Stellar Populations, Butcher-Oemler Effect, Star Formation in Clusters

InternationalAstronomicalUnion,2003O.Engvold,ed.

StellarPopulations,Butcher-OemlerEffect,StarFormationinClusters

BiancaM.Poggianti

OsservatorioAstronomicodiPadova-INAF,Italy

Abstract.InthisbriefoverviewofthestellarpopulationsofgalaxiesinclustersIhighlightsomeofthemostrecentresults,includingthedownsiz-ingofthek+apopulationandthestarformation–localdensityrelation.Iconcludediscussingsomeopenquestionsregardingtheinterpretationoftheobservationalresults,andspeculatingonthecurrentmeaningofthewords“primordial”and“environmental”.

1.Introduction

Studyingtheevolutionaryhistoriesofgalaxiesinclustershasalwaysbeenanendeavourtounderstandhowgalaxies(allgalaxies)cametobeasweobservethem.Inrecentyears,thisquestcanbebetterdescribedasanefforttocom-prehendhowgalaxiesformedandevolvedina“changing”environment.Infact,thecommonframeworkforthesestudiesisnowahierarchicaluniverse,inwhichgalaxiescanandoftenchangeenvironmentduringtheirevolution.Whenagalaxyinfallsintoacluster,itcandosoasasingleisolatedgalaxy,asapair,aspartofagrouporasmemberofanothermergingcluster.Agalaxycanexperienceverydifferentenvironmentalconditionsthroughoutitsevolution.Amajorchallengefortoday’sresearchistoidentifytheeffectsproducedbythevariousenvironmentsontheobservablepropertiesofgalaxies.Itisespeciallyhardtodiscriminatebetweenminorandmajorinfluences.Ifallenvironmentsarepotentiallyexpectedtohavesomeimpactontheirgalaxies,wewouldliketodiscernsecondaryeffects(“cosmetics”)fromtheprimarycausesthatdeterminehowgalaxiesare.

Inthissearchwecanrelyonsomeobjectiveandmeasurableproperties,suchasmassinstarsandgas,theirchemicalcontent,theirmotionwithinthegalaxyandthestructurestheydesign.Thestarformationactivityisoneofthemostoutstandingcharacteristics.Thoughaprecisemeasurementofthecurrentandpaststarformationactivityisnoeasytaskforanygalaxy,starformationaffectstheaccessibleobservables,suchascolorsandspectra,insuchamanifestwaythatisoftenthemostimmediateevidenceforevolution.

InthiscontributionIfocusonthebuild-upofthestellarpopulationsinclus-tergalaxiesandontheevolutionwithredshiftoftheirstarformationactivity.Thesubjecthasbeenveryrichinresultsandpaperspublishedduringthepast25years,anditisimpossibletogiveanexhaustivesummaryinashorttext.Iwillthereforeonlytouchuponsomeoftheissues,andhighlightjustacoupleofthemostrecentresults.Overviewsontheevolutionofthestellarpopulationsinclus-1

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tergalaxiescanbefoundinEllingson2003,Poggianti2003aandseveralreviewsinthethirdvolumeoftheCarnegieObservatoriesAstrophysicsSeries:Clus-tersofGalaxies:ProbesofCosmologicalStructureandGalaxyEvolution(eds.J.S.Mulchaey,A.Dressler,andA.Oemler(Pasadena:CarnegieObservatories,http://www.ociw.edu/ociw/symposia/series/symposium3/proceedings.html).2.

Whereitallstarted....andwhereweare

TheButcher-Oemlereffectistheexcessofgalaxiesbluerthanthecolor-magnituderedsequenceinclustersatz>0.1−0.2ascomparedtotherichestnearbyclus-ters(Butcher&Oemler1984).

Theoriginofthescatterinthefractionofbluegalaxiesfromclustertoclus-teratanygivenredshift,andtheconsequentsearchforcorrelationsbetweenthebluefractionfBandtheglobalclusterproperties(Smailetal.1998,Margonineretal.2001,Metevieretal.2000);thetrendwithredshiftoffBinopticalandX-rayselectedclustersamplesandthedependenceonclusterselection(Andreon&Ettori1999,Ellingsonetal.2001,Kodama&Bower2001,Fairleyetal.2002);thechangeingalaxycolorswithclustercentricdistance(Pimbbletetal.2002);thenatureofthegalaxiesgivingrisetotheButcher-Oemlereffect(DeProprisetal.2003);thesearesomeoftheaspectsrelatedtotheButcher-Oemlereffectthatarestillsubjectofongoinginvestigation,witnessingtheinterestinfullyunderstandingtheconsequencesandthecauseoftheseminalButcher&Oemlerfindings.

Putinaslightlydifferentway,theButcher-Oemlereffectisthepresenceoflargenumbersofbluegalaxiesinrichclustersmoredistantthanthoseinthelocaluniverse.ThishigheraveragelevelofactivityinthepasthasbeenconfirmedandgreatlyclarifiedbyspectroscopicsurveysandbyHubbleSpaceTelescopemorphologicalstudiesofgalaxiesindistantclusters(Couchetal.1994,1998,Dressleretal.1997,Baloghetal.1997,1998,1999,Poggiantietal.1999,vanDokkumetal.1999,2000,Fabricantetal.2000,Ellingsonetal.2001,Postmanetal.2001,Smailetal.2001,Lubinetal.2002).

Nomatterhowtheevolutioninthepopulationsofclustergalaxiesisob-served,ifusingthebluecolors,thespectralfeaturesortheproportionofspiralversusearly-typegalaxies:allthethreetypesofobservationscontributetode-lineateapictureinwhichtheevolutionofgalaxiesinclustersisstrong,giventhatalargefractionofthematz=0haveevolvedfromstar-forming,late-typegalaxiestopassivelyevolving,early-typegalaxieswithinarelativelyshortperiodoftime(Kodama&Smail2001,vanDokkum&Franx2001).

Atleastthreephenomenalikelyplayaroleinthisevolution(Poggiantietal.1999,Ellingsonetal.2001,Kodama&Bower2001):thedeclininginfallrateofgalaxiesontoclustersatlowerzpredictedbyhierarchicalcosmologicalmodels(Bower1991,Kauffmann1995);theevolutionwithzoftheaveragestarformationrate(SFR)inthe“field”galaxiesthatinfallintoclusters;andthedeclineofstarformationintheclustergalaxieslikelyduetosomephysicalprocess(orprocesses)thatactswhengalaxiesinfallinthedenserenvironment.Infact,thoughtheuniverseasawholeseemstoevolvetowardsaprogressivelylowerstarformationactivityintheMadauplot,suchatrendappearstobeacceleratedinclusters(Kodama&Bower2001).

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ThepaststarformationactivityasseenfromtheabsorptionlinesThemostcitedresultobtainedfromspectraofgalaxiesindistantclustersisthepresenceof“k+a”spectra,withnoemissionlinesandstrongBalmerlinesinabsorptionindicatingavigorousstarformationactivitythatwasterminatedatsomepointduringthelast1-1.5Gyr(Dressler&Gunn1983,Couch&Sharples1987).Sincesuchspectrainequivocallyidentifypost-starburstorpost-starforminggalaxies,theirexcessindistantclustersascomparedtothefieldatsimilarredshiftsisstrong,ifnotthestrongest,evidencethatstarformationistruncatedbythedenseenvironment(Dressleretal.1999,butseeBaloghetal.1999foradifferentview).K+agalaxieshavebeenthoroughlydiscussedelsewhere(e.g.Poggianti2003bandreferencestherein),butitisworthhighlightinghereaveryrecentresultfromTranetal.(2003).Onthebasisofthegalaxyvelocitydisper-sionsandradii,theseauthorsfindthatthedescendantsofthek+a’sobservedatz∼0.8mustbeamongthemostmassiveearly-typegalaxiesinclusterstoday,whilethemaximumluminosityandgalaxyvelocitydispersionofclusterk+agalaxiesdecreasetowardslowerredshifts.AnotherresultpointinginthesamedirectionispresentedinPoggiantietal.(2003),whereweanalyzedthelumi-nositydistributionofk+agalaxiesintheComacluster:nok+aasluminousasinthedistantclustersisobserved,whilek+a’sarefoundtobeasignificantfractionofthedwarfgalaxypopulation.

TheongoingstarformationactivityasseenfromtheemissionlinesIntheabsenceofasignificantAGNionizingradiation,emissionlinesarepresentinaspectrumonlywhenthereisongoingstarformationactivity.Inagreementwiththeotherphotometricandspectroscopicsignsofcurrentactivity,asignificantfractionofeventhemostluminousgalaxiesindistantclustershavebeenobservedtohaveemissionlinesindicatingtheyareactivelyformingstars(Dressleretal.1999,Postmanetal.2001),thoughthefractionofemissionlinegalaxiesremainslowerinclustersthaninthefieldatallredshifts.

Whetherthestarformationactivityinclustersisenhancedinsomeoftheinfallinggalaxies,andhowrelevantthisisfortheglobalclusterpopulationsisstilldebated.Besidesconsiderationsbasedonopticalobservationsinfavorandagainstcluster-triggeredstarbursts,astrikingresultcomesfromdust-freewavelengths:thelargenumbersofLuminousInfraredstarburstgalaxiesdetectedbyISOCAMindistantclustersseemtobetoohightobeconsistentwithsimpleaccretionofstarburstgalaxiesfromthefield(Ducetal.submitted,Coiaetal.submitted).

Finally,severalworkshavebeenrecentlydevotedtothestudyoftheemis-sionlinetrendwithclustercentricradius.Baloghetal.(1997,1998)haveshownhowthemeanequivalentwidthof[Oii]increasesradiallyindistantclusters,failingtoreachthemeanfieldvalueevenasfaroutasseveralMpcfromtheclustercentre.Anextensionoftheseradialstudieshasbeencarriedoutusingthelargedatasetsofthe2dF(Lewisetal.2002)andSloan(Gomezetal.2003)spectroscopicsurveys.AsshowninFig.1,themeanEW(Hα)increasesatlowerprojectedlocalsurfacedensityofgalaxies(and,correspondingly,largerdistancefromclustercentre)downtoadensityof∼1galaxyperMpc2,andapproachesthefieldvalueat∼3timesthevirialradiusofthecluster.

ThemeanEW,orSFR,usedinthesestudiesisthemeanoverallgalaxies(withandwithoutemissionlines,ofanymorphologicaltype),includingthepas-

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Figure1.Left-FromLewisetal.2002(2dF):µ∗,thestarforma-tionratenormalizedtoL∗,isproportionaltotheEW(Hα).Itstrendasafunctionofthelocalgalaxydensityisshownforallclusters(solidpoints)andforclusterswithσgreaterthanorlessthan800kms−1(tri-anglesandcrosses,respectively).Thehorizontalsolidlinerepresentsthefieldvalue.Right-FromGomezetal.2003(Sloan):theshadedarearepresentsthedistributionofstarformationrates,correctedforextinction.Themedianisshownasasolidline.Thetopoftheshadedareaisthe75thpercentile,whilethebottomisthe25thpercentile.siveearly-typegalaxiesthatdominatetheclustercoresandthehighestdensityregions.AmeantotalEWdecreasingwithdensityandtowardstheclustercen-treisthereforenotsurprisinginthisrespect.WhetherthetrendofmeanEWsimplyreflectsadifferentmorphologicalmixasafunctionoflocaldensity,andwhetheritcanbefullyexplainedbythemorphology-densityrelation(Dressleretal.1997)isstillunclear(Lewisetal.2002,Gomezetal.2003).

TheoriginoftheobservedEWtrendremainsinmyopiniononeofthemostinterestingquestionstobeanswered,togetherwiththeoriginofthemorphology-densityrelationandtherelationbetweenthetwo.Asystematicvariationofgalaxypropertieswiththeenvironmentdoesnotnecessarilyimplya“transfor-mation”duetoanenvironmentalprocessbeginningtoactonagalaxywhenthisentersanenvironmentforthefirsttime.Inprinciple,onecannotexcludethatthecorrelationofstarformationwithlocaldensitycanbepartlyorfullyexplainedasan“imprinting”onthegalaxystarformationhistoryestablishedintheearlyuniverseasafunctionofthelocaldensity.Forexample,themostmas-siveellipticalsinthedensestregionstoday(theclustercores),thatformedmostoftheirstarsatveryhighredshifts,wereinthedensestregionsalsoatz>3anditisthen,bythoseconditions,thattheirstarformationhistorywasdecided.ThesegalaxiescontributetotheshapeoftheEW-densityrelationtodayassome

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ofthegalaxieswithEW=0(devoidofSF),anditisthereforelogicaltoconcludethatatleastpartoftheSF-densityrelationhasbeenestablishedveryearlyon.Similarly,themorphology-density(MD)relationmusthavea“primordial”com-ponent,sinceatallredshiftsthemostmassiveearly-typegalaxiesarefoundinthehighestdensityregions.However,theMDrelationasweobserveittodayisnotfullyestablishedathighredshift,becauseweobservehowitevolvesinclus-ters,withlatetypespiralsbeingtransformedintoearly-typegalaxies(Dressleretal.1997,Lubinetal.2002).

Itisprobablytimefortheastronomicalcommunityworkingonclusterstoreconsiderthemeaningofwordssuchas“primordial”and“environmental”.By“primordial”,dowemeananythingthattookplaceatz>3,oranythingthatwasalreadyengravedinthegalaxy“destiny”bytheearlyconditionsinthelocationwheretheyformedtheirfirststars?Andby“environmental”,dowemeananyphysicalprocessthataffectsgalaxiesoncetheyentera“new”environment,implyingthattheirevolutionwouldhavebeendifferentiftheydidnotbecomepartofit?Ordoweconsideraneffectenvironmentalanytimeithastodowithsomethingexternaltothegalaxyitself?Inthiscase,whatdoweconsiderasthe“galaxyitself”,anythingthatwillbepartofthatgalaxybyz=0?Canaprocessbeconsideredanenvironmentaleffectevenifitonlyaffectedthegalaxyatz>3?Re-thinkingthedefinitionofthewords“primordial”and“environmental”isnotjustasemanticexercise,itisprobablyusefultomakeprogressinanerainwhichwebelievegalaxiescanchangeenvironment,buttoday’senvironmentisnotnecessarilyunrelatedwiththeenvironmentsofyesterday.

Acknowledgments.IthanktheIAUandtheorganizersofthisJointDis-cussiononclustersfortheirkindinvitationandforgenerouslysupportingmyparticipationwithaIAUtravelgrant.References

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