The Luminosity Distribution of Globular Clusters in Dwarf Galaxies

CLUSTERSINDWARFGALAXIES

SidneyvandenBergh

arXiv:0704.2226v1 [astro-ph] 17 Apr 2007DominionAstrophysicalObservatory,HerzbergInstituteofAstrophysics,NationalResearch

CouncilofCanada,5071WestSaanichRoad,Victoria,BC,V9E2E7,Canada

sidney.vandenbergh@nrc.gc.ca

ABSTRACT

ThemajorityoftheglobularclustersassociatedwiththeSagittariusdwarfgalaxyarefaint.Inthisrespectitdifferssignificantlyfromtheglobularclus-tersystemssurroundingtypicalgiantgalaxies.TheobservationthatmostofglobularclustersintheouterhalooftheGalaxyarealsosub-luminousmaybeunderstoodbyassumingthattheseclustersoncealsobelongedtofaintcluster-richdwarfsystemsthatweresubsequentlycapturedanddestroyedbytheMilkyWaySystem.

Subjectheadings:galaxies:dwarf-galaxies:starclusters-globularclusters:general

1.INTRODUCTION

Ithasbeenknownformanyyears(Harris1991,Harris2001)thatmostglobularclustersystemshavearoughlyGaussianluminositydistributionsthatpeaksslightlyabove,Mv=-7.ThefactthatthefiveglobularclustersassociatedwiththeFornaxdwarfspheroidalhaveMv=-7.1±0.5(vandenBergh2000,p.224)appearedtosupportthenotionthattheluminositydistributionofglobularclustersmightbeindependentofparentgalaxyluminosity.However,thisconclusionwasrecentlychallengedbyvandenBergh(2006),whouseddatabySharinaetal.(2005),toshowthattheluminositydistributionoftypicalglobularclustersembeddedindwarfgalaxieshavingMv>−16seemedtodifferdramaticallyfromthatoftheglobularclusterssurroundinggiantgalaxieshavingMv<−16.InparticularitwasfoundthattheluminositydistributionofglobularclustersinsomedwarfgalaxiesappearedtoincreasemonotonicallydowntothecompletenesslimitofthedataatMv∼−5.

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2.

THESAGITTARIUSDWARF

RecentlyJord´anetal.(2007)havepointedoutthattheresultsbyvandenBergh(2006)aremainlybasedonthedatabySharinaetal.(2005),whomaynothavesufficientlytakenintoaccountthepotentialcontaminationoftheirlistofcandidateglobularclusters.Theirresultsmustthereforeberegardedasbeingquiteuncertainuntilclustercandidateshavebeenconfirmedspectroscopically.FortunatelysuchacheckisalreadyavailableusingtheglobularclustersthatappeartobeassociatedwiththenearbySagittariusdwarf.Table1listsinformationontheseclustersdrawnfromrecentcompilationsbyvandenBergh&Mackey(2004)andMackey&vandenBergh(2005),supplementedbyinformationonthenewlydiscoveredobjectWhiting1(Carraroetal.2007).Fortheglobularslistedinthistableonefinds=−5.8(or=−5.2ifNGC6715=M(whichmaybethenucleusoftheSagittariusdwarf)isexcluded.InotherwordstheglobularsassociatedwiththeSagittariusdwarfareclearlylessluminousthanthosesurroundingtypicalgiantgalaxies.AKolmogorov-Smirnovtestshowsthatthereisonlya3%probabilitythattheglobularclustersinSagittarius,andthoseinthemainbodyoftheGalaxywithGalactocentricdistances<15kpc(vandenBergh2006),weredrawnfromthesameparentdistributionofluminosities.IfMisexcludedfromthesamplethenanS-Ktestgivesaprobabilityofonly1%thattheGalacticandSagittariusglobularclustersamplesweredrawnfromasimilarparentdistribution.ThisresultsuggeststhattheSagittariusdwarfglobularclusterresemblesthosetypicaloftheclustersystemssurroundingfaintgalaxiesthatwerestudiedbyvandenBergh(2006).

3.DISCUSSION

Itwouldbeimportanttoknowiftheluminositydistributionofglobularclustersisuni-versal,orifitisafunctionofparentgalaxyluminosity(mass).AnalysisofthedatapresentedbySharinaetal.stronglysuggeststhattheluminositydistributionofglobularclustersingalaxieswithMv>−16differsdramaticallyfromthatofthemuchmorethoroughlystudiedglobularclustersystemssurroundingmassivegalaxieswithMv<−16.Thatthisdifferenceisnotanartifactproducedbydistance-dependentselectioneffectsisshownbythefactthattheSagittariussystem(whichisthenearestknowndwarf)exhibitsthesametypeofglobularclusterluminositydistributionthatisobservedinglobularclustersystemsassociatedwithmoredistantdwarfgalaxies.Inthisconnectionitisofinteresttonotethatbothdwarfgalax-ies(Tolstoyetal.2003),andsomeoftheglobularclustersassociatedwiththem(Sbordoneetal.2005),havemetallicitysignaturesthatdiffersignificantlyfromthoseencounteredinmoreluminoussystems.Boththedwarfspheroidalsandtheglobularsassociatedwiththem

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arefoundtohavethesamelowalpha-elementtoironratioandthesamelowNi/Feratio.Itisofinteresttonote(Sbordone2005)thattheouterhalocluster(Rgc=18.5kpc)RuprechtNo.106sharesthesamepeculiarmetallicitysignature.ThissuggeststhattheanomalouslylowluminosityofmanyoftheglobularclustersintheouterGalactichalo(vandenBergh&Mackey2004)maybeduetothefactthattheseobjectswerestrippedfromdwarfgalaxiesthathavesubsequentlybeentidallydisrupted.

Insummaryitappearsthatboththeanomalousluminositydistributionofglobularclus-tersintheouterhalo,andtheirmetallicitysignature,mightbeunderstoodbyassumingthattheseobjcetsoriginatedindwarfgalaxiesthatweresubsequentlycapturedand/ordisruptedbyGalactictidalforces.

Iamparticularlyindebtedtotheanonymousrefereeforhintingatthepossibleevolu-tionaryconnectionbetweentheanomalousluminositydistributionofouterhaloglobularsandtheiranomalousabundancesignature.

REFERENCES

Carraro,G.,Zinn,R.&Bidin,C.M.2007,A&A(inpress)=astro-ph/07022253Harris,W.E.1991,ARAA,29,3

Harris,W.E.2001inStarClusters(28thSaas-FeeAdvancedCourse)ed.,L.Labhardt&

B.Binggeli(Berlin:Springer),p.223Jord´an,A.etal.2007,ApJS(inpress)=astro-ph/0702496Mackey,A.D.&vandenBergh,S.2005,MNRAS,360,631Sharina,M.E.,Puzia,T.H.&Marakov,D.I.2005,A&A,442,85

Tolstoy,E.,Venn,K.A.,Shetrone,M.,Primas,F.,Hill,V.,Kaufer,A.&Szeifert,T.2003,

AJ125,707Sbordone,L.,Bonifacio,P.,Marconi,G.,Buonanno,R.,&Saggia,S.2005,A&A,437,905vandenBergh,S.2000,TheGalaxiesoftheLocalGroup,Cambridge:CambridgeUniversity

PressvandenBergh,S.2006,AJ,131,304

vandenBergh,S.&Mackey,A.D.2004,MNRAS,3,713

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Table1.GlobularclustersthatareprobablyassociatedwiththeSagittariusdwarfgalaxy

Name

Mv