1997Aj 113.1652F the Astronomical Journal

1997AJ 113.1652F tial stagesofgalaxyformation. Observationsofglobular probe ofchemicalenrichmentand starformationintheini- provide auniquetoolforstudyinggalaxyformationandevo- of thecollapseandmergermodels forellipticalgalaxyfor- clusters (GCs)mayenableusto distinguishbetweenvariants lution atearlytimes.Inparticular,theymayprovidethebest fossils” andliketheircounterpartsinpalaeontologythey 1652 Astron.J.113 (5),May1997 THE ASTRONOMICALJOURNAL Globular clustershavelongbeenrecognizedas‘‘galactic © American Astronomical Society • Provided by theNASA Astrophysics Data System formation process.©1997AmericanAstronomicalSociety. [S0004-6256(97)03505-X] analog ofthemetal-poorhaloGCsinspirals.Thedisk spiralsmayrepresentathirdphaseofthis contemporaneously withthegalaxystars.Inthissensemetal-richGCsinellipticalgalaxiesare to thebimodalGCmetallicitydistributions.Themetal-poorGCsareformedatanearlystageincollapse indigenous GCsformedintwodistinctphasesofstarformationfromgasdifferingmetallicity,givingrise that thevastmajorityofGCsingEandcDgalaxieshaveprobablyformedsitu.Wespeculatethese of theprotogalacticcloud.Themetal-richGCsformedoutmoreenrichedgas,roughly nearby galaxiesappearstocontributetheGCpopulationinouterpartsofcDgalaxies,butwesuggest ApJ, 384,50)isunlikelytoaccountfortheGCsystemsinthesegalaxies.TidalstrippingofGCsfrom metal-poor GCsatlargegalactocentricradii.Wecriticallyreviewcurrentideasfortheoriginofin Fourth, wefindsteepermetallicitygradientsinhighSgalaxies.Thisisduetothegreaternumberof galaxies isthattheirGCsarepreferentiallylocatedintheoutergalaxyregionsrelativetounderlying high specificfrequency,S)onlyhaveshallow,extendedradialdistributions.Acharacteristicof giant elliptical(gE)andcDgalaxiesconcludethatthegaseousmergermodelofAshman&Zepf(1992, high Sgalaxieshaveproportionatelymoremetal-poorGCs,perunitgalaxylight,thanlowgalaxies. starlight. Third,wefindthattheratioofmetal-richtometal-poorGCscorrelateswithS.Inotherwords, early-type galaxies.However,thegalaxieswithrelativelypopulousGCsystemsfortheirluminosity(called galaxy luminosity.Second,theslopeofGCsystemradialsurfacedensityvariesconsiderablyin with thegalaxyfieldstars.Themeanmetallicityofmetal-poorpopulationislargelyindependent well withparentgalaxyluminositybutthemeanmetallicityofmetal-pooronesdoesnot.Thisindicates with bimodalGCmetallicitydistributions,wefindthatthemeanofmetal-richGCscorrelates properties ofGCsystems.First,whenweseparatethemetal-richandmetal-poorsubpopulations,ingalaxies multimodal GCmetallicitydistributionsseeninmassiveearly-typegalaxies.Weexploretheoriginofthese Perhaps themostnoteworthyofrecentfindingsinextragalacticglobularcluster(GC)researchare that themetal-richGCsarecloselycoupledtogalaxyandshareacommonchemicalenrichmenthistory distinct GCpopulations,theimplicationsforgalaxyformationandevolution,identifyseveralnew N N N N Lick Observatory,UniversityofCalifornia,SantaCruz,California95064,andSchoolPhysicsSpaceResearch, ON THEORIGINOFGLOBULARCLUSTERSINELLIPTICALANDcDGALAXIES 1. INTRODUCTION Jet PropulsionLaboratory,4800OakGroveDrive,Pasadena,California91109 Lick Observatory,UniversityofCalifornia,SantaCruz,California95064 of Birmingham,Edgbaston,BirminghamB152TT,UnitedKingdom Received 1997January16;revisedFebruary18 Electronic mail:[email protected] Electronic mail:[email protected] Electronic mail:[email protected] 0004-6256/97/113(5)/1652/14/$ 10.00 VOLUME 113,NUMBER5 Duncan A.Forbes Carl J.Grillmair Jean P.Brodie ABSTRACT many oftheearlymodelshave beenruledout,someofthe These modelsfocusongascloud physics,includingmetal- have soughttoexplaintheformation mechanismforGCs. past (Kormendy1990). licity, coolingandstarformation rates,shocks,etc.Although models (e.g.,Fall&Rees1985;MurrayLin1992;Kumai understand theformationofGCsthemselves.Avariety mation, adistinctionthathasbeendifficulttomakeinthe et al.1993a;Harris&Pudritz1994;VietriPesce1995) The firststepinusingGCstoprobegalaxyformationis © 1997Am.Astron. Soc.1652 MAY 1997 1997AJ 113.1652F 04(Mv+15) plane relationtothatofelliptical galaxies(Hoessel,etal. the mainbodiesofcDgalaxies haveasimilarfundamental- best-studied. TheirGCsmayhave hadasimilaroriginand tion andthenumberofGCsmoreaccuratelyreflects but notalwayscDs)are“anomalous”inthesensethat bright centralgalaxieswithhighSvalues(theseareusually temperature. Hissample,of5cDsand14giantellipticals resided (McLaughlinetal.1994).Initialevidencethatsuch the galaxiesthemselvessharesome commonproperties,e.g., contain themostpopulousGC systems; whichareoftenthe with thesefindings. than onemassivehighSgalaxyperclusterisconsistent local clustermassdensity.Thefactthatthereisnevermore they havetoomanyGCsfortheirluminosity.Rather,these evolutionary historiesthangEs,theirGCsystemsmayactu- neous samplethanwaspreviouslyavailable,Blakeslee correlations didinfactexistwaspresentedbyWestetal. tics oftheclustergalaxiesinwhichparentgalaxy between thepropertiesofaGCsystemandcharacteris- higher Svaluesthanfieldellipticals;Harris1991).How- pected forsometime(e.g.,clusterellipticalshaveonaverage early-type galaxies(i.e.,gEandcDgalaxies).These galaxies are“underluminous”fortheirclustercentricposi- ally havehadasimilarorigin.Blakesleeconcludedthat dispersion, thelocaldensityofbrightgalaxiesandX-ray ever, itwasgenerallybelievedthattherewerenocorrelations tion hasbeenthesubjectofdebate. identified andindeedthequestionofwhethertheyare most outstandingprobleminglobularclustersystemre- tems hasbeendescribedbyMcLaughlinetal.(1994)as“the S valuesatallgalactocentricradii(McLaughlin,etal. higher globalSvaluesbuttheyshowevidenceforhigh S galaxies.Furthermore,thesegalaxiesdonotsimplyhave for —22>My>—23.5severalgalaxieshave5^—15,i.e.,a tudes -\9>M>-2(e.g.,Kissler-Patig1996).However, ity ofM=—15(Harris&vandenBergh1981).Theresult- vention istonormalizetheGCcountsperabsoluteluminos- roughly withtheparentgalaxy’sluminosity.Ausefulcon- This mayprovideconstraintsforthefurtherrefinementof although cDgalaxieshaveundoubtlyhadmorecomplex anomalous ormerelyrepresenttheextremeofsomedistribu- factor ofthreetimesasmanyGCsperstarlight“normal” this way,S~5formostearly-typegalaxieswithmagni- ing “specificfrequency,”5^=AX10',isprobably formed byexaminingthesystemicpropertiesofGCsystems. more recentmodelsmayeventuallybeacceptedasapre- (gEs), coverthewholerangefrom5^—4to—10.Therefore, (1996) hasshownthat5^iscorrelatedwithclustervelocity (1995). Morerecently,basedonalargerandmorehomoge- search.” TheoriginoftheseGCsystemshasyettobeclearly a measureofGCformationefficiency.Whennormalizedin GC andgalaxyformationmodels. approach, andattempttodeterminewherewhenGCs scription forGCformation.Inthispaperwetakeadifferent 1987). First,we presentseveralrecentdevelopments inthe 1994). Findinganexplanationforthe“high”SGCsys- 1653 FORBESETAL.:THEORIGINOFGLOBULARCLUSTERS N N N N N N v v N N Environmental effectsonGCsystemshavebeensus- In thispaperwefocusontheoriginofGCsinmassive The numberofGCsinanearly-typegalaxyscales © American Astronomical Society • Provided