehotgastemperatureingalaxygroupsisdeterminedbythedensityandmassofthegroup,whichinturnisdominatedbydarkmatter.
Itisalsonoteworthythatthevelocitydispersionswithinlargegalaxiesingroups
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tendtobecomparabletothevelocitydispersionsofthegalaxiesintheirgroups:E/S0galaxieshavetypicalvelocitydispersions(atL∗)of250kms−1,whilegiantspiralstypicallyhaveequivalentvelocitydispersionsof150-200kms−1(Burstein,etal.1997).Obviously,thistrendbreaksdownatthehighermassesofclusterswiththeirmuchhighervelocitydispersion,butthereisevidencethatclustersrepresenttheendpointoftheclusteringofgroupsindenseenvironments(Blumenthaletal.1984).Withingalaxygroups,the“darkmatter/visiblematterconspiracy”correspondingtothesimilarityofstellarmotionsanddarkmattervelocitydispersionthereforeappliestoboththeenvironmentofgalaxiesandtothegalaxiesthemselves(cf.Burstein&Rubin1985).
Thisviewisconsistentwiththehierarchical-clustering-merging(HCM)picture(Burstein,etal.1997)inwhichE/S0galaxiesareformedindenser,moremassiveenvironments,whilespiralsareformedinlowermass,lessdenseenvironments.ThenewwrinklehereissomethingthattheHCMscenariohastroublepredictingquantitatively(owingtoalackofphysicalunderstandingoftheoriginoftheinitialmassfunctionforstars).Namely,galaxiesputmuchoftheirbaryonicmassinto“hot”storage.Someofthishotgaswillbeinstorageinthehaloofthegalaxiesthemselves,whilesomewillbewithinthegeneralgroupmedium.
ThefactthatmuchofthishotgascancoolefficientlytocoldgasinlessthanaHubbletimecanhaveimportantobservationalimplications.Theinteractionofhotintragroupgaswiththecoldgasingalaxiesmovingwithinthegroupscanleadtocomplexhydrodynamicaleffectsincludingshocksandphasetransitions,withsubsequentcooling.Thisinteractionofhotandcoldgascanleadtoseveralinterestinggalacticphenomena(e.g.,tidaltails,galacticfountains,small,HI-richdwarfgalaxiesinorbitabouttheirparentgalaxies,suchasintheLocalGroup).Infact,shocksthatformintheinteractionregionleadtohigherdensitiesforthehotgas,withconsequentlyshortercoolingtimes.Quantitativelyaddressingthese
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complicationswillinvolvedetailedmodelingwithyet-to-be-writtenN-bodyhydrocodes,butitisreasonabletoassumethatthecoolingofthishotgasanditsinteractionswithextantgalaxiescansignificantlyaffecttheevolutionofthesegalaxies.Wenowexploresomeoftheplausibleobservationalconsequencesofthisinteraction.
4.ObservationalConsequencesofGalaxyInteractionswithIntragroupGasForthefollowing,wemakethreeassumptions:a)hotgasexistsinallgalaxygroups;b)
muchofthisgascancoolinmuchlessthanaHubbletime;andc)thecoolingrateisevenfasterwhenthereareshocksassociatedwiththeinterstellarmediumingalaxies.Theseassumptionsleadtoawiderangeoflinkedobservationalimplications.
1.Thedensity/morphologyrelationforgalaxies:Ithaslongbeenknown(e.g.Dressler1980)thatgalaxymorphologycorrelateswithlocalinitialconditionssuchasmassdensity.Almostcertainly,partofthiscorrelationiscausal,sincehigherdensityimpliesmoremajormerging,whichprobablyleadstoellipticals.Atthesametime,higherinitialdensitiesleadtogroupswithhighervelocitydispersionandgastemperature.WhilethegasinS-richgroupswillthereforebeabletocoolinlessthanaHubbletime,thehottergasE/S0-richgroupsmaybetoohot(∼107K,cf.JSM00andFig.1)toefficientlycool.
2.Galaxywarps,tidaltailsandHImasssurfacedensitydistributions:Mostedge-onspiraldiskshavewarpsthatpersistwellintotheirextendedHIdistribution(Sancisi1976),whichisconsistentwiththewarpmodeldevelopedtomatchtheHIdistribution(Bosma1981).Surprisingly,thereisevidencethattheratioofmasssurfacedensity(mostlydarkmatter)toHIsurfacedensityintheouterregionsofspiralsisaconstantoverseveralscalelengths(Bosma1981;Hoekstra,Sancisi,&vanAlbada2001),withasharpoutercut-offintheHIsurfacedensity(Hoekstraetal.).SomeHI-dominatedtidaltailsextending
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fromspiralgalaxieshavemoregasthanlikelyjustfromthegalaxiesfromwhichtheyareextended(Hibbard&Yun2001).
