A Search for the Most Massive Galaxies. II. Struct

arXiv:0809.2602v1[astro-ph]15Sep2008Mon.Not.R.Astron.Soc.000,1–21(0000)Printed15September2008(MNLATEXstylefilev2.2)ASearchfortheMostMassiveGalaxies.II.Structure,EnvironmentandFormationM.Bernardi1,J.B.Hyde1,A.Fritz1,R.K.Sheth1,K.Gebhardt2&R.C.Nichol31DepartmentofPhysicsandAstronomy,UniversityofPennsylvania,209South33rdStr.,Philadelphia,PA19104,USA2AstronomyDepartment,UniversityofTexas,1UniversityStation,C1400,Austin,TX78712,USA3InstituteofCosmologyandGravitation,MercantileHouse,HampshireTerrace,Univ.ofPortsmouth,Portsmouth,PO12EG,UK15September2008ABSTRACTWestudyasampleof43early-typegalaxies,selectedfromtheSloanDigitalSkySurvey(SDSS)becausetheyappearedtohavevelocitydispersionsofσ≥350kms−1.High-resolutionphotometryintheSDSSipassbandusingtheHigh-ResolutionChanneloftheAdvancedCameraforSurveysonboardtheHubbleSpaceTelescopeshowsthatjustlessthanhalfofthesampleismadeupofsuperpositionsoftwoorthreegalaxies,sothereportedvelocitydispersionisincorrect.Theotherhalfofthesampleismadeupofsingleobjectswithgenuinelylargevelocitydispersions.Noneoftheseobjectshasσlargerthan426±30kms−1.Theseobjectsdefineratherdifferentsize-,mass-anddensity-luminosityrelationsthanthebulkoftheearly-typegalaxypopulation:fortheirluminosities,theyarethesmallest,mostmassiveanddensestgalaxiesintheUniverse.Althoughtheslopesofthescalingrelationstheydefineareratherdifferentfromthoseofthebulkofthepopulation,theylieapproximatelyparalleltothoseofthebulkatfixedσ.Thissuggeststhattheseobjectsaresimplythelarge-σextremesoftheearly-typepopulation–theyarenototherwiseunusual.Theseobjectsappeartobeoftwodistincttypes:thelessluminous(Mr−23)objectsareratherflattened,andtheirpropertiessuggestsomeamountofrotationalsupport.WhilethismaycomplicateinterpretationoftheSDSSvelocitydispersionestimate,andhenceestimatesoftheirdynamicalmassanddensity,wearguethattheseobjectsareextremelydensefortheirluminosities,suggestingmergerhistorieswithabnormallylargeamountsofgaseousdissipation.Themoreluminousobjects(Mr−23)tendtoberoundandtolieinoratthecentersofclusters.Theircircularisophotes,largevelocitydispersions,andenvironmentsareconsistentwiththehypothesisthattheyareBCGs.ModelsinwhichBCGsformfrompredominantlyradialmergershavinglittleangularmomentumpredictthattheyshouldbeprolate.Ifviewedalongthemajoraxis,suchobjectswouldappeartohaveabnormallylargevelocitydispersionsfortheirsizes,andtobeabnormallyroundfortheirluminosities.Thisistrueoftheobjectsinoursampleonceweaccountforthefactthatthemostluminousgalaxies(Mr−23.5),andBCGs,becomeslightlylessroundwithincreasingluminosity.Thus,theshapesofthemostluminousgalaxiessuggestthattheyformedfromradialmergers,andtheshapesofthemostluminousobjectsinourbig-σsamplesuggestthattheyarethedensestoftheseobjects,viewedalongthemajoraxis.Keywords:galaxies:ellipticalandlenticular,cD—galaxies:evolution—galaxies:kinematicsanddynamics—galaxies:structure—galaxies:stellarcontent1INTRODUCTIONThemostmassivegalaxiesmayplaceinterestingconstraintsonmodelsofgalaxyformation(e.g.DeLuciaetal.2006;Almeidaetal.2007).Butwhichobservableoneshoulduseasaproxyformassisdebatable.Ifluminosityisagoodproxy,thentheBrightestClusterGalaxiesshouldbethemostmassivegalaxies;thishasledtoconsiderableinterestintheirproperties(Scott1957;Sandage1976;Thuan&Ro-manishin1981;Malumuth&Kirschner1981;Hoesseletal.1987;Schombert1987,1988;Oegerle&Hoessel1991;Lauer&Postman1994;Postman&Lauer1995;Crawfordetal.1999;Laineetal.2003;Laueretal.2007;Bernardietal.2007a).2Bernardietal.SDSSJ112626.6+003620.7SDSSJ083551.2+392621.7SingleSDSSJ013431.5+131436.4SingleSDSSJ162332.4+450032.0SingleSDSSJ010803.2+151333.6SingleSDSSJ083445.2+355142.0SingleSDSSJ091944.2+562201.1SingleSDSSJ155944.2+005236.8Figure1.Surfacebrightnessisophotesoftheobjectsinoursample,whichprovidethebasisfordeterminingwhichobjectsaresingles.Thetwopanelsforeachobjectshowalarger5×5arcsecregionwithlogarithmicallyspacedisophotes,anda1×1arcsecregionwithlinearlyspacedisophotes.ThicksolidcircleshowsthesizeoftheSDSSfiber;structurewithinthiscircleislikelytohavecontributedtotheestimatedvelocitydispersion,whereasstructuresoutsideithavenot.However,velocitydispersionissometimesusedasasur-rogateformass(thevirialtheoremhasmass∝Rσ2),soitisinterestingtoaskifasampleselectedonthebasisofvelocitydispersionalsocontainsthemostmassivegalaxies.Suchasampleisalsointerestinginviewofthefactthatblackholemasscorrelatesstronglyandtightlywithvelocitydispersion(Ferrarese&Merritt2000;Gebhardtetal.2000),sogalax-ieswithlargeσareexpectedtohostthemostmassiveblackholes.Withthisinmind,Bernardietal.(2006)culledasampleof∼100objectswithσ350kms−1fromtheFirstDataReleaseoftheSloadDigitalSkySurvey(SDSSDR1;Abaza-jianetal.2003).Thetotalareafromwhichtheseobjectswereselectedisabout2000deg2;foraspatiallyflatUni-versewithΩm=0.3andHubbleconstantH0=70kms−1,whichweassumeinwhatfollows,thiscorrespondstoaco-movingvolumeofabout3.34×108Mpc3outtoz=0.3.Someoftheseobjectsturnedouttobeobjectsinsuper-position,evidenceforwhichcameprimarilyfromthespectra(seeBernardietal.2006fordetails).Arandomsubsetoftheothers,43objectsinall,wasobserved