by theNASA Astrophysics Data System :: -1 refer tothemetal-poor,intermediatemetallicityandmetal- A=620± 130.Webelievethistobemoreaccuratethan populations thathavedifferent metallicities orages,some rich GCpopulations.Thedifferencebetweenthemetallicity NGC 1399and4486(M87)therehavebeenclaimsfor bution. Theobservedmetallicitydistributionseffectivelyrule We havetakendistancemoduliandthenumberofGCs combination of both. Thisisthewell-knownage-metallicity of themetal-richandmetal-poor peakshasameanvalueand galaxies fromtheliteraturearesummarizedinTable1.For formation episodesfromdifferentmetallicitygas;asingle distributions suggeststhatGCshaveformedindistinctstar there arenowahandfulofconvincingcases(seeTable1for low centralcusps(e.g.,Faberetal.1996;Carollo The oneexceptionisNGC1404,forwhichwetakeNqq vide importantconstraintsonanymodelforGCformation. that haverecentlybeendiscoveredorconfirmed.Theypro- ping andinsituGCformation).Fourth,wepresentacase properties ofGCsystemsearly-typegalaxies.Third,we a trimodalmetallicitydistribution.Inthesecaseswewill out asimplemonolithiccollapseandprovidestrongcon- a summary).TheexistenceofmultimodalGCmetallicity In thissensetheyaregenerallyboxy,slowrotatorswithshal- ies discussedinthissectionareluminousgEorcDgalaxies. Hamabe valuewhichgivesM=—22.7.Mostofthegalax- the averageofabovetwovalues.Hereweadopt cluster, Hamabe(1993)findsV°=10.75whichisroughly those givenintheRC3.TheoneexceptionhereisNGC Faber etal.(1989)andaregenerallywithinafewtenthsof the numberquotedbyHarris(1996),i.e.,950±140(seealso from therecentHSTstudyofForbesetal.(1997b),i.e., ideas. the originofGCsandmakeseveralpredictionstotestthese present ourviewsandsupportingdataforwhatwebelieve the frameworkofvariousmodelsproposedtoexplain the originofGCs.Second,wediscusstheseresultswithin speaking, onesofcolor.Thebimodal colorsrepresentGC statistical errorofA[Fe/H]=1.0±0.1 dex. straint foranyGCformationmodel.Thedata11gE/cD star formationepisodewouldgiverisetoaunimodaldistri- galaxies. Finally,wesummarizeourpreferredscenariofor are themostviableformationmechanisms(i.e.,tidalstrip- (metallicity) distributionsinearly-typegalaxieswasweak, and 11.16(RC3).InaphotometricstudyoftheHydra (A) directlyfromtheMcMasterCatalog(Harris1996). study ofGCsystemsthatprovideimportantconstraintson 3311, inwhichthevaluesareV^=10.14(Faberetal.1989) Sec. 3.5).Extinction-correctedVmagnitudescomefrom study ofthecDgalaxyNGC1399andFornaxcluster 1997). WeassumeA=75kmsMpc. gc v T gc 0 2.1 MultimodalGlobularClusterMetallicityDistributions 2. RECENTDEVELOPMENTSINEXTRAGALACTICGLOBULAR The metallicitydistributionsquoted inTable1are,strictly Although theinitialevidenceformultimodalGCcolor In thissectionwehighlightseveralaspectsofGCsystems CLUSTER STUDIES 1653 1997AJ 113.1652F bimodal metallicity distributions.Thetwometallicity peaks 4486 (M87).Severalinteresting resultswerefoundbyGei- 4472 (M49)inVirgo(Geisleret al1996).Althoughitisnot has M=—2\.Lessluminousellipticalswith>2\ CCD photometry,thebeststudiedGCsystemisthatofNGC mean offsetbetweenGCsandfieldstarsis0.5dex,as With asimilarnumberofGCsineachsubpopulation,the a highSgalaxy(5^=5.5± 1.7), itisagiantelliptical generally observed.ThelowestluminositygalaxyinTable1 metal-rich GCshaveasimilarmetallicitytothegalaxystars, tallicity ofGCsisabout0.5dexlowerthanfieldstarsingEs rived anage-metallicityrelationforMilkyWayhaloGCsin sler etalwhichmayalsohold true forotherellipticalswith (M= —22.1)thatissomewhat moreluminousthanNGC also showametallicityoffsetbetweenGCandgalaxystars and themetal-poorGCsareabout1dexmoremetal-poor. can explainthelong-standingobservationthatmeanme- investigate thisissue. Zepf 1996).Forexample,Chaboyeretal(1992)havede- probably besomecomponentduetoageeffects(seealso and redGCsinanellipticalgalaxyareneededtofurther which GCswithA[Fe/H]=1dexareseparatedinageby colors mostlyreflectGCmetallicitybutinrealitytherewill population isyoungerby—13Gyr.Ontheotherhand,for (Forbes etal1996). sler etal1996)canbeextendedtoothergalaxies,thenwe GC system.Inthediscussionbelowwewillassumethat the redpopulationismoremetal-richthanblueoneby contemporaneous populationsA(U—7)~0.2indicatesthat for atypicalcolordifferenceofA(V-/)~0.2,theblue ity andthattheredpopulationisveryold(i.e.,15Gyr),then able given,forexample,ourknowledgeoftheMilkyWay we assumethatthetwopopulationshavesamemetallic- degeneracy thataffectstheinterpretationofopticalcolors.If Harris etal1992,10Forbes1997a). 2 Ostrovetal.1993,3Forbesal1997b,4Seeker1995,5Zepf6Geisler1996,7Lee&89 metal-poor globularclusters;(10)Sourceofmetallicity(i.e.,broad-bandcolorsorWashingtonphotometry);(11)Metallicityreference(1Bridgesetal1996b, frequency; (7)Metallicityofpeaks(*indicatesmarginalsignificance);(8)Percentageglobularclustersineachpeak;(9)Numbermetal-richto IC 1459E331.7 N5846 E032.3 N5128 EOp28.3 N4494 E030.8 (e.g., Harris1991).Thisthensimplyarisesbecausethe N4486 EO/cD31.0 N4472 E231.0 N3923 E331.9 N3311 EO/cD33.4 N1404 El31.0 N1399 El/cD31.0 ~4Gyrs. Highsignal-to-noisespectraofasampleblue (1) Galaxyname;(2)type;(3)DistancemodulusfromHarris(1996);(4)AbsoluteVmagnitude;(5)Totalnumberofglobularclusters;(6)Specific N1052 E431.3 ~ 1dexinmetallicity.Thelatterappearsmuchmorereason- Galaxy Type{m-M)MNS[Fe/H]Percent/NSourceRef. (1) (2)(3)(4)(5)(6)(7)(8)(9)(10)(11) v N v 1654 FORBESETAL.\THEORIGINOFGLOBULARCLUSTERS vGCNRP In termsofthenumberGCsobservedwithhighquality If thecharacteristicsofGCsysteminNGC4472(Gei- © American Astronomical Society • Provided by theNASA Astrophysics Data System -21.8 -22.6 3120±18502.8±1.7 -22.0 1700±4002.7±0.6 -21.0 1400±3505.4±1.3 -22.5 13000±50014±1 -22.7 6300±19005.5±1.7 -22.3 4300±10005.0±1.2 -22.7 12400±500011±4 -21.1 620±1302.3±0.4 -21.5 5340±178013±4 -21.0 600±702.4±0.2 Table 1.Multimodalmetallicitydistributions. Notes toTable1 0 4 parent galaxy.The Z—Lrelationholdsimportant cluesabout the relationissameasgalaxy Z—Lrelation(Brodie have nowconfirmedthatsuch a relationshipoftheformZ GCs havehadasimilarchemical enrichmenthistorytotheir metallicity wasindependentofgalaxyluminosity.Larger puted byAshman&Bird(1993).TheyclaimedthattheGC this relation,atleastforlowluminositygalaxies,wasdis- tallicity dataofBrodie&Huchra(1991).Theexistence Bergh (1975)andlatersupportedbythespectroscopicme- & Huchra1991).Takentogether theseresultsindicatethat galaxy samples(Durrelletal1996;Forbes1996b) galaxy luminosity(Z—L)wasfirstsuggestedbyvanden radius. Themetal-poorGCsaresimplyoffsetfromthefield fective radiusofabout100"(Benderetal1992). process (similartothehaloofourGalaxy).Themeanmetal- of 227"and274",respectively.Thegalaxyitselfhasanef- centrated thanthemetal-poorGCs,andhaveeffectiveradii lying fieldstarmetallicityoverawiderangeingalactocentric licity ofthemetal-richpopulationcloselymatchesunder- relative mixofthetwopopulationswithradius.Thereislittle rich GCsinNGC4472.)Theoverallradialmetallicitygra- ocL, doesindeedexist,over 10 magnitudes.Theslopeof or noevidenceforametallicitygradientwithineitherGC indicates thereareabout4200metal-poorand2100metal- peak. (AsgiveninTable1,foratotalpopulationof6300this that A[Fe/H]=1.2.Theycanbewellrepresentedbytwo in NGC4472arelocatedat[Fe/H]=-1.25and-0.05,so stars by1.2dex.Themetal-richGCsaremorecentrallycon- dient oftheGCsystemisalmostentirelyduetochanging subpopulation suggestingalargelydissipationlessformation about 2/3inthemetal-poorpeakand1/3metal-rich Gaussians, bothwiththesamedispersionof0.38dex, 2.2 TheGlobularClusterMetallicity—GalaxyLuminosity A relationshipbetweenGCmeanmetallicityandparent -1.35,-0.75,-0.1 45,40,15 -1.5,-0.8,-0.2 35,40,25 -1.25,-0.05 65,35 -1.3,-0.3* 60,40 -1.6,-0.2* 60,40 -1.1,-0.4* 50,50 -0.95,+ 0.055,45 -0.5,+ 0.1570,30 -1.2,-0.2 25,75 -1.2,-0.1 60,40 -0.8,0.0* 70,30 Relation 0.4 V-I8 3.0 V-I10 0.7 Wash9 0.7 V-I8 0.3 Wash7 0.5 Wash6 0.8 Wash5 0.4 Wash4 0.7 B-I3 0.7 Wash2 1.0 B-I1 1654 1655 FORBES ET AL.: THE ORIGIN OF GLOBULAR CLUSTERS 1655