TheseobservationsofspiralgalaxiesareconsistentwithastrongcontinuinginterplaybetweencoldHIingalacticdisksandacooling,hotintragroupmedium.Inparticular,hotgasmaycooltoformapartoftheHIdisk,ortheremaybeapressureequilibriumbetweenthecolderdiskmaterialandthehotgas.Tidally-extrudedHIfromgalaxiescanpossiblystimulateformationofHIfromthehotgasthatsurroundsthegalaxies,likelyformingtheHIinpressureequilibriumwiththehotgas.Onlydetailedhydromodelingcantellusmoreabouthowthisinteractionmightwork,buttheprimafacieevidenceisthatthereissimplytoomuchmassinHItidaltailstohavecomejustfromtheouter,lowHIsurfacedensitypartsofspirals.TheobservedsharpcutoffsintheHIdisksofgalaxiesmightalsoarisefromdynamicalpressureorcoolingfromthehotgassurroundingthegalaxy.
3.HighvelocityHIclouds:TheseexistnearourGalaxy,withtheirdistancesandoriginstillhotlydebated(Blitzetal.1997;Sembach2002a,b).Recentobservations(cf.Sembach2002a,b)indicatethatthehighvelocitycloudsnearourGalaxyareenvelopedinhotgasoftemperature∼106K.Similarkindsof“highvelocityclouds”areobservednearthedisksofotherspiralgalaxies(Sancisi,etal.2001),suggestingthatthisphenomenonisnotisolated.Itispossiblethatthesecoldcloudscouldarisefromthermalcoolingstimulatedbythepassageofdwarfgalaxiesorbyanoutflowingwinddrivenbystarformationingalaxydisks.Similarly,whenwelookattheGalacticpolesinbothdirections,weseeHIgasinfallingtowardtheGalacticplane(cf.Tolbert1971).Thisisconsistentwiththeideathatalocalblowoutforagalaxydiskcanstimulatecoldgastoformfromthehotgasinthegalaxy’shalo,andfallbacktotheGalacticplane.
4.UltravioletspectroscopicobservationsofhighionizationabsorptionfeaturesindistantobjectsshowthepresenceofhotgasnearourGalaxyalongdifferentlinesofsight
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(e.g.,Dixon,Davidsen,&Ferguson1995;Savageetal.2002;Sembach2002b)aswellasinothergalaxygroups(e.g.,Tripp2002).AspointedoutbyMulchaey,etal.(1996b),thisispreciselywhathotgasinS-richgroups(orinourgalactichaloand/orinourLocalGroup)wouldproduce.
5.GalaxygroupswhichshowstrongtidalinteractionsamongtheirmembersoftenhaveasubstantialfractionoftheirHIoutsidetheirgalaxies.Forexample,theM81grouphas25%ofitsHIgasinitsintragroupmedium,ratherthaninthegalaxies(Appleton,Davies,&Stepenson1981).ThisdoesnotincludetheHIgasfromtheintragroupmediumthathasalreadybeendumpedonM82andNGC3077andformedstarsthere.ItisunlikelythattidaltailsalonecanproducesuchahighpercentageofHIgasinthegeneralgroupmedium(again,theyarisefromtherelativelylowdensityouterpartsofgalaxies).MuchoftheintragroupgasintheM81groupcouldhavecomefromcoolingofthehotgasinthatgroup,stimulatedbythetidalinteractionsofthegalaxies,aswellaswithsubsequentstarformationinM82andNGC3077.
6.Thesolutionofthe“G-dwarfproblem”inourGalaxymostlikelyrequiresaconstantinfalloflowmetallicitygas(cf.Chiappina,Matteucci,&Gratton1997).Similarly,semi-analyticmodelsofgalaxyevolutionseemtoimplytheneedforcontinuousinfallofgasfromoutsidegalaxiestoproducethegalaxiesweseetoday(cf.Sembach2002a).Hotgasinthehaloortheimmediateenvironmentofspiralgalaxies,whichisofmoderatelylowmetallicity,andwhichcancooltocoldgas,canprovidejustsuchareservoir.
7.SpiralarmsappearinHIdisksofspiralsoutsidethedisksofstars(e.g.,Bosma1981).Giventhatgalaxiesmoveatroughlythespeedofsoundthoughtheirintragroupgas,theshocksthatareproducedcanhelpstimulatespiralarmformationintheHIgasintheouterpartsofgalaxies.
