Masses of star clusters in the nuclei of bulge-less spiral g

In the last decade star clusters have been found in the centers of spiral galaxies across all Hubble types. We here present a spectroscopic study of the exceptionally bright (10^6 - 10^8 Lsun) but compact (Re ~ 5 pc) nuclear star clusters in very late type

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aMassesofstarclustersinthenucleiofbulge-lessspiralgalaxiesC.J.Walcher1,R.P.vanderMarel2,D.McLaughlin2,H.-W.Rix1,T.B¨oker3,N.H¨aring1,L.C.Ho4,M.Sarzi5,J.C.Shields6ABSTRACTInthelastdecadestarclustershavebeenfoundinthecentersofspiralgalaxiesacrossallHubbletypes.Weherepresentaspectroscopicstudyoftheexception-allybright(106–108L⊙)butcompact(relatetypespiralswithUVESattheVLT.We～ nd5pc)thenuclearvelocitystardispersionsclustersinofverythenineclustersinoursampletorangefrom13to34kms http://doc.guandang.netingphotometricdatafromtheHST/WFPC2andsphericallysymmetricdynamicalmodelswedeterminemassesbetween8×105and6×107M⊙.Themasstolightratiosrangefrom0.2to1.5intheIbandThisindicatesayoungmeanageformostclusters,inagreementwithpreviousstudies.Giventheirhighmassesandsmallsizeswe ndthatnuclearclustersareamongtheobjectswiththehighestmeansurfacedensityknown(upto105M⊙pc 2).Fromtheirdynamicalpropertiesweinferthat,ratherthansmallbulges,thecloseststructuralkinofnuclearclustersappeartobemassivecompactstarclusters.Thisincludessuchdi erentobjectsasglobularclusters,”superstarclusters”,ultracompactdwarfgalaxiesandthenucleiofdwarfellipticalgalaxies.Itisachallengetoexplainwhy,despitethewildlydi erentcurrentenvironments,alldi erenttypesofmassivestarclustersshareverysimilarandstructuralproperties.ApossibleexplanationlinksUCDsandmassiveglobularclusterstonuclearstarclustersthroughstrippingofnu-cleateddwarfgalaxiesinamergerevent.Theextremepropertiesofthistype

ofclusterswouldthenbeaconsequenceoftheirlocationinthecentersoftheirrespectivehostgalaxies.

In the last decade star clusters have been found in the centers of spiral galaxies across all Hubble types. We here present a spectroscopic study of the exceptionally bright (10^6 - 10^8 Lsun) but compact (Re ~ 5 pc) nuclear star clusters in very late type

Subjectheadings:galaxies:starclusters;galaxies:nuclei;galaxies:structure;galaxies:spiral

1.Introduction

InthelastdecadetheimagequalityofHSThasboostedthestudyofthecentersofspiralgalaxies.TheseobservationshaveshownthatstarclustersareacommonfeatureinthenucleiofspiralgalaxiesofallHubbletypes(Phillipsetal.1996;Matthews&Gallagher1997;Carolloetal.1997;Carollo,Stiavelli&Mack1998;B¨okeretal.2002),frombulge-dominatedgalaxiestobulge-lessgalaxies.Nuclearstarclustersarehardtoobserveinearlytypespiralsagainstthebrightbulge.Howeverinlate-type,bulge-lessgalaxies,theystandoutwellagainstthelowsurfacebrightnessdisk.TheHSTsurveybyB¨okeretal.(2002,hereafterB02)hasshownthat75%ofalllatetypespiralshostsuchanuclearclusterintheirphotometriccenter.Weusethetermnuclearcluster(NC)foraluminousandcompactstarclusterneartheoverallphotometriccenterofthegalaxy.Whilethee ectiveradii(～5pc,B¨okeretal.2004,hereafterB04)arecomparabletoglobularclusters,theluminositiesexceedthoseofthemostluminousMilkyWayglobularclustersbyupto2ordersofmagnitude.

Intheabsenceofanyobviousbulge(B¨oker,Stanek&vanderMarel2003),NCsrepresenttheclosestthingtoacentral“hotcomponent”.Inmostgalaxyformationscenariosthebulgeatthecenteristhe“trashbinofviolentrelaxation”,whereakinematicallyhotstellarcomponenthasformedeitherthroughexternalpotentialperturbations,suchasearlymergersoffragments,orperhapsthroughinternale ectssuchasviolentbarinstabilities.Bulge-lessgalaxiesincontrast,musthavelivedveryshelteredlives,astheircentral“trashbin”isvirtuallyempty.ThediscoveryofalmostubiquitousNCsatthesamephysicallocationintheirhostgalaxyasabulgewouldhave,bringsupthequestionwhetherbulgesandNCsarerelated.AthinkablescenariowouldbethatNCsareprotobulgesthatgrowbyrepeatedaccretionofgasandsubsequentstarformation.ThepaceofthatgrowthandtheresultingsizeofthecentralcomponentwouldthendetermineifwecallthecentralcomponentaNCorasmallbulge.

WhilethehighspatialresolutionofHSTallowssurfacephotometryoftheclusters,otherfundamentalparameterssuchase.g.ageandmassremainlargelyunknown.Severalcasestudiesintheliteraturehoweverhaverevealedmassive,youngobjects.M33isthenearestScgalaxyhostingaNCanditsnucleushasbeenextensivelystudiedinthepastdecade(e.g.Davidge2000;Long,Charles&Dubus2002;Stephens&Frogel2002).Despitesomedi erencesinthedetails,allstudiesonM33agreethatthereissomepopulationyoungerthan0.5Gyrinthecentralparsecandthatstarformationhasvariedsigni cantlyoverthe

In the last decade star clusters have been found in the centers of spiral galaxies across all Hubble types. We here present a spectroscopic study of the exceptionally bright (10^6 - 10^8 Lsun) but compact (Re ~ 5 pc) nuclear star clusters in very late type

pastseveralGyrs.ThemassofthecentralclusterinM33wasestimatedfromadetailedpopulationanalysisinGordonetal.(1999)tobe5×105M⊙,consistentwiththeupperlimitderivedfromthevelocitydispersionbyKormendy&McClure(1993)of2×106M⊙.FromHαrotationcurvesoflatetypespirals,Matthews&Gallagher(2002) ndthatthevelocityo setsatthepositionof vesemi-stellarnuclei—certainlytobeidenti edwithNCs—areconsistentwithmassesof≈106–107M⊙.Theyalsopointoutthatthelocationoftheclusterandthedynamicalcenterofthegalaxydonotalwayscoincide.TheonlydirectmassdeterminationforaNCfromameasurementofthestellarvelocitydispersionanddetaileddynamicalmodelingwasdonefortheNCinIC342byB¨oker,vanderMarel&Vacca(1999);they nd6×106M⊙,withaK-bandmass-to-lightratioof0.05insolarunits.Thederivedmeanageis≤108years.B¨okeretal.(2001)alsostudiedtheNCinNGC4449usingpopulationsynthesismodels.Theyestimateanageof≈107yearsinagreemen …… 此处隐藏：40026字，全部文档内容请下载后查看。喜欢就下载吧 ……