clusters
MichaelFellhauer
Inst.f¨urTheor.Phys.&Astrophys.Univ.Kiel,Leibnizstr.15,
24098Kiel,Germany
Abstract
Youngstarclustersininteractinggalaxiesareoftenfoundingroupsorclustersofstarclusterscontainingupto100singleclusters.Inourprojectwestudythefuturefateoftheseclustersofstarclusters.Wefindthatthestarclustersmergeontimescalesofafewdynamicalcrossingtimesofthesuper-cluster.Theresultingmergerobjecthassimilaritieswithobserveddwarfellipticals(dE).Furthermore,ifde-structiveprocessesliketidalheating,dynamicalfrictionorinteractionwithdiscorbulgeoftheparentgalaxyaretakenintoaccountourmergerobjectsmayevolveintoobjectsresemblingdwarfspheroidalgalaxies(dSph),withouttheneedofahighdarkmattercontent.
1Introduction
Systemsofinteractinggalaxies(likeTheAntennaeorStephan’sQuintet)showintensestarformation.Inthetidaltailsonefindsespeciallyyoungmassivestarclusters(Reff≈2−10pc;M≈104−107M⊙;seee.g.Whitmoreetal.(1999)).Thesestarclustersarenotdistributeduniformlybutareclusteredintogroups,somecontainingmorethanahundredclusters.Withthisprojectweinvestigatethefuturefateoftheseclustersofstarclusters(whatwecall“super-cluster”).FollowingtheargumentsofKroupa(1998)thesesuper-clustersmaybeboundentities,meaningthatthestarclustersinthesuper-clusterarenotonlyboundtothetidalfieldoftheparentgalaxybutalsoboundtoeachother.Therefore,thestarclustersareverylikelytomergewitheachotheronafewdynamicaltimesofthesuper-cluster.Thismergingprocessandtheresultingmergerobjectarethemajortopicofthisproject.
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2M.Fellhauer
2Setup
Wesimulatesuper-clusterscontainingN0=20starclusters.EachofthemisrepresentedbyaPlummersphere,containing100,000particles,atotalmassof106M⊙,ascale-length(Plummerradius)ofRpl=6pcandacrossingtimeof1.4Myr.AllstarclustersaredistributedintoanencompassingPlummer
sc=50,75,150,300pc,andthissuper-spherewithvariedscale-length,Rpl
clusterisplacedonacircularorbitatdistanceD=5,10,20,30,50,100kpcaroundananalyticgalacticpotentialwithanasymptoticallyflatrotationcurvewithvc=220km/sandRgal=4kpc.Forthediscussionoftheresults
sc=wedefinetwodimensionlessparametersforoursystems.α=Rpl/Rpl
0.02,0.04,0.08,0.12istheratiobetweenthescale-lengthofthesingleclus-sc/Rterandthescale-lengthofthesuper-cluster.β=Rcuttidal=0.2–2.7is
theratiobetweenthesizeandthetidalradiusofthesuper-cluster.Sev-eralrandomrealizationsofeachparametercombinationareperformedtoreducestochasticerrors.ThesimulationsarecarriedoutwiththeparticlemeshcodeSuperbox(Fellhaueretal.2000)whichhastheabilitytotracemanyobjectswithdifferentlevelsofhighresolutionsub-grids.Whiletheoutermostgridcoversthewholeorbitaroundthegalacticcentrewiththelowestresolution,themediumsizedgridsofeachstarclustercoverthesizeofthesuper-clusterandallintermediateinteractionsbetweentheclustersinmediumresolution(10–50pc),andtheinnermosthighresolution(1pc)gridscovereachsinglecluster.Bothmediumandhighresolutiongridsstayfocusedonthesimulatedobjectsandtravelwiththemthroughthesimula-tionarea.
3CompactMergerObjects
Adetaileddescriptionofthemergingtime-scalesandtheefficiencyofthemergerprocesswillappearinFellhaueretal.(2001).