8.TherelationshipbetweenkinematicvelocityfieldsandHIdistributionsinspiral
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galaxiesiscomplicated.Whilethekinematicvelocityfieldsinmanynon-interactingspiralgalaxiesaretypicallyveryregular,theangulardistributionofHIgas(and,tosomeextent,thediskstars)canbeveryirregular(e.g.,Bosma1981).Inextremecases,theHIdensitydistributionwithinaspiralgalaxydiskcanbeverylopsided,whilepreservingaregularvelocityfield.Thereareexceptionalcasesinwhichthekinematicvelocityfieldscanalsobelopsided(Swaters,etal.1999).Ifcoldgasisbeingproducedbycoolingofhottergasduetointeractionswith,forexample,gasproducedbystarformationinthegalaxydisks,thiscouldeasilyleadtoahighlyvariablegasdensitywithoutsignificantlyaffectingthekinematicvelocityfield.Thisalsosuggeststhatthedisksformfromtheinsideout(highergasdensityinthegalaxyinteriors),producingbluecolorgradientsowingbothtolowermetallicityandyoungerstarsasafunctionofincreasingradius(cf.Courteau&Holtzman1995).
Ofcourse,theaboveisnotanexhaustivelistofobservationalconsequencesofourassumptions.Moreover,noteveryoneoftheseobservationsisnecessarilyaconsequenceofourassumptions.Rather,wesuggestthatalloftheobservationslistedhereareconsistentwithourassumptions.Basedonthisdiscussion,wehaveestionsforfutureinvestigation:
i.Densityinfluencestheformationandevolutionofgalaxiesinmorethanjustdefiningthecircumstancesoftheirbirth.Rather,thetemperatureofthehotgastheyproduceintheirgroupsalsocandictatetheirsubsequentevolution.Hydromodelingofgalaxiesmusttakeintoaccounttheenvironmentsinwhichwefindthem,whicharegroup-dominated.
ii.Observationsindicatearangeingasmassingalaxygroups(JSM00),someofwhichmaybeobservationally-driven,butsomeofwhichmaybereal.Separately,spiralgalaxiesarefoundwithawiderangeinsurfacebrightness(cf.Impeyetal.1996).Therecouldbearelationshipbetweengasmass/gastemperaturerangesingalaxygroupsandthesurfacebrightnessoftheirspiralgalaxiesthatshouldbeexplored.
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iii.Similarly,HIcontentandpropertiesofspiralgalaxieswoulddependonthegalaxygroupsinwhichwefindthem.TheknownlargevariationofHIpropertiesamongspiralgalaxies(Bosma1981;Hoekstra,Sancisi,&vanAlbada2001)makesthispredictiononlytestablewithalargestatisticalsampleofgalaxiesinawiderangeofgroupenvironments.Unfortunately,mostofthenearby,largespiralswhichhavebeenwell-studiedfortheirHIdistributionarepartoftheComa-SculptorCloud,whichiscompletelycomprisedofS-richgroups(Tully1987).
iv.Ofcourse,directdetectionofhotgasinlowdispersiongalaxygroupswoulddirectlyproveourassumptions.Unfortunately,theverylongexposuretimesnecessaryforthistohappenmakesuchobservationshardtodo.
IfthewaygalaxyformationandevolutionproceedsaswehaveoutlinedinthisLetteriscorrect,thentwokeypiecesofinformationaremissingfromhigh-zgalaxyimages:thethree-dimensionaldistributionofdarkmatterandthekindsofhotgasthatarefoundateachstageofgalaxyformationandevolution.Knowingthetemperatureanddensityofhotgasingroupsasafunctionofredshiftwouldallowonetomakebetterobservationalpredictionsfordifferentrangesoftime.
Inclosing,itisclearthatmanyofthepossibleobservationalconsequenceswehaveidentifiedcanbemodelednumerically.Inprinciple,N-bodyhydrocodescanadequatelymodeltheinteractionsofhotgasingalacticgroupswiththedisksofspiralgalaxies.However,thestarformationprocessandthewaythatstarformationrelatestogalaxy(andsurroundinggroup)propertiesmayturnouttobeanessentialelement.Untilweunderstandthephysicsthatproducestheinitialmassfunctionsofstars,anyhypothesessuchasoursthatinvolveinteractionofhotgasingalaxygroupswithcoldgasandstarformationingalaxiesmustremainatleastpartlyspeculative.
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WewishtothankDr.GeorgeCoyne,SJ,Dr.Jose’Funes,SJ,Dr.EnricoCorsiniandProfessorFrancescoBertolafororganizingtheJune2000meetingon“GalaxyDisksandDisksinGalaxies”,andconversationswithRenzoSancisi,whichgreatlystimulatedourideas.
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Fig.1.—Thelogarithmofthecoolingrates(inyears)ofhotgasasafunctionofthelogarithmofitstemperature(givenbothintermsofKandinkeV),forfourvaluesofmetallicityrelativetosolar(z);1.0,0.1,0.01and0.00.Dottedlinesaredrawnbetweenthez=1.0andz=0.1curvesatvaluesofLogT=5.78and6.5,correspondingtotherangeofobservedvelocitydispersionsinlow-σgalaxygroups,foragasdensityofnH=10−3cm−3.Adottedlineatacoolingtimeof1Gyrisdrawntoguidetheeye.
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