Fornax compact object survey FCOS On the nature of

arXiv:astro-ph/0401610v129Jan2004Astronomy&Astrophysicsmanuscriptno.mieskeFebruary2,2008(DOI:willbeinsertedbyhandlater)FornaxcompactobjectsurveyFCOS:OnthenatureofUltraCompactDwarfgalaxiesSteffenMieske1,2,MichaelHilker1,andLeopoldoInfante21SternwartederUniversit¨atBonn,AufdemH¨ugel71,53121Bonn,Germany2DepartamentodeAstronom´ıayAstrof´ısica,P.UniversidadCat´olica,Casilla104,Santiago22,ChileReceived21November2003/Accepted16January2004Abstract.TheresultsoftheFornaxCompactObjectSurvey(FCOS)arepresented,complementingthefirstpartoftheFCOSdescribedinMieskeetal.(2002).TheFCOSaimsatinvestigatingthenatureoftheUltraCompactDwarfgalaxies(UCDs)recentlydiscoveredinthecenteroftheFornaxcluster(Drinkwateretal.2000a).280unresolvedobjectswithcolours0(V−I)1.5maginthemagnitudespacecoveringUCDsandbrightglobularclusters(18V21mag)wereobservedspectroscopically.54newFornaxmemberswerefound,plusfiveofthesevenalreadyknownUCDs.Theirdistributioninradialvelocity,colour,magnitudeandspacewasinvestigated.Itisfoundthatbrightcompactobjects(V20orMV−11.4mag),includingtheUCDs,haveahighermeanvelocitythanfaintcompactobjects(V20mag)at96%confidence.ThemeanvelocityofthebrightcompactobjectsisconsistentwiththatofthedwarfgalaxypopulationinFornax,butinconsistentwiththatofNGC1399’sglobularclustersystemat93.5%confidence.ThecompactobjectsfollowacolourmagnituderelationwithaslopeverysimilartothatofnormaldEs,butshiftedabout0.2magredwards.ThemagnitudedistributionofcompactobjectsshowsafluenttransitionbetweenUCDsandGCs.Aslightoverpopulationof8±4objectsforV20magisdetectedwithrespecttotheextrapolationofNGC1399’sGCluminosityfunction.Thespatialdistributionofbrightcompactobjectsisincomparisontothefaintonesmoreextendedat88%confidence.Allourfindingsareconsistentwiththethreshingscenario(Bekkietal.2003),suggestingthatasubstantialfractionofcompactFornaxmembersbrighterthanV≃20magcouldbecreatedbythreshingdE,Ns.FainterthanV≃20mag,themajorityoftheobjectsseemtobegenuineGCs.Ourresultsarealsoconsistentwithmergedstellarsuper-clusters(Fellhauer&Kroupa2002)asanalternativeexplanationforthebrightcompactobjects.Keywords.galaxies:clusters:individual:Fornaxcluster–galaxies:dwarf–galaxies:fundamentalparameters–galaxies:kinematicsanddynamics–galaxies:starclusters–globularclusters:general1.Introduction1.1.“Normal”dwarfgalaxiesThefaintendofthegalaxyluminosityfunctionismainlypopulatedbythedwarfellipticalgalaxies(dE)andthedwarfspheroidals(dSph,thefaintestdwarfgalaxiesintheLocalGroup).Theyarethemostnumeroustypeofgalaxiesinthenearbyuniverse,havingabsolutemagnitudesfainterthanMV≃−17mag.Theyfollowatightmagnitude-surfacebrightness(M–μ)relationinthesensethatcentralsurfacebrightnessincreaseswithincreasingtotalluminosity(Ferguson&Sandage1988,1989,Drinkwateretal.2001a,Hilkeretal.2003).AconsequenceofthisM–μrelationisthatthesizeofdEsSendoffprintrequeststo:S.MieskeCorrespondenceto:[e-mail]ithtotalluminosity,withtheeffectiveradiusbeingoftheorderofafewhundredpc.ForphotometricstudiesofdEsingalaxyclustersthismeansthattheapproximateangularsizeofcandidatedwarfgalaxiesisknown.Thismakesthemorphologicalseparationbetweenclusterandback-orforegroundgalaxieseasier.1.2.UltraCompactDwarfgalaxies(UCDs)However,dwarfgalaxieswithsizesmuchsmallerthannormaldEsatcomparabletotalluminositiesaremor-phologicallymisclassifiedeitherasstarsordistantbackgroundgalaxieswhenusingtheM–μrelationasamembershipcriterion.Inordertotestthemagnitudeofsuchabias,amongotherscientifictasks,Drinkwateretal.(2000aand2000b)performedtheall-objectFornaxClusterSpectroscopicSurvey(FCSS)downtoV≃19.5(MV≃−11.9)mag,usingthe2dFspectrographmountedontheAnglo-AustralianTelescope.Theywereable2S.Mieskeetal.:OnthenatureofUltraCompactDwarfstoconfirmtheexistenceofatightmagnitude-surfacebrightnessrelationforthemajorityoftheFornaxdwarfgalaxies.InadditiontheydiscoveredatotalofsevenunresolvedFornaxmembers,beingmuchsmallerthannormaldwarfgalaxiesatcomparablemagnitudes(Drinkwateretal.2000aand2003,Karicketal.2003).TwooftheUCDshadfirstbeendiscoveredbyHilkeretal.(1999).Drinkwateretal.namedtheseobjects“UltraCompactDwarfgalaxies”(UCDs).Theyarelocatedinthecentralprojected30′oftheFornaxclusteraroundNGC1399andcoveraluminosityrangeof18V19.5mag,or−13.4MV−11.9mag((m−M)=31.4mag,Ferrarreseetal.2000).ThismakesthembrighterthanthebrightestMilkyWayglobularclusterωCentauri(MV≃−10.2mag),butbyfarfainterthanthecompactdwarfellipticalgalaxyM32(MV≃−16mag,seeGraham2002).TheUCDsdofallintotheluminosityrangeofdwarfgalaxynuclei(Lotzetal.2001).1.3.PossibleoriginsofUCDsThreedifferentoriginsfortheUCDsareindiscussion:(1)theyarethebrightestglobularclusters(GCs)ofNGC1399’sveryrichglobularclustersystem(GCS)(e.g.Mieskeetal.2002,Dirschetal.2003);(2)theyaretheremnantnucleiofstrippeddwarfgalaxieswhichhavelosttheirouterpartsinthecourseoftidalinteractionwiththeFornaxcluster’spotential(Bekkietal.2003);(3)theyareformedfromtheamalgamationofstellarsuper-clustersthatwerecreatedincollisionsbetweengas-richgalaxies(Fellhauer&Kroupa2002,Kroupa1998,Marastonetal.2004).Regardingpossibility(1):NGC13