Theresultingmergerobjectsarecompact,sphericalanddenseobjects.Theyhavehalf-massradiifrom40–150pcandsizesof120–1,000pc.Themassoftheseobjectsresemblesbetween75and95%ofthetotalmassoftheinitialsuper-cluster.Centraldensitiesarederivedbyfittingexponentialpro-filestotheinnerparts.Therangeofcentraldensitiesis100–1,000M⊙/pc3withexponentialscalelengthsofabout10pc.Intheouterpartsasteeppowerlaw(r−3.5)fitsthedensitybest.Thesurface-densitycanbewellfittedwithade-Vaucouleurprofile.Thevelocitydispersioninthecentralregionis
Buildingdwarfgalaxies3
about30km/sbutmeasuredalongtheline-of-sightthroughthecentreonederivesabout20km/s.Allthisdatasuggeststhatourmergerobjectshavesimilarpropertiestonucleateddwarfgalaxies(dE,N).Becausethemergerprocesslastsforaboutadozencrossingtimesofthesuper-cluster(forsmallα’sthiscansumuptomorethan1Gyr)themergerobjectissurroundedbyseveralstarclusterswhicharestillintheprocessofmergingorareonnear-circularorbitsaroundthesuper-clusterandslowlydecayingthroughdynamicalfriction.Therefore,ourmodelcanaccountforhighspecificclus-terfrequencies(SN),whicharefoundfordE,N-galaxies.
Furthermore,oursimplemodelsdonotaccountfordestructiveprocessesliketidalheatingoneccentricorbits,dynamicalfrictionintheparentgalaxyanddecayoforbit,shocksduetodiscorbulgepassages.Alltheseprocesseswouldleadtoamass-lossofthemergerobjectandadecreaseinitscen-traldensity.Translatedtoobservations,ourobjectswouldtransformintodE,noNwithtime,whilethenumberofremainingclustersisalsodecreasing.ObservationsshowthatdE,NhavehigherSNthandE,noN.
Ourobjectsalsoshowextendedtidaltailsalongthewholeorbitaroundthegalacticcentre.Escapedstarclustersarefoundinthesetidalfeatures.
4DissolvedMergerObject
Intwotestcalculationsthesuper-clusterwassetuponaneccentricorbit(Dapo=60kpc,Dperi=30kpc)sothattidalheatingcouldactonthemergerobjects.Themergerobjectsgetdissolvedwithtime(mass-loss≈5%perperigalacticonpassage).Duetoadisruptiveinteractionwithasecondobjectthecentraldensitiesdropdownfromseveralhundredto0.3–0.002M⊙/pc3,butcanstillbefittedwithanexponentialintheinnermostpart.Butintheouterpartsthematerialisnotboundtotheobjectanymore.Ithasa“fluffy”structureextendingafewkpc.Thesurface-densitycannotbefittedwithade-Vaucouleurprofileanymore.Themeasuredcentralvelocitydispersionsareintheorderof1–3km/s,butmeasuredalongtheline-of-sightthroughthecentreandtheouterunboundpartsonederivesabout10km/s.Comparedwithobservationstheseobjectslooklikedwarfspheroidalgalaxies(c.f.Klessen&Kroupa1998).StarclustersremainingaroundtheseobjectscanaccountfortheveryhighSNofsomedSph-galaxies.
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5Conclusions
Withoursimulationsweareabletoshow,thatthestarclustersinasuper-clustermergeefficiently.Comparingourparameterswithobservationswepredictanexponentialdecreasewithtimeofthenumberofclustersiftheobservedsuper-clustersareboundentities.
TheresultingmergerobjecthassimilarpropertieslikedE,N-galaxies,andifdestructiveprocessesaretakenintoaccountthenextstepintheevo-lutionofourobjectswouldbedE,noN.ThisalsogivesanaturalexplanationwhydE,NhavehigherSNthandE,noN.
ThefinalstageoftheevolutionwouldbeadSph-galaxy,suggestingthatatleastsomeoftheseobjectsmaynothaveapredictedhighdarkmattercontent.
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