Fig. 1. Globular cluster mean metallicity-galaxy luminosity relation. The Fig. 2. Globular cluster surface density slope vs specific frequency for el- upper panel shows the mean metallicity of the metal-rich population from liptical galaxies. The power-law slope of the globular cluster surface density Table 1. The solid line is a weighted fit to the data. A good correlation profile and the specific frequency (SN) are taken from the list of Kissler- between the metallicity of the metal-rich globular clusters and the parent Patig (1996). For low SN galaxies, there is a wide range of density slopes, galaxy luminosity is present. The lower panel shows the mean metallicity of but high SN galaxies only have flat, extended globular cluster distributions. the metal-poor population. The solid line is the metal-rich relation offset lower by 1 dex in [Fe/H] (i.e., the average offset between the metal-rich and metal-poor populations). There is little or no correlation between the metal- ity difference between the two peaks). There is considerably licity of the metal-poor globular clusters and the parent galaxy luminosity. more scatter in the Z—L relation for the metal-poor data points. This is unlikely to be caused by errors or calibration problems and so the difference in scatter must be a real ef- GC and galaxy formation processes, but the large scatter, fect. Either there is virtually no Z—L relation for metal-poor which may be mostly observational error, makes it difficult GCs or there is a ‘second parameter effect’ causing the sig- to reach any definitive conclusions. nificant scatter. Figure 1 clearly indicates that [Fe/H] for the In the previous studies of the GC Z—L relation, the GC metal-rich GCs is more closely coupled to the luminosity of metallicity was taken to be the mean of the entire GC sys- the galaxy than for the metal-poor GCs. This result will be tem. We now know that in several cases, the GC system is discussed further in Sec. 3.4. better represented by two GC mean metallicities correspond- ing to the metal-rich and metal-poor subpopulations. Al- 2.3 The Spatial Distribution of Globular Clusters though observational errors may dominate, there are other reasons for expecting some scatter in the Z—L relation. In It has been known for sometime that the radial distribu- particular, some of the scatter will be due to changing pro- tion of GCs is often more extended than the starlight of the portions of the metal-poor and metal-rich GC populations parent galaxy (see Harris 1991 for a review). The GC radial from one galaxy to the next, which is further complicated by surface density profile is usually fit by: observations that cover different galactocentric radii (metal- a poor GCs are more radially extended than the metal-rich P = Por , (1) ones). In Fig. 1 we plot the mean metallicity of the two where a is the radial slope. Kissler-Patig (1996) has col- subpopulations, listed in Table 1, against the galaxy luminos- lected measurements of a form the literature. After exclud- ity. We have assumed errors of ±0.1 for Washington color ing the SO galaxies, we show in Fig. 2 this slope versus the photometry, ±0.15 for B — I and ±0.25 for V—I colors SN value as given by Kissler-Patig. This figure shows that which reflect the scatter in the color-metallicity relations. A low SN galaxies have a wide range of GC radial slopes, weighted fit for the metal-rich GC peak gives [Fe/H] whereas high SN galaxies only have relatively flat slopes. For =—0.21 (±0.06) Mv—4.6(± 1.4). The scatter of the SN>1 the mean slope is a=-1.43±0.12 (statistical error) metal-rich data points about the relation is less than the es- and for SN^1 it is a= —1.76±0.09. There are no cases timated error bars, suggesting that our assumed metallicity known of high SN galaxies with steep GC radial slopes. This errors are overestimated. In the lower panel, for the metal- ‘zone of avoidance’ is unlikely to be due to selection effects poor GCs, we have simply offset the metal-rich relation as high SN galaxies have numerous GCs and so are relatively downward by A[Fe/H]= 1.0 dex (i.e., the average metallic- easy to find and study. Thus another property of high SN

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1997AJ 113.1652F 1/4 1/4 located atlargeradiiinthemainbody(rpart)of ris 1991;McLaughlinetal.1993).Inotherwords,acharac- been measuredoversimilargalactocentricradiiforahandful tions andcanthereforeonlybe discussed onamorequahta- below. Mostothermodelshavenotmadesuchclearpredic- Zepf &Ashman(1993),makesdetailedpredictionsforGC cooling flows,(3)intraclusterglobularclusters,(4)insitu merger). or sometertiaryGCprocesshasoccurred(e.g.,theaccretion lapse. Eitherthecollapsewasmorecomplex(e.g.,episodic) type galaxieseffectivelyrulesoutasimplemonolithiccol- ence ofmorethanonepopulationGCsinmassiveearly- units (hierarchicalclustering)playaroleintheformation majority viewisthatbothcollapseandthemergingofsub- into the“collapse”or“merger”picture.Thepresent for theformationofgalaxieshavetraditionallybeendivided galaxies (e.g.,West1993)seemstoberuledout.Theories Peebles &Dicke1968)andGCformationthatisunrelatedto matter inGCs,formationwellbeforethegalaxyitself(e.g., their parentgalaxy(seeHarris1991)andtheabsenceofdark formed. GiventhenumerouscorrelationsbetweenGCsand models, GCsformaftermostoftheparentgalaxyhas the sametimeasgalaxyitself,andfortertiaryformation axy. Insecondaryformationmodels,GCsformatessentially mary formationmodels,GCsmayformwellbeforethegal- into threetypes—primary,secondary,andtertiary.Inthepri- govski 1989)thisimpliesthatGCsintheenvelopearealso uct oftheirclusterenvironment(e.g.,Kormendy&Djor- origin. AscDenvelopesaregenerallythoughttobeaprod- outer GCsandtheenvelopeofcDgalaxieshaveasimilar lope (e.g.,McLaughlinetal.1993).Thissuggeststhatthe evidence forachangeinslopeattheradiusofcDenve- galaxy. InNGC4486(M87)theGCradialprofileshows teristic ofhighSgalaxiesisthattheirGCsarepreferentially GC systemisgenerallyflatterthanthegalaxystarlight(Har- of galaxies.Intheinner(i.e.,r)regionscDgalaxies, light slope.Unfortunately,thestarlightslopeshaveonly ence