Bright and Dark Matter in Elliptical Galaxies Mass

arXiv:astro-ph/0612732v310May2007Mon.Not.R.Astron.Soc.000,000–000(0000)Printed5February2008(MNLATEXstylefilev1.4)BrightandDarkMatterinEllipticalGalaxies:MassandVelocityDistributionsfromSelf-consistentHydrodynamicalSimulationsJ.O˜norbe1,R.Dom´ınguez-Tenreiro1,A.S´aiz1⋆,andA.Serna21DepartamentodeF´ısicaTe´orica,C-XI.UniversidadAut´onomadeMadrid,Madrid,E-28049,Spain2DepartamentodeF´ısicayA.C.,UniversidadMiguelHern´andez,Elche,Spain5February2008ABSTRACTWehaveanalysedthemassandvelocitydistributionsoftwosamplesofrelaxedelliptical-like-objects(ELOs)identified,atz=0,inasetofself-consistenthydro-dynamicalsimulationsoperatinginthecontextofaconcordancecosmologicalmodel.ELOshavebeenidentifiedasthosevirtualgalaxieshavingaprominent,dynamicallyrelaxedstellarspheroidalcomponent,withnoextendeddiscsandverylowgascontent.Ouranalysisshowsthattheyareembeddedinextended,massivedarkmatterhaloes,andtheyalsohaveanextendedcoronaofhotdiffusegas.Darkmatterhaloeshaveex-periencedadiabaticcontractionalongtheirassemblyprocess.TherelativeELOdark-tobright-masscontentandspacedistributionsshowbrokenhomology,andtheyareconsistentwithobservationalresultsonthedarkmatterfractionatthecentralregions,aswellasonthegradientsofthemass-to-lightratioprofilesforboxyellipticals,asafunctionoftheirstellarmasses.Theseresultsindicatethatmassiveellipticalsmissstars(i.e.,baryons)attheircentralregions,ascomparedwithlessmassiveones.Oursimulationsindicatethatthesemissingbaryonscouldbefoundbeyondthevirialradiiasahot,diffuseplasma.Thismasshomologybreakingcouldhaveimportantimplica-tionstoexplainthephysicaloriginoftheFundamentalPlanerelation.TheprojectedstellarmassprofilesofourvirtualellipticalscanbewellfitbytheS´ersicformula,withshapeparametersnthatagree,onceastellarmass-to-lightratioindependentofposi-tionisassumed,withthoseobtainedfromsurfacebrightnessprofilesofellipticals.Theagreementincludestheempiricalcorrelationsofnwithsize,luminosityandvelocitydispersion.Thetotalmassdensityprofilesshowapower-lawbehaviouroveralarger/rvirinterval,consistentwithdataonmassivelensellipticalsatshorterradii.Thevelocitydispersionprofilesshowkinematicalsegregation,withnosystematicmassde-pendence(i.e.,nodynamicalhomologybreaking)andapositiveanisotropy(i.e.,radialorbits),roughlyindependentoftheradialdistanceoutsidethecentralregions.TheLOSvelocitydispersionprofilesaredeclining.Theseresultsgive,forthefirsttimefromcosmologicalsimulations,aratherdetailedinsightintotheintrinsicmassandvelocitydistributionsofthedark,stellarandgaseouscomponentsofvirtualellipti-cals.Theconsistencywithobservationsstronglysuggeststhattheycouldalsodescribeimportantintrinsiccharacteristicsofrealellipticals,aswellassomeoftheirpropertiesrecentlyinferredfromobservationaldata(forexample,downsizing,theappearanceofbluecores,theincreaseofthestellarmasscontributedbytheellipticalpopulationaszdecreases).Keywords:galaxies:ellipticalandlenticular,cD-galaxies:haloes-galaxies:kine-maticsanddynamics-galaxies:structure-darkmatter-hydrodynamics⋆Currentaddress:Dept.ofPhysics,MahidolUniversity,Bangkok10400,Thailand1INTRODUCTIONAmongallgalaxyfamilies,ellipticalgalaxies(EGs)arethesimplestonesandthosethatshowthemostpreciseempiri-c0000RAS2O˜norbeetal.calregularities,suchas:theverysimilarshapestheirsurfacebrightnessprofilesshow(i.e.,theS´ersiclaw,S´ersic1968),thepower-lawformoftheirthree-dimensionalmassdensityprofilesemergingfromsomestronglensinganalyses(Koop-mansetal.2006),andtherelationsamongsomeoftheirobservableparameters.Theimportanceoftheseregularitiesliesinthattheyverylikelyencodealotofrelevantinfor-mationsonthephysicalprocessesunderlyingthehistoriesofthemassassembly,theratesofdissipationandtheratesofstarformation(SF)ofEGs.Despiteoftheirinterest,veryfewisknown,bothfromthetheoreticalorobservationalpointsofview,aboutthemassorvelocitydistributionsofthedifferentellipticalsmasscomponents(stars,hotandcoldgasanddarkmatter).Therehasbeen,nevertheless,animportantrecentprogressonthephotometriccharacterisationofEGs,and,infact,authorsnowagreethattheS´ersiclawadequatelydescribestheop-ticalsurfacebrightnessprofilesofmostofthem(Caon,Ca-paccioli,&D’Onofrio1993;Trujillo,Graham,&Caon2001;Bertin,Ciotti,&DelPrincipe2002).TheS´ersiclawcanbewrittenIlight(R)=Ilight0exp[−bn(R/Rlighte)1/n],(1)whereIlight(R)isthesurfacebrightnessatprojecteddistanceRfromtheellipticalscentre,Rlighteistheeffectivehalf-lightradius,encompassinghalfthetotalgalaxylumi-nosity,bn≃2n−1/3+0.009876/n,andnistheS´ersicshapeparameter.Puttingn=4thelargelyuseddeVaucouleursR1/4law(deVaucouleurs1948)isrecovered.Itisgenerallyassumedthatgalaxiesofanytypeareembeddedinmassivehaloesofdarkmatter.However,fromtheobservationalpointofview,theimportanceandthedis-tributionofdarkmatterinEGsisstillamatterofaliv-ingdebate.Dataonstellarkinematicsfromintegrated-lightspectraareveryscarcebeyond2Rlighte,makingitdifficulteventoestablishthepresenceofadarkmatterhalo(Kron-awitteretal.2000;Magorrian&Ballantyne2001)throughthismethod.Otherwise,thelackofmasstracersatlargerdistancesthatcanbeinterpretedwithoutanyambiguity,hashistoricallyhamperedthepr