Internal Dynamics, Structure and Formation of Dwar

arXiv:astro-ph/0206153v223Oct2002AJinpressInternalDynamics,StructureandFormationofDwarfEllipticalGalaxies:I.AKeck/HSTStudyofSixVirgoClusterDwarfsM.GehaUCO/LickObservatory,UniversityofCalifornia,SantaCruz,1156HighStreet,SantaCruz,[e-mail]1,2HerzbergInstituteofAstrophysics,NationalResearchCouncilofCanada,5071WestSaanichRoad,Victoria,B.C.,[e-mail]eScienceInstitute,3700SanMartinDr.,Baltimore,[e-mail]iththeKeckII10-metertelescope∗andEchelleSpectrographandImagerispresentedforsixVirgoClusterdwarfelliptical(dE)galaxiesintheabsolutemagnituderange−15.7≤MV≤−17.2.Themeanline-of-sightvelocityandvelocitydispersionareresolvedasafunctionofradiusalongthemajoraxisofeachgalaxy,nearlydoublingthetotalnumberofdEswithspatially-resolvedstellarkinematics.Noneoftheobservedobjectsshowsevidenceofstrongrotation:upperlimitsonvrot/σ,theratioofthemaximumrotationalvelocitytothemeanvelocitydispersion,arewellbelowthose1HerzbergFellow2Permanentaddress:UCO/LickObservatory,UniversityofCalifornia,SantaCruz,1156HighStreet,SantaCruz,CA95064∗DatapresentedhereinwereobtainedattheW.M.KeckObservatory,whichisoperatedasascientificpartnershipamongtheCaliforniaInstituteofTechnology,theUniversityofCaliforniaandtheNationalAeronauticsandSpaceAdministration.TheObservatorywasmadepossiblebythegenerousfinancialsupportoftheW.M.KeckFoundation.–2–expectedforrotationally-flattenedobjects.SuchlimitsplacestrongconstraintsondEgalaxyformationmodels.AlthoughthesegalaxiescontinuethetrendoflowrotationvelocitiesobservedinLocalGroupdEs,theyareincontrasttorecentobservationsoflargerotationvelocitiesinslightlybrighterclusterdEs.UsingsurfacephotometryfromHubbleSpaceTelescope†WideFieldPlanetaryCamera2imagesandspherically-symmetricdynamicalmodels,wedetermineglobalmass-to-lightratios3≤ΥV≤6.Theseratiosarecomparabletothoseexpectedforanoldtointermediate-agestellarpopulationandarebroadlyconsistentwiththeobserved(V−I)colorsofthegalaxies.ThesedEgalaxiesthereforedonotrequireasignificantdarkmattercomponentinsideaneffectiveradius.Weareabletoruleoutcentralblackholesmoremassivethan∼107M⊙.ForthefivenucleateddEsinoursample,kinematicandphotometricpropertiesweredeterminedforthecentralnucleusseparatelyfromtheunderlyinghostdEgalaxy.ThesenucleiareasbrightorbrighterthanthemostluminousGalacticglobularclustersandlieneartheregionofFundamentalPlanespaceoccupiedbyglobularclusters.Inthisspace,theVirgodEgalaxieslieinthesamegeneralregionasLocalGroupandothernearbydEs,althoughnon-rotatingdEsappeartohaveaslightlyhighermeanmassandmass-to-lightratiothanrotatingdEs;thedEgalaxiesoccupyaplaneparallelto,butoffsetfrom,thatoccupiedbynormalellipticalgalaxies.Subjectheadings:galaxies:dwarf—galaxies:kinematicsanddynamics1.IntroductionDwarfellipticalgalaxies(dEs)arethemostcommongalaxytypebynumberintheLocalUniverse,dominatingthegalaxyluminosityfunctionofnearbyclusters.Yetthesegalaxiesremainamongthemostpoorlystudiedgalaxiesduetotheirfaintluminosities,MV≥−18,andcharacter-isticloweffectivesurfacebrightnessμV,eff22magarcsec−2(Ferguson&Binggeli1994).Unlikebrighter,classicalellipticalgalaxieswhosesurfacebrightnessprofilesarewellfitbythedeVau-couleursr1/4law(deVaucouleurs1948),dEshavebrightnessprofilesthatarecharacterizedbySersicprofiles(S´ersic1968)withindicesrangingbetweenn=1–3(wheren=1correspondstoanexponentiallawandn=4toanr1/4law)makingthemappearmorediffusethanclassicalellipticalsofthesametotalmagnitude(Bingelli&Jerjen1998).IntheVirgoCluster,themajorityofdEsbrighterthanMV∼−16containcompactcentralnuclei;fainterthanMV∼−12mostdEsshownosignofanucleus(Sandage,Bingelli,&Tammann1985).Nucleitypicallycontain5%to20%ofthetotalgalaxylightandareslightlyresolvedatthedistanceofVirgobyHubbleSpaceTelescope(HST)imaging(Milleretal.1998).†BasedonobservationswiththeNASA/ESAHubbleSpaceTelescope,obtainedattheSpaceTelescopeScienceInstitute,whichisoperatedbytheAssociationofUniversitiesforResearchinAstronomy,Inc.,underNASAcontractNAS5-26555.–3–Inhierarchicalmodelsofgalaxyformation,dwarfgalaxiesformoutofsmalldensityfluctuationsintheearlyUniverseandarepredictedtobelessspatiallyclusteredthannormalellipticalorspiralgalaxies(Dekel&Silk1986).However,dwarfellipticalgalaxiesarepreferentiallyfoundindenseclusterenvironments,moresothaneitherellipticalsorspirals(Bingellietal.1987);therearefew,ifany,examplesofisolateddEs.Thus,currentmodelsfavordEformationfromaprogenitorgalaxypopulation.TheproposedprogenitorsofdEsarespiralorirregulargalaxieswhicharemorphologicallytransformedintodEsthroughtheprocessesofgalaxyharassmentandinteraction(Moore,Lake,&Katz1998).DetailedinternalkinematicsofdEsareapowerfulobservationaltoolwithwhichtotestthesescenarios.Untilrecently,radialvelocitydispersionprofileswereonlyavailablefortheLocalGroupdEsandtwoofthebrightestdEsintheVirgoCluster(Bender&Nieto1990;Bender,Paquet,&Nieto1991;Held,Mould,&deZeeuw1990).Inaddition,ahandfulofglobalvelocitydispersionmea-surementsexistedfordEsinvariousenvironmentsoutsidetheLocalGroup(Peterson&Caldwell1993).Thes