betweentheGCradialslopeandunderlyingstar- tial distribution(seealsoKaisleretal.1996). galaxies isthattheirGCsystemshaveaveryextendedspa- esis onthegrounds thatellipticalshavemanymore GCsper tive basis. galaxies. ThemergermodelofAshman&Zepf(1992)and GC formation,and(5)tidalstrippingofGCsfromnearby giant ellipticalandcDgalaxies:(1)agaseousmerger,(2) of existingGCsorthecreationnewinagaseous and subsequentevolutionofearly-typegalaxies.Thepres- associated withsomeaspectofthecluster. system formationonamacroscopiclevelwhicharetested 1656 FORBESETAL:THEORIGINOFGLOBULARCLUSTERS N Fall &Rees(1988)characterizedGCformationmodels Van denBergh(1984)arguedagainst themergerhypoth- Below wediscussspecificmodelsfortheoriginofGCsin A moreusefulquantitytoexaminewouldbethediffer- © American Astronomical Society • Provided by theNASA Astrophysics Data System 3. MODELSFORTHEORIGINOFGLOBULARCLUSTERS 3.1 TheGaseousMergerModel IN gEANDcDGALAXIES jects) itseemsplausiblethattheseobjectswillevolveinto be disruptedbyhighvelocitycollisions. Forbes etal.1996a).Furthermore,thismodelcannotapply models predictamass-metalficityrelationforindividualGCs that GCsforminthecollisionofhighvelocitygas.Their metal-poor ellipticals.Theseissuesandothershavebeendis- minous) ellipticalsarenotmadebysimplymergingseveral exceeds thenumberofmetal-richGCs.TheGCmetallicity- fewer innumber.Column9ofTable1showsthatthisis the lowerluminosityprogenitorandwouldthereforealsobe unit fightthanspirals.Anadditionalproblemisthatapurely massive gas-richgalaxiesorseveral~L*galaxies.ForcD bona fideGCs.ThustheAshman&Zepf(1992)modelap- The metal-richGCsareobservedtobemorecentrallycon- presence ofmetal-poorGCsfromtheprogenitorspiralsand to theGCsystemsofdwarfgalaxieswhichwouldprobably merger, themodelsofKumaietal.(1993a,1993b)propose from thegasofmergerprogenitors.Foragaseous cussed byvandenBergh(1990). galaxy luminosityrelationalsosuggeststhatmetal-rich(lu- generally notthecase,i.e.,numberofmetal-poorGCs lation suggeststhatthemetal-poorGCswouldcomefrom GCs. Inparticular,theGCmetallicity-galaxyluminosityre- created) cannotexplaintheratiosofmetal-poortometal-rich tively metal-rich withpeaksat[Fe/H]=—0.5 and+0.15 model. ThecaseofNGC3311 provides aparticularlystrong lous GCsystemswiththeAshman &Zepf(1992)merger mentioned someproblemsintrying toaccountforthepopu- to several“differentepochsof[globular]clusterformation man &Zepf(1992)hypothesisthegEshavebeenformedby massive early-typegalaxies.IntheframeworkofAsh- merging galaxies. pears consistentwiththeobservationsofGCsincurrently issues remain(suchasdestructionoftheverylowmassob- with thepropertiesofprotoglobularclusters.Althoughsome ing galaxies(e.g.,Whitmoreetal.1993;Schweizer is thediscoveryofprotoglobularclustersincurrentlymerg- model. Oneofthemostnoteworthysuccessesthismodel centrated thanthemetal-poorones,asexpectedintheir metal-rich onescreatedfromthegasofmergedgalaxies. model thebimodalGCmetallicitydistributionsaredueto context ofGCformationingEandcDgalaxies.Intheir man (1993).Belowwediscussthismodelindetailthe which isnotsupportedbytheobservationaldata(see,e.g., constraint foranyGCformation modelasitsGCsarerela- cD galaxies? and thusaspreadinclusterages.”Cangaseousmerger galaxies theyadvocatemultiplemergerswhichinturnleads of theidentifiedobjectsgenerallyappeartobeconsistent stellar mergeroftwoellipticals(inwhichnonewGCsare attention isthatofAshman&Zepf(1992)andAsh- subsequent GCformation,asdescribedbyAshman&Zepf (1992), accountforthepropertiesofGCsystemsingEand 1996). Thesizes,magnitudes,colors,andmassdistribution 1994) andbrightestclustergalaxies (Blakeslee1996)have Previous studiesofhighSgalaxies(McLaughlinetal. Here weareinterestedintheoriginofGCsmost Another mergermodelthathasbeenthefocusofmuch Schweizer (1987)hassuggestedthatnewGCsarecreated N 1656 1997AJ 113.1652F ents ofellipticals withhighSvalueswillbeshallower than Ashman (1993)predictthatthe “colour[metallicity]gradi- metal-poor GCssignificantlyexceed themetal-richones. Table 1wegivetheratioofnumbermetal-richto by themergerofseveralnormalSgalaxies,majority • Theradialmetallicityslope increaseswithS.Zepf& direct contradictiontothatexpected intheAshman&Zepf galaxies isthattheyhaverelativelymoremetal-poorGCs,in creases, asshowninFig.3.ThusapropertyofhighS galaxies. Table1showsthatthisratio(column9)isgener- vary from~1fornormalSelhpticalsto4high metal-poor GCs(thisisequivalenttoA5=A/A^asde- the metal-richGCsshouldbemorenumerous(byafactorof metal-rich population.InordertoexplainhighSgalaxies ally lessthan1.Furthermore,inthemergermodelthisratio fined byAshman&Zepf1992).Thisratioisexpectedto GCs mustbecreatedinthemergerevent.Thisimpliesthat model, indecreasingorderofsignificance. the expectationsofAshman&Zepf(1992)merger merger modelofAshman&Zepf(1992)predictsanoppositetrendtothat lar clustersincreasesthespecificfrequency(S)valuedecreases.HighS should increasewithincreasing5^aslargernumbersofnew • Themetal-poorGCsaremorenumerousthanthe detail someoftheobservationaldatawhicharecontraryto count forthe‘metal-poor’peakof[Fe/H]~—0.5.Herewe galaxy (whichtypicallyhavemean[Fe/H]=-1.5)canac- galaxies havemoremetal-poorglobularclustersthanlowSones.The lower left.Thedatashowthatastheratioofmetal-richtometal-poorglobu- Fig. 3.Specificfrequencyvstheratioofmetal-richtometal-poorglobular (1992) model.ForthecDgalaxies inTable1thenumberof (metal-rich) GCsareformed.Insteadwefindthatitde- seen, i.e.,anincreasingNIratiowithhigherSvalues. clusters. ThedatapointforNGC5846(N/N=3.0,S2.S±\.l)isnot ~3X) thanthemetal-poorGCs.Thisisnotcase.In (Seeker etal.1995).ItishardtoseehowtheGCsinaspiral shown fordisplaypurposes.AtypicalerrorbarNINisinthe 1657 FORBESETAL.\THEORIGINOFGLOBULARCLUSTERS N N N N N newold N N N RPN RPN RP © American Astronomical Society • Provided by theNASA Astrophysics Data System t this hne. talhcity gradientof—0.58.It reproduces thegeneraltrend for NGC4472(i.e.,S=5 .5 andA[Fe/H]/AlogR metalhcity gradientofA[Fe/H]/Alog/?=-0.58dex/".The these additionalGCsaredistributedwiththesamesurface the numberofmetal-richGCsremainsconstant).Thuswe tween agalaxyofS=5.(i.e.,NGC4472)andonewith Furthermore, wewillassumethattheonlydifferencebe- dicted relationwouldhaveagradient oftheoppositesignto dashed lineshowninFig.4simplyconnectsthedatapoint density distributionasiscurrentlyseen,thenweexpecta can predictthemetalhcityslopeofagalaxywith^=11.If S= 11isanincreaseinthenumberofmetal-poorGCs(i.e., changing relativemixofmetal-richandmetal-poorGCs. metalhcity gradientsinelhpticalsareduesolelytothe for NGC4472(M49),wewillassumethattheobservedGC NGC 4472(Geisleretal.1996).Asappearstobethecase metallicity slopewithincreasingSbasedonthedatafor that themergermodelpredictionfailedtomatchob- problem forthemergermodel.Geisleretal.