Near-infrared surface photometry of early-type spi

arXiv:astro-ph/9802140v220Feb1998A&Amanuscriptno.(willbeinsertedbyhandlater)Yourthesauruscodesare:11.06.2;11.11.1;11.19.2;11.19.5;11.19.6ASTRONOMYANDASTROPHYSICS1.2.2008Near-infraredsurfacephotometryofearly-typespiralgalaxies:II.M/Lratiosanddarkhalos.⋆G.Moriondo1,C.Giovanardi2,andL.K.Hunt31DipartimentodiAstronomiaeScienzadellospazio,L.E.Fermi5,I-50125Firenze,Italy2OsservatorioAstrofisicodiArcetri,L.E.Fermi5,I-50125Firenze,Italy3C.A.I.S.M.I.,L.E.Fermi5,I-50125Firenze,ItalyReceived;acceptedAbstract.Wepresentmassdistributionsobtainedfromnear-infrared(NIR)surfacebrightnessdecompositionsandrotationcurvefittingofasampleofearly-typespi-ralgalaxies.Bulgeanddiskmass-to-light(M/L)ratios,darkhaloparameters,andthemodifiedNewtoniandy-namics(MOND)accelerationparameterarederived.WefindthatthemeandiskNIRM/Lishigherthanthatofthebulge,andcomparisonwithstellarpopulationsyn-thesismodelsimpliesthatearly-typespiralbulgesare,onaverage,youngerandmoremetalrichthandisks.NIRdiskM/Lisfoundtodependondiskluminosity,consis-tentlywithpreviouslyreportedtrendsinellipticalsandspirals,andwithcolddarkmattermodelsfordiskforma-tion.Withintheopticalradius,themeanratioofstellartodarkmatteris2andthetypicaldarkhalomassis1011M⊙.ThevalueoftheMONDaccelerationparameterthatbestaccommodatesthesampleasawholeis1.3·10−8cms−2.Keywords:Galaxies:spirals–Galaxies:fundamentalparameters–Galaxies:structure–Galaxies:kinematicsanddynamics–Galaxies:stellarcontent1.IntroductionThecomparisonoftheluminosityprofilesandrotationcurvesofdiskgalaxiesprovidedthefirstevidencefordarkmatterinouterpartsofgalaxies(e.g.,vanAlbada&San-cisi1986).Theluminousmaterial,eitherintheformofstarsorneutralgas,isnotabletoreproducetheapproxi-mateflatnessoftherotationcurvesatlargeradii,implyingSendoffprintrequeststo:G.Moriondo⋆BasedonobservationstakenatTIRGO(Gornergrat,Switzerland).TIRGOisoperatedbyCAISMI–CNR,Arcetri,Firenze,Italy.thattheNewtoniantheoryofgravitationneedstobemod-ifiedorthatthemassintheouterregionsofmostspiralsisdominatedbyadarkcomponent.Thephysicalextentofsucha“darkhalo”cannotbeinferredfromtherotationcurves,sincetheselastareflatevenwheretheluminousmaterialceasestobedetected.Hence,thetotalmassinspiralgalaxiesisnotyetwellknown.Thepropertiesofdarkmatter(DM)andthestructureofdarkhalosasderivedfromrotationcurveshavepro-foundimplicationsoncosmologicalissuessuchasgalaxyformation.RecentN-bodysimulationsofcolddarkmat-terhalostogetherwithadiabaticinfallmodelshaveshownthatmatchingobservedrotationcurvesrequiresasystem-aticincreaseofdiskmass-to-lightratioswithluminosity(Navarroetal.1996).Suchatrendisobservedinlargebodiesofrotationcurvedata(Broeils1992;Persicetal.1996–hereafterPSS),andfurthercomparisonsbetweentheoreticalpredictionsandobservationsmayhelpclarifythenatureofdarkmatteringalaxies.Thepresenceofadarkhalomakesglobalmass-to-light(M/L)ratiosderivedfromrotationcurvesmuchhigherthanthoseexpectedfromstellarpopulations,andpro-ducesradialgradientsinthetotalM/L.ThecontributionofDMtendstobelargerforlow-luminositysystems,asthefractionofDMtoluminousmatterwithintheopticalradiusappearstodecreasewithincreasinggalaxylumi-nosity(Kormendy1990;Saluccietal.1991).Inluminoussystems,therefore,themassdistributionintheinnerfewdiskscalelengthscanbelargelyascribedtothebulgeanddiskstars.Consequently,whenstellargalaxycompo-nentscanbeaccuratelyisolatedwithsurfacephotome-try,estimatesoftheirM/Lfromrotationcurvesprovideusefulconstraintsfortheage,abundance,andstarforma-tionhistoryoftheirstellarpopulations.RecentpopulationsynthesismodelspredicttrendsofM/Lwithmetallicityandwithwavelength(Worthey1994;Bruzual&Charlot2Moriondoetal.:NIRphotometryofearly-typespirals.II1993),andcanbeusedtoinferandcomparepropertiesofthestellarpopulationsinbulgesanddisks.Wehavereportedinapreviouspaper(Moriondoetal.1997–hereafterPaperI)theresultsoftwo-dimensionalnear-infrared(NIR)surfacebrightnessdecompositionsforasampleofearly-typespirals.NIRwavelengths,especiallywhencombinedwiththoseintheoptical,areapowerfuldiagnostictoolsincetheytracemoreaccuratelythestellarmasscontentandminimizethecomplicationsofextinc-tion.Inthispaper,weapplythebulge+diskdecomposi-tionresultstotheanalysisoftherotationcurvesforoursample.InSect.2wederivetheradialmassdistributionsandevaluatethecontributionofthebulge,disk,anddarkhalototheobservedrotationcurves;asanalternativetodarkmatterhalos,wealsoderiveavalueofthemodi-fiedNewtoniandynamicscriticalaccelerationparameter.TheresultingNIRM/Lratiosoftheluminouscompo-nentsareanalyzedinSect.3,andcomparedwiththoseatopticalwavelengthsandwithpopulationsynthesismod-els.Finally,weassesstheimportanceofdarkhalos,andexploretrendsofM/Lwithluminosityinthecontextofthepropertiesofthefundamentalplaneforgravitationallyboundobjects(e.g.,Bursteinetal.1997).2.Dataanalysis2.1.SurfacephotometryandstructuraldecompositionThetechniquesusedtoreduceandanalyzethephotomet-ricdataaredetailedinPaperI.Briefly,wedecomposedthesurfacebrightnessdistributionsofeachsamplegalaxyintothetwocomponentsofbulgeanddisk.Twoindependentmethodswereused:a)aparametricfitassumingagen-eralizedexpone