Origin of the metallicity distribution of the NGC 5128 stellar halo

Recent {\it Hubble Space Telescope} photometry in the nearby elliptical galaxy NGC 5128 shows that its halo field star population is dominated by moderately metal-rich stars, with a peak at [m/H] $\simeq$ -0.4 and with a very small fraction of metal-poor (

Mon.Not.R.Astron.Soc.000,1–??(1994)Printed2February2008

aTEXstyle lev1.4)(MNL

OriginofthemetallicitydistributionoftheNGC5128

stellarhalo

K.Bekki,1WilliamE.Harris2,4,andGretchenL.H.Harris3,4

1School34

2Department

arXiv:astro-ph/0212545v1 27 Dec 2002

ofPhysics,UniversityofNewSouthWales,Sydney2052,NSW,Australia

ofPhysics&Astronomy,McMasterUniversity,HamiltonONL8S4M1,Canada

DepartmentofPhysics,UniversityofWaterloo,WaterlooONN2L3G1,Canada

VisitingFellow,ResearchSchoolofAstronomy&Astrophysics,AustralianNationalUniversity,Weston,ACT2611,Australia

AcceptedReceivedinoriginalform2001

ABSTRACT

RecentHubbleSpaceTelescopephotometryinthenearbyellipticalgalaxyNGC5128showsthatitshalo eldstarpopulationisdominatedbymoderatelymetal-richstars,withapeakat[m/H] 0.4andwithaverysmallfractionofmetal-poor([m/H]< 1.0)stars.Inordertoinvestigatethephysicalprocesseswhichmayhaveproducedthismetallicitydistributionfunction(MDF),weconsideramodelinwhichNGC5128isformedbymergingoftwomajorspiralgalaxies.We ndthatthehaloofanellipticalformedthiswayispredominantlypopulatedbymoderatelymetal-richstarswith[m/H]～ 0.4whichwereinitiallywithintheouterpartsofthetwomergingdiscsandweretidallystrippedduringthemerger.TomatchtheNGC5128data,we ndthattheprogenitorspiraldiscsmusthaverathersteepmetallicitygradientssimilartotheonede nedbytheMilkyWayopenclusters,aswellassparsemetal-poorhaloes(5%orlessofthediscmass).Veryfewstarsfromthecentralbulgesofthespiralgalaxiesendupinthehalo,sotheresultsarenotsensitivetotherelativesizes(bulge-to-discratios)ormetallicitiesoftheinitialbulges.Finally,wediscussthee ectsontheglobularclustersystem(GCS).Theemergentellipticalwillendupwithmetal-poorhaloclustersfromtheoriginalspiralhaloes,butwithmoderatelymetal-richhalostarsfromtheprogenitordiscs,thuscreatingameano setbetweentheMDFsofthehalostarsandtheGCS.RemainingquestionsyettobeansweredconcernthetotalsizeoftheGCSpopulation(the“speci cfrequencyproblem”)andtheobservedexistenceofmetal-richglobularclustersinlargenumbersintheouterhaloesofgiantellipticals.Wealsodiscusspossibledi erencesintheMDFsofstellarhaloesofgalaxiesofdi erentHubbletype.

Keywords:galaxies:abundances—galaxies:ellipticalandlenticular,cD—galax-ies:evolution—galaxies:individual(NGC5128)—galaxies:stellarcontent—globularclusters:general

1INTRODUCTION

Physicalpropertiesofmetal-de cientstellarhaloesingalax-iesareconsideredtoprovidevitalcluestotheunderstand-ingofearlydynamicalandchemicalevolutionofgalaxies.Inparticular,thedetailedinvestigationofstructural,kine-matic,andchemicalpropertiesofsucha“fossilrecord”com-ponent(i.e.,stellarhalo)oftheGalaxyhasrevealedapossi-blescenarioastohowtheGalaxyhasdevelopeditsdynami-calstructuressuchasbulge,thinandthickdiscs(e.g.,Free-man1987;Majewski1993;vandenBergh1996).Becauseoftheobservationaldi cultiesinrevealingthreedimensionalstructureandkinematicsofhalostars(withrespecttotheir

c1994RAS

hostgalaxy)ineachoftheLocalGroupgalaxiesotherthan

theMilkyWay,themetallicitydistributionfunction(here-afterMDF)ofhalostarsinthesegalaxieshasservedtogivesomeconstraintsonthepaststarformationandchem-icalevolutionhistoriesofthesesystems(Mould&Kristian1986;Durrell,Harris,&Pritchet1994,2001;Pritchet&vandenBergh1998;Christian&Heasley1986:Reitzel,Guhathakurta,&Gould1998;Grillmairetal.1996;Hanetal.1997;Martinez-Delagado&Aparicio1998).Thestel-larhaloofM31appearstobedominatedbyamoderatelyhigh-metallicitypopulationwith[m/H]～ 0.5(cf.theref-erencescitedabove),astrikinglydi erentenrichmentlevelthanintheMilkyWaystellarhalo,andastrongindicatorof

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