(1996)found variation forNGC4472. S elhpticals.ThistrendisindirectcontradictiontotheZepf S ellipticalstohavesteepermetallicityslopesthannormal though thereisconsiderablescatter,thetrendforhigh 4 weplotthe[Fe/H]metalhcitygradientsversusS.Al- surprisingly well.Notethatthe Ashman &Zepf(1992)pre- values fromtheliteratureandUstedtheminTable2.InFig. man 1993).WehavecollectedmetallicitygradientsandS formed nearthecenterandmetal-poorGCswouldbe those withnormalSvalues.”Thiswouldbesobecausein data fromForbesetal1996b,6Geisler1996,7Lee&1993, et al1995,4Zepf5Unpublishedmetallicitygradientsusing gradient (index/")-*statisticalerror,includesdatafromAjharetal. served GCmetallicityinbothabsolutevalueandradial & Ashman(1993)predictionandappearstobeageneral a highSgalaxy,largenumbersofmetal-richGCsshouldbe 8 Forbesetal1997a). (1994); (6)Reference(1Ostrovetal.1993,2Forbesal1997b,3Seeker V magnitude;(4)Specificfrequency;(5)Globularclustermeanmetallicity (1) Galaxyname;(2)DistancemodulusfromHarris(1996);(3)Absolute — 0.41)tothatexpectedfor an S=11galaxywithame- “puffed up”tolargegalactocentricradii(seeZepf&Ash- N N N N N N N N N N N Galaxy (m—M)M N5846 N4494 N4486 N4472 N4365 N4278 N3311 N4406 N3923 N1404 N1399 v We havemadeacrudepredictionforthevariationof (1) (2)(3) 32.3 30.8 31.0 31.0 31.0 31.4 30.0 31.9 33.4 31.0 31.0 Table 2.Metallicitygradientslopes. -22.6 -21.0 -22.5 -22.0 -21.8 -19.9 -21.1 -21.5 -22.7 -22.3 -22.7 Notes toTable2 5.5±1.7 2.8±1.7 5.4±1.3 5.2±0.6 5.0±0.4 5.0±1.2 2.3±0.4 14±1 11 ±1.4 11±4 13±4 S A[Fe/H]/AlogRRef. (4) (5)(6) N f -0.13±0.03 -0.71 ±0.05 -0.65±0.17 -0.41 ±0.03 -0.33 ±0.09 -0.41±0.13 -0.64±0.05 -0.49±0.19 -0.78±0.06* -0.36±0.13 -0.34±0.18 1657 CM UDLO 1658 FORBES ET AL.\ THE ORIGIN OF GLOBULAR CLUSTERS 1658

(M49) both lie in the Virgo cluster and have essentially the same luminosity, and yet they have quite different SN values (13 and 5.5, respectively). In the merger hypothesis, this would indicate that the GCs in NGC 4486 formed ~3 X more efficiently than those in NGC 4472. • The GC surface density profiles of normal SN galaxies are not always flatter than the underlying starlight. Zepf & Ashman (1993) “expect the globular cluster system to be noticeably flatter than the galaxy” for normal (i.e., not high SN) galaxies. As shown in Fig. 2, galaxies with 7 have a wide range in GC surface density slopes suggesting a wide range in the difference between GC and starlight slopes. Kissler-Patig et al. (1996) have studied several galaxies in the Fornax cluster with SN<1. All have GC slopes that are consistent with the starlight slopes—none are noticeably flatter contrary to the prediction of Zepf & Ashman (1993). • The main body and envelope of cD galaxies appear to be distinct. For high SN galaxies, Zepf & Ashman (1993) predict in general that “the properties of the galaxy and the globular cluster system should be similar.” Both the main body and the cD envelope should be formed by the same mergers. In practice, the envelopes of cD galaxies are generally found to be structurally distinct from the rest of the Fig. 4. Globular cluster radial metallicity gradient vs specific frequency. galaxy (e.g., Mackie 1992). Furthermore, in both NGC 1399 The data suggest that high SN galaxies (from Table 2) have steeper radial and NGC 4486 (M87) the velocity dispersion of the GCs is metallicity slopes than low SN ones. The dashed line is the expected trend if well in excess of the stellar velocity dispersion at large radii an increase in SN corresponds to only larger numbers of metal-poor globular (Grillmair et al. 1994b; Huchra & Brodie 1987; Mould et al. clusters which are placed with the same radial distribution as seen in NGC 4472 (see text for details). The zeropoint is taken to be the current values for 1990); for NGC 1399 the GC velocity dispersion in the cD NGC 4472 (i.e., 5^=5.5 and A[Fe/H]/A log R= -0.41). The merger model envelope is close to that for the galaxies in the cluster. of Ashman & Zepf (1992) predicts an opposite trend to that seen, i.e., To summarize, the gaseous merger model as proposed by shallower metallicity gradients in high SN galaxies. Ashman & Zepf (1992) for GC formation in massive early- type galaxies faces some serious difficulties. We note that

• High SN galaxies do not have peaky central GC densi- some of these difficulties have been identified by Ashman & ties. The gas in a merger quickly dissipates to the galaxy Zepf (1997) and possible solutions suggested. However, it is center, where large numbers of new GCs should form. The our view that the disagreements between the observational expected higher GC surface densities in the centers of high data and the expectations from the Ashman & Zepf merger SN galaxies are not seen. In fact the surface density flattens model are still substantial. In particular, the model cannot off in log space giving a GC system “core” (e.g., Grillmair explain the large numbers of excess GCs present in high et al. 1986; Lauer & Kormendy 1986; Grillmair et al. 5^ cD galaxies and appears to be in conflict with GC trends 1994a). The size of this core is larger in more luminous in normal SN giant ellipticals. galaxies and this does not appear to be due to destruction An open question is whether the Ashman & Zepf (1992) effects over time (Forbes et al. 1996b). model can explain the origin of GCs in low luminosity ellip- • Multiple mergers will not produce distinct metallicity ticals. It is currently not clear whether such galaxies are peaks. Based solely in luminosity terms, roughly 10 L* gal- smaller versions of gEs or if they form a distinct population axies would be required to form a cD galaxy or the more (e.g., Carollo et al. 1997). Using the list of Kissler-Patig massive ellipticals, from multiple mergers. In general these (1996), we find ellipticals with - 19.5>My> — 21.0 have = galaxies will have slightly different GC mean metallicities 5At 4.6±0.7. Whitmore et al. (1997) show that in the best- and the gas (from which new GCs form) should be enriched studied merging galaxies the number of new GCs created is by varying amounts. Therefore, unless all of the merging equal to or less than the number of original GCs from the galaxies are similar, we would expect a broad, top-hat type progenitor spirals. This means that the SN values, after 15 distribution covering a range of metallicity, but instead dis- Gyrs, are —2-3. Globular clusters need to form with much tinct metallicity peaks are seen. In the case of NGC 4472 higher efficiency in a gaseous merger if the SN values of (although not a cD it has a similar luminosity to a cD galaxy) merger remnants are to match those of elliptical galaxies. So the metal-rich and metal-poor peaks have a similar disper- the argument that there are too many GCs in ellipticals to be sion when fitted with a Gaussian. explained by mergers (van den Bergh 1984) may still be • Galaxies with the same luminosity should have similar valid. If the GC systems of low luminosity ellipticals are not SN values. If galaxies are made from multiple mergers, why the result of a merger, then currently merging galaxies must do some apparently create GCs much more efficiently than form a galaxy with characteristics different from today’s el- others? For example, NGC 4486 (M87) and NGC 4472 lipticals (see also van den Bergh 1995).

