Morphological studies of the galaxy populations in

arXiv:astro-ph/9711019v14Nov1997MorphologicalStudiesoftheGalaxyPopulationsinDistant“Butcher–Oemler”ClusterswithHST.II.AC103,AC118,andAC114atz=0.31.1WarrickJ.CouchSchoolofPhysics,UniversityofNewSouthWales,Sydney2052,AustraliaAmy.J.Barger2InstituteofAstronomy,MadingleyRd,CambridgeCB3OHA,UKIanSmailDepartmentofPhysics,UniversityofDurham,SouthRd,DurhamDH13LE,UKRichardS.EllisInstituteofAstronomy,MadingleyRd,CambridgeCB3OHA,UKandRayM.SharplesDepartmentofPhysics,UniversityofDurham,SouthRd,DurhamDH13LE,UKABSTRACTWepresentnewresultsofaprogramtostudythedetailedmorphologiesofgalaxiesinintermediateredshiftclustersandhenceunderstandthephysicaloriginoftheenhancedstarformationseenintheseenvironmentsatearlierepochs.Deep,highresolutionimageryhasbeenobtainedof3richclusters,AC103,AC118&AC114atz=0.31,throughtheR(F702W)filteroftheWideFieldCamera(WFPC-2)oftheHubbleSpaceTelescope(HST).ForAC103andAC118,single3–orbitpointingscoveringacentral∼0.5×0.5h−1Mpcareahavebeenobtained.ForAC114,amosaicoffour6–orbitpointingshasprovidedcoverageofacentral∼1.2×0.7h−1Mpcarea,allowingforthefirsttimeastudyextendingtotheouterregionsofamoredistantcluster.Morphologicalclassificationhasbeenconductedvisuallyusingtheseimages,withHubbletypesplusevidenceofdynamicalinteractionsand/orstructuralabnormalitiesrecordedforallgalaxiesdowntoR702=22.25inAC103&AC118,andR702=23.00inAC114.NewspectroscopylimitedatK′≤18hasalsobeenacquired,providingwithinourWFPC-2imagesatotalsampleof129confirmedclustermembersofwhich117havesecurestarformationclassifications(eg.starburst,post–starburst,Hδ–strong).OurstudyrevealsthatthemixofHubbletypeswithinthedistantclustersis,overall,significantlydifferenttothatseeninthesamehigh–densityenvironments1BasedonobservationsobtainedwiththeNASA/ESAHubbleSpaceTelescopewhichisoperatedbySTSCIfortheAssociationofUniversitiesforResearchinAstronomy,Inc.,underNASAcontractNAS5-26555.2presentaddress:InstituteforAstronomy,UniversityofHawaii,2680WoodlawnDr.,Honolulu,HI96822–2–nearby:thereareproportionallymorespiralgalaxiesandfewerE/S0galaxiesattheseearlierepochs.Withinthecore(r∼200h−1kpc)regionsofthe3clusters,spiralscoveringthefullSa–Sdm/Irrrangearepresentinnumbersuptoafactorof∼4higherthanthatseeninpresent–dayclusters,thehighestfractionbeingobservedintheleastmassiveclusterwithinoursample,AC103.Onlyinthevirialisedcoreofourmostmassive,regularcluster,AC114,doweseemorphologicalfractionsapproachingthoseofthepresentepoch.However,outsidethecoreofthisclusterthespiralfractionrisesto∼60%–3timesabovethepresent–daylevel.Dynamicalinteractionsarealsowidespreadthroughoutthepopulationsofallthreeclusterswith∼20%ofthemembers,onaverage,showingmorphologicalevidenceofthisphenomenon.Thevarioussubsetsof‘active’clustermembersshowemergentmorphologicaltrends.Themajorityofbluegalaxieseitherundergoingastarburstorseenshortly(0.5Gyr)thereafteraresystemsinvolvedinmajormergers.Thesegalaxies,however,aregenerallyofmodestluminosity(L∼L∗+1mag)eveninthisbrightenedphase;intheirfadedstatetheyappeardestinedtobecomedwarfs,toofainttobeincludedinmagnitude–limitedspectroscopicsamplessuchasours.Clustermemberswithongoingstarformationtypicalofpresent-dayspiralsaremostlynormalSb–Sdm/IrrHubbletypes.Galaxiesidentifiedashavingcompletedtheirlastmajorepisodeofstarformation1–2Gyrpriortotheepochofobservationareconspicuousbytheircommonalityinmorphology,allbeingearly–type(S0–Sb)disksystems.TheredHδ–strongobjects,interpretedfromtheircolorsandspectraasbeingtheremnantsofsecondarystarformationinolddormantsystems,havemorphologiesconsistentwiththispicture,beingamixtureofnormalEandS0galaxies.Incombination,theseresultspointtoclustergalaxyevolutionbeingdrivenbynotonebutseveraldifferentphysicalprocesses.Theonlyconspicuousoneisgalaxy–galaxyinteractionsandmerging.However,ourstudyrevealsmanygalaxieswherethisprocesscouldnothavebeenresponsibleforalteringthecourseoftheirstarformationactivity.Furthermore,thenormaldiskmorphologyofthemajorityofthesepreviouslyactivegalaxiesindicatetheoperationofprocess(es)whichhaltstarformationbutleavethebasicdiskstructureintactandlargelyunperturbed.Thistendstofavourmechanismswhichaffectthegassupply(eg.ram-pressurestripping,galaxyinfall)ratherthanprocesseslike“galaxyharassment”wherewesimplydonotseethepopulationofseverelydistortedspiralsexpectedifitsoperationiswidespread.Subjectheadings:cosmology:observations–galaxies:evolution–galaxies:photometry,spectroscopy–clustersofgalaxiesReceived;accepted–3–1.IntroductionOneofthelongest–standingandmoststrikingpiecesofevidenceforgalaxyevolutionistheso–called“Butcher-Oemler”(BO)effectwherebythecoresofrich,regularclustersatz∼0.2areobservedtocontainmanymorebluegalaxiesthanintheirpresent–daycounterparts(Butcher&Oemler1978,1984;Couch&Newell1984;seealsoRakos&Schombert1995).Yet,understandinghowclusterpopulationscanbesodifferentatrecentlook–backtimesanddeterminingthephysicalprocessesresponsiblefortheirsubsequenttransformationremainanimportantchallengeinobservationalcosmology.Thisseriesofpapersprovidesanimportantcontributiontowardstheresolutionoftheseissuesthroughcomprehensiveanalys