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1997AJ 113.1652F 5 -1 ronment. ThissuggestsasimilarGCformationmechanism rell etal(1996)foundthatGCswithM^IOMhavea Warren 1997).Thisis,however,unlikelytobethesourceof known correlationsofGCswiththeirparentgalaxy(e.g.,the hancements outsideofgalaxies,itwouldnotaccountforthe Even ifamechanismcouldcreateGCsinlocaldensityen- ping. CoolingflowGCformationhasbeendiscussedabove. favor oftheirintraclusterglobularcluster(IGC)hypothesis. ture (oncecorrectedforthedistanceofgalaxyfrom have beenidentifiedinseveralgalaxieswithoutcooling bular clustersseeninNGC1275weretheresultofM that coolsandcondensesfromcluster-widehotgas(Fabian resent afewSearle-Zinnfragments whereaslargegalaxies ris &Pudritz1994).Inthiscase adwarfellipticalmayrep- Zinn (1978)asthebuildingblocks ofgalaxies(seealsoHar- The power-lawisidenticaltothatforgiantmolecularclouds for allgalaxies,includingdwarfs,giantsandcDgalaxies. markably independentofgalaxytype,luminosity,andenvi- power-law massdistributionofthesameform,whichisre- with possiblyasmallcontributionfromothersources.Dur- in situformationformostoftheGCsgEandcDgalaxies, the entire~10,000excessGCsrequiredinhighSgalaxies cooling flows,localdensityenhancementsandtidalstrip- They suggestedthreepossiblesourcesfortheseIGCs— trend forthenumberofGCstocorrelatewithX-raytempera- cluster ofgalaxiesratherthanwithanyindividualgalaxy.A works forfurtherdetails. enon.” SimilarviewswereputforthbyGrillmairetal. that coolingflowsareresponsibleforthehighSphenom- GC formationandconcludedthat“itseemsmostunlikely et al.1984).Richeral(1993)proposedthattheprotoglo- clusters. ThelargestGMCsmay havesimilarmassesand et al.1994;Harris1995;Durrell1996)advocate discuss tidalstrippingbelow. and metallicityisstillanimportantconstraint.Wefurther discoveries ofintergalacticplanetarynebulae(Theuns& GCs maybemoreviable,especiallyinlightoftherecent GC metallicity-galaxyluminosityrelation).Tidallystripped dynamical centerofthecluster)wastakenasevidencein of GCsthatareassociatedwiththepotentialwell flows (e.g.,Whitmoreetal.1993,1997;Schweizer densities totheprimordialfragments proposedbySearle& GMCs areassociatedwiththeformationofprotoglobular galaxy NGC3921,Schweizeretal.(1996)concludedthat are madeupof numerous suchfragments. (1994b) andHarrisetal.(1995).Wereferthereadertothese (GMCs) inourGalaxy.Inastudyofthecurrentlymerging ~300Af yrcoolingflow.Subsequently,protoglobulars 1659 FORBESETAL/.THEORIGINOFGLOBULARCLUSTERS 1996). Bridgesetal.(1996a)havediscussedcoolingflow 0 N N o West etal.(1995)proposedthatthereexistsapopulation It hasbeensuggestedthatGCscouldforminthematerial Harris andco-workers(e.g.,1991;McLaughlin © American Astronomical Society • Provided by theNASA Astrophysics Data System 3.3 IntraclusterGlobularClusters 3.4 InSituFormation 3.2 CoolingFlows 5 n m -1 know, atleastfortheGCmetallicity-galaxyluminosityrela- relations betweentheGCsystemandparentgalaxysug- that theGCsystemstendtobemorespatiallyextendedthan required toformthe“excess”~10,000GCsseen(assuming rotation dependingonthedegreeofdissipationduring regions ofNGC1399(whichwouldbedominatedbythe dicated bythenowwell-establishedbimodalcolordistribu- explain thepresenceoftwodistinctGCspopulations(asin- modification tothestandardcollapsepictureisrequired lated, ifatall,withgalaxyluminosity.Thisrelaxesthecon- tion (Sec.2.2),thatthemetal-poorGCsarepoorlycorre- curred justbeforethemaincollapsephase.Butwenow gest thattheepochofthis“pre-galaxy”GCformationoc- the galaxyfieldstars(Harris1991).Thevariousknowncor- the starlightsuggestthatGCsformedbeforebulkof the meanthanunderlyingstarlightatagivenradiusand galaxies, theobservationsthatGCsaremoremetal-poorin galaxy itselfwasinamostlygaseousphase.Inearly-type GCs incDgalaxieswereformedatanearlyepochwhenthe each GChasamassof10M).Thusapparentlymostthe Zepf 1992;Larson1993)about10Moofgaswouldbe is thatwithaGCformationefficiencyof^1%(Ashman& the localambientdensityishigh, alargerfractionofthe whereas thedynamicalcollapse timevariesas^p~.Soif the numberofpre-galaxyGCs may dependonlocaldensity. than metal-richones.Thephysical mechanismforregulating that highSgalaxieshaverelativelymoremetal-poorGCs evidence tosupportthisideaisshowninFig.3whichshows with ahigherfractionofpre-galaxy(metal-poor)GCs.Some initial phase.ThushighSgalaxiesmaysimplybethose value associatedwiththemasveryfewfieldstarsforminthe collapse andthepresenceoftorques. GCs formedinthesecond(galaxy)phasemayshowsome ity dispersionsthanthegalaxystarsinouterregions the starsinmainbodyofNGC1399andwithnomea- metal-poor ones)hadamuchhighervelocitydispersionthan mair etal.(1994b)foundthatasampleofGCsintheouter the pre-galaxyGCstohaveanearlysphericaldistribution, to bimodalGCmetallicitydistributions.Wewouldexpect tion indifferentmetallicitygascouldbeexpectedtogiverise vast majorityoffieldstarsalsoform.Suchepisodicforma- to producethemetal-richGCs.Inthis“galaxy”phase ther collapse.Asecondepisodeofstarformationisrequired then theremainingmetal-enrichedgasshouldundergofur- field starswillform,enrichingtheprotogalacticgas.Ifonly lapse. we discussourviewforGCformationinamultiphasecol- tions) whichrulesoutasimplemonolithiccollapse.Below initial materialturns intoGCsleavinglessgasfor thesecond In generalthestarformation time scalevariesas^p, a smallfractionofthetotalgasisusedinthisinitialphase, straint ontheepochofformationfortheseGCs.Afurther surable rotation.InNGC4486,GCsalsohavehigherveloc- sion. Thereissomeevidencetosupportthisprediction.Grill- show littleornorotationandhavealargevelocitydisper- (Huchra &Brodie1987;Mouldetal.1990).Themetal-rich o N N For cDgalaxies,anargumentinfavorofsituformation We alsoexpectthepre-galaxyGCstohaveahighS At earlytimes,low-metallicitypre-galaxyGCsandsome N 1659 1997AJ 113.1652F We speculatethatthediskGCs inspiralsrepresentathird halo atmuchthesametime,and maythereforebethespiral Way haloGCs(Chaboyeretal1992)indicatesthatthe be veryinterestingifitcouldextendedtoincludelarge galaxies). galaxy analogtothemetal-rich GCpopulationinellipticals. This indicatesthathalostarsand GCswereformedinthe metallicities similartothoseofhalostars(Carney1993). collapse pictureabove,thehaloGCshavekinematicsand with thediskandhalocomponents.Withreferenceto general, therearetwodistinctpopulationsofGCsassociated Gyrs agoandthesecondphase~5later. this casethefirstepochofstarformationoccurredabout12 cal GroupCarinagalaxybySmecker-Haneetal(1996).In tended dormantphasecomesfromarecentstudyoftheLo- metal-rich GCsformed~4Gyrsafterthemetal-poorones. [Fe/H] =—0.2,thentheage-metallicityrelationforMilky mean of[Fe/H]=—1.2andmetal-richoneswitha process andthelengthofdormantphase.Ifwecrudely would suggestsomeunderlyinguniformityintheformation most largegalaxies.Ifconfirmedbylargersamples,this tween thetwoGCpopulationsisabout1dexin[Fe/H]for galaxies. Itisinterestingthatthemetallicitydifferencebe- phase. Theirmodelwasappliedtosmallgalaxies;itwould first phase,willonlypartiallycoolbeforebeingre-heatedby propose thatthehotgasinitiallyheatedbySNetypeIIin quent starformationmaybestalledbyseveralGyrs.They model inwhich,afterthefirststarformationphase,subse- cently, Burkert&Ruiz-Lapuente(1997)haveproposeda turning itonagainatsomelaterstageinthecollapse.Re- lack ofawell-understoodmechanismforinitiatingGC(and the massofprotocloudisnotamajorfactor. not findastrongtrendforthenumberofmetal-poorGCs the totalmassofprotogalacticcloud.However,wedo possibility, isthatthenumberofpre-galaxyGCsdependson those withmorelocallydenseand/orclumpygas.Another Some evidenceforearlystarformationfollowedbyanex- assume thatalllargegalaxieshavemetal-poorGCswitha episode ofGCformation.HighSgalaxiescouldsimplybe stage ofthecollapseprocess (whichisabsentingE SNe typela.Thisre-heatingdelaysthesecondstarformation star) formationinthepre-galaxyphase,turningitoffand (see Table1)toscalewithgalaxyluminosity,suggestingthat 1660 N In theMilkyWay,andbyinferencespiralgalaxiesin Perhaps themainproblemwiththiscollapsepictureis © American Astronomical Society • Provided by theNASA Astrophysics Data System FORBES ETAL.:THEORIGINOFGLOBULARCLUSTERS -1 primary inkpc;(5)Radialvelocitydifferencefromkms;(6)ObservedabsoluteVmagnitude;(7) column 8. Central velocitydispersion;(8)PredictedVmagnitudefromcolumn7;(9)GCmetallicity (1) Galaxyname;(2)type;(3)DistancemodulusfromHarris(1996);(4)Projectedseparation N5846A cE232.36380-19.22.230-20.5±0.5-0.8±0.3 N4486B cEO31.070200-17.72.301-21.0±0.5-0.7±0.3 N1404 El31.045500-21.12.353-21.0±0.5-0.7±0.3 Galaxy Type(m-M)(AR)(AV)Mlogo-[Fe/H] vpredictpredict (1) (2)(3)(4)(5)(6)(7)(8)(9) Table 3.Tidallystrippedcandidates. Notes toTable3 NGC 5846A(cE2)near5846inacompactgroup. Fornax; NGC4486B(cEO)near4486inVirgo,and bors. Theyare:NGC1404(El)locatednear1399in 4472, theglobal5^is5.5,whereaslocalSexceeds30 modeled inasimplewaybyMuzzioandcollaborators(e.g., timates oftheuncertainty, aregiveninTable 3.Thepre- luminosity ofthegalaxyandGC metallicity,alongwithes- metallicity ofthetidallystripped GCs.Thepredictedoriginal lation fromForbesetal.(1996b) wecanestimatethemean largely unaffectedbytidalstrippingwecanestimatethe pact secondarygalaxy.Asthecentralvelocitydispersionis idea issupportedbythesimulationsofAguliar&White Faber (1973)suggestedthatthetwocompactellipticalswere undergone atidalinteractionwiththeirmoremassiveneigh- envelope oftheprimarygalaxy(Hausman&Ostriker1978). Fully-accreted galaxieswillhavetheirorbitalenergycon- S value,similartothatofthecDenvelopeM87(S tric radius(e.g.,Forbesetal.1996a).InthecaseofNGC ies tendtobemoreextendedradiallythantheunderlying important pointtobemade.TheGCsinatleastsomegalax- total numberofGCsisincreased,theSvalueunaffected be removedinthesameproportionssothat,although tured asignificantnumberofGCsfromotherVirgocluster Forte etal.(1982)proposedthatNGC4486(M87)hadcap- thors anddiscussedbyvandenBergh(1990).Forexample, original luminosityofthecompaniongalaxyfromL actually tightlyboundcoresoftidallystrippedgalaxies.This verted intointernalheat,whichactsto“puff-up”theouter galaxy couldbeaccretedbyitsmoremassiveneighbor. only theoutermostGCsandifstrongenough,whole forward againstthishypothesisisthatGCsandstarlightwill Muzzio etal.1984;1987).Themainargumentput galaxies. TheexchangeofGCsbetweengalaxieshasbeen galaxy bytidalstrippinghasbeensuggestedseveralau- stripped fromtheouterpartsofagalaxymayhavehigh at 90kpc(McLaughlinetal.1994).ThusGCstidally (1986) whoshowthattidalinteractionresultsinamorecom- starlight sothatthelocalSvalueincreaseswithgalactocen- — 25).Tidaleffectswould,intheirweakestform,remove (van denBergh1984;Harris1991).However,thereisan 1996). Furthermore,usingtheGC metallicity-luniinosityre- — crscalingrelationofnormal ellipticals (e.g.,Faberetal. N N N N The transferofGCsfromsmallgalaxiestoahighS In Table3welistthreenearbygalaxiesthatmayhave N 3.5 TidalStripping 1660 1997AJ 113.1652F r Washington photometry,isclaimedtohavethreepeaksat For My=-21.08,weestimatethatNGC1404hadanorigi- the weightedaverageoftwoground-basedstudies.Ifwe the oppositesidetoNGC1404(allowingsubtractionofGCs radial samplingsofthethreestudies. Whatevertheexplana- This possibilityisexploredfurtherinSec.4below. metallicity isestimatedtobe40%ofthetotalnumberGCs metallicity peakat[Fe/H]—-0.8(Ostrovetal.1993),as lost —730GCswithameanmetallicityof[Fe/H]=—0.7 nal GCpopulationof1350±270.SoNGC1404mayhave with S=5±1,thenwecanestimatethenumberofmissing will adoptthisvalue(asgiveninTable1)whichiscloseto offers amorereliableestimateofthenumberGCsandwe from foregroundstarsandbackgroundgalaxies,aback- over ground-basedstudiesofverylowcontaminationrates calculated thenumbertobeA=620±130(5^=2.3 Hanes &Harris(1986)fromground-basedstudies.Fora NGC 1404havebeenmadebyRichtleretal.(1992)and 4486B, andNGC5846A,respectively. predicted metallicitiesoftheacquiredGCs(aslistedinTable massive galaxy.Figure5showsthemetallicitydistribution these GCscanbeidentifiedbytheirmetallicityinthemore ping hasoccurreditremovedverylittlegalaxystarlight hand, NGC1404hasanobservedluminositythatisconsis- in NGC1399havebeentidallystrippedfrom1404. large fractionoftheintermediatemetallicitypopulationGCs GCs. Thus: assume thatNGC1404oncehada“normal”GCpopulation associated withNGC1399).WebelievethattheHSTdata ground fieldandapointing—10areminfromNGC1399on tively. MostrecentlyForbesetal.(1997b),usingHSTdata, distance modulusof31.0,theyfoundN=880±120(5^ [Fe/H] =-0.7±0.3.EstimatesofthenumberGCsin of GCsinNGC1399,4486,and5846.Wealsoshowthe tent withitsvelocitydispersion,suggestingthatiftidalstrip- of GCswith[Fe/H] 0.8,whichinturncould bethose not crucialtoouranalysis,only that thereexistsomefraction tion, thepresenceofanintermediate metallicitypopulationis colors. Geisleretal.(1996)discussed thispossiblecontra- veal onlythemetal-poorandmetal-richpeaksfromV—I However, wenotethatcentral(Whitmoreetal.1995)and [Fe/H] =—1.35,—0.8,and—0.1(Lee&Geisler1993). 3) fromthecompaniongalaxies,i.e.,NGC1404, axies hasbeenremovedviatidalstripping,thenperhaps dieted luminosity,suggeststhatNGC4486Bhaslostabout diction andsuggestedthatitcould beduetothedifferent shown inFig.5.ThenumberofGCswiththisintermediate slightly off-center(Elson&Santiago1996)HSTimagesre- (see Table1),i.e.,about2100±700.Itisplausiblethata (but itmaystillhavestrippedoffouterGCs). — 95%ofitsstarsandNGC5846A—70%.Ontheother ±0.3. Ontheotherhand,NGC1399revealsanintermediate ±0.4). TheForbesetal.(1997b)datahadtheadvantages 1661 FORBESETAL.:THEORIGINOFGLOBULARCLUSTERS = 3.2±0.4)andA^gc^190±80(5^=0.7±0.3),respec- n GC GC _04(Mv+15) The GCmetallicitydistributioninNGC4486,from For NGC1404,thepredictedGCmeanmetallicityis If somefractionoftheGCsystemcompaniongal- M .=5x10-A/'(2) © American Astronomical Society • Provided by theNASA Astrophysics Data System 10 HST imaging.UsingtheHarrisvalue,thereareabout780 broad peaks,themetal-poorpeakrangingfrom—1.3 4486B tohaveanoriginalpopulationof1250±250GCs. M=-21.45 and -21.08,L=3.2and2.3X10 L for respectively. Wewillassumethat Disequaltothecurrent Forbes etal.(1997a)estimateatotalof4670±1270from is estimatedtobe3120±1850(Harris1996).Wenotethat NGC 5846AGCs,predictedfromitsoriginalluminosity,is may havealsocontributedGCstoNGC4486. fraction oftheintermediatemetallicityGCs.Othergalaxies S valueof5±1andM=—21.0,wewouldexpectNGC mediate metallicityisabout40%or5200±200.Foranormal Lee &Geisler(1993)thatthefractionofGCsatthisinter- physical separationisquiteprobably largerthan45kpc).For pericenter maybesomewhatsmaller, andthatthecurrent projected separationof9.7'or 45 kpc(notethattheorbital NGC 1399.ThetidalradiusRinkpcisgivenby radius beyondwhichmaterialhasbeentidallystrippedby NGC 1399whichhasS=13±4. tidally strippedofGCs,whichhavesincebeenacquiredby centered intheclusterX-rayemissionwhichextendsoutto Virgo cluster.AsketchoftheinnerregionsFornax is morecompactandperhapsdynamicallyolderthanthe relatively nearbyat16Mpcandwell-studied.Furthermoreit to searchfortheeffectsoftidalstripping.Theclusteritselfis time inhighvelocitydispersionclustersisshorter,individual understood inthecontextoftidalstripping.Ascrossing tributed tothemetal-poorpeakinNGC5846. Thus tidallystrippedGCsfromNGC5846Amayhavecon- overall GCsysteminNGC4486butmayrepresentsome Thus NGC4486Bhasnotcontributedsignificantlytothe where Disthedistanceofclosestapproach,andmasses crude estimateofthetidalradiusinNGC1404,i.e., tical NGC1404lieswithintheX-rayenvelopeof1399 cluster isshowninFig.6.ThecDgalaxy,NGC1399, increasing theoccurrenceofinteractionsaccordingly. as theestimateoforiginalGCpopulationinNGC5846A. [Fe/H]= -0.8±0.3.ThetotalnumberofGCsinNGC5846 of thestrippedGCsfromNGC4486Bis[Fe/H]=-0.7 of NGC1399and1404 are denotedbyMand, and appearstohaveanabnormallylowSvalueof2.3 at least38'or175kpc(Beebeetal.1996).Thenearbyellip- galaxies canpassclosetotheclustercentermanytimes, axy densityforclustergalaxies,andnotedthatthiscanbe stripped fromNGC4486B.Thepredictedmeanmetallicity < [Fe/H]<-0.9(Forbesetal.1997a).Themetallicityof 13,000±500 (Harris1996).Weestimatefromthedataof ±0.4. InSec.4.5wenotedthatNGC1404mayhavebeen ±463 metal-poorGCs.Thiscompareswith800±150GCs ±0.3. ThetotalnumberofGCsintheNGC4486systemis v y o Nv N x2 N lB The GCmetallicitydistributioninNGC5846hastwo Following theargumentsofFaber(1973)wecanmakea The Fornaxclusterofgalaxiesisagoodregioninwhich Blakeslee (1996)foundacorrelationof5^withlocalgal- R =D{’i.5MIM)~,(3) l2 4. ACASESTUDY:THEFORNAXCLUSTER 1661 1997AJ 113.1652F -1 higher thanthestellarvelocitydispersionmeasuredinNGC locity dispersionofnearly400kms.Thisvalueismuch Patig etalwe fist theirSvalues,luminosities, predicted magnitudes fromFaberetal(1989) andNfromKissler- by Kissler-Patigetal(1996).Using (m-M)=31.0,Vband of severalFornaxclustergalaxies havebeenstudiedrecently contribute totheGCsystemof NGC 1399?TheGCsystems nax clusterbeentidallystripped ofGCswhichmayalso galaxies. contributes toboththeGCsystemandstellarenvelopeofcD GC resultandlendingsupporttotheideathattidalstripping nebulae atdistances—25kpc,increasingconfidenceinthe NGC 1399)ofitsparentgalaxy.Amaboldietal(1994)find or allofthespecificangularmomentum(withrespectto within thedarkmatterhaloofNGC1399,sincematerial ies isnaturallyconsistentwithatidalstrippingscenario dispersion measuredforgalaxiesoftheFornaxcluster.The mair etal(1994b)foundthatasampleof50GCsinNGC the expectationsfromtidalstrippinghypothesis.Grill- the outerGCsisaplausiblemechanismtoexplainab- to thatestimatedaboveandsuggeststidalstrippingof galactocentric distanceofabout18kpc.Thisradiusissimilar estimate thattheGCsystemofNGC1404terminatesata that ofNGC1404,thenthetidalradiusbecomesR ics thatM/L~50-100M/LforNGC1399(Grillmair There isevidencefromX-rayobservationsandGCkinemat- have thesameMILratiothentidalradiusR=15kpc. NGC 1399and1404,respectively.Ifbothgalaxies ping. mediate metallicityglobularclustersmayhavebeenacquiredbytidalstrip- have beenconvertedintometallicity.Theexpectedmeanmetallicityoftid- Fig. 5.Globularclustermetallicitydistributioninthreecentraldominant a remarkablysimilarvelocitydispersionamongtheplanetary similarities invelocitydispersionsofGCsandFornaxgalax- et al1994b).IfNGC1399hasaMILsome10-20times 5846A aremarkedbeloweachdistribution.Incasesomeoftheinter- ally strippedglobularclustersfromNGC1404,4486B,and Giesler etal(1996)forNGC4486andForbes(1997a)5846 galaxies. ThephotometricdatafromOstrovetal(1993)forNGC1399, stripped fromapassingorinfallinggalaxywillretainmost sence ofGCsbeyond18kpc. — 10kpc.Richtleretal(1992)andForbes(1997b) 1399 atsomewhatsmallerradii,butissimilartothevelocity 1399 (withameangalactocentricradius—40kpc)hadve- 1662 N GC o Have anyothergalaxies,besidesNGC1404,intheFor- Globular clustervelocitydispersionsalsoappeartomatch © American Astronomical Society • Provided by theNASA Astrophysics Data System FORBES ETAL.:THEORIGINOFGLOBULARCLUSTERS ■2-10 1-2-10 boring galaxiesandamuchsmaller cDenvelopethanNGC the twocDgalaxies. in thenumberofaccretedgalaxies (andtheirGCs)between have ahighermeanvelocitydispersionthanthegalaxystars Fornax galaxies.Thelattergalaxies(NGC1427,1374,1379, to normalSgalaxieshaveawiderangeofGCradialslopes becomes truncatedandtheGCsurfacedensityslopemay Fig. 6.SchematicsketchofthegalaxiesincentralregionsFornax dynamical youthoftheVirgocluster willleadtodifferences are alsosomenoteabledifferencesbetweenNGC4486 In Virgo,thecentercD(NGC4486)isalsoembeddedwithin ing starlightwithintheerrors,whereasNGC1399,hasaGC measured theradialslopesforNGC1399andfourother will tendtoremovetheoutermostGCssothatGCsystem tended thanthestarlightwithaflatterslope,tidalstripping parison oftheGCsurfacedensityslopeandthatun- Fornax galaxieshaveundergonesometidalstripping. the trendwithprojecteddistancemayindicatethatseveral mean GCmetallicitiesandprojecteddistancesfromNGC cluster. TheX-rayemissionextendsfromthecDgalaxy(NGC1399)outto an extensiveX-rayenvelope.TheGCsintheouterregions and 1387)haveGCslopesthatarethesameasunderly- and starlightradialprofiles.Kissler-Patigetal(1996)have and presumablyawiderangeinthedifferencebetweenGC derlying starlight.InitiallyaGCsystemmaybemoreex- errors areconsistentwithaconstantSvalue.Nevertheless with distancefromtheclustercenter,althoughformal (Huchra &Brodie1987;Mouldetal1990).However,there similarities betweentheFornaxclusterandVirgocluster. slope thatisnoticeablyflatterthanthestarlight. approach thatofthestellarprofile.Figure2showslow at least38'. (M87) andNGC1399.4486 hasfarfewernearneigh- 1399. Thiscombinedwiththehigher velocitydispersionand 1399 inTable4.4seemstoindicatethatSincreases N N N In thecontextofabovediscussion,therearemany Further evidencefortidalstrippingcomesfromacom- The FornaxCluster NGC 1404 % NGC 1389 % NGC 1386 I • NGC1374 • NGC1375 • NGC1373 1662 1997AJ 113.1652F radial abundancegradientintheGCsystemfromthisphase. rich). Theywillhavesomerotation dependingonthedegree have essentiallythesamemetallicity (i.e.,relativelymetal- maining gas.Inthesecondphase,aftergascloudhas Both thefieldstarsandGCsprovideenrichmentforre- First, intheearlystagesofcollapse,thereoccursrapid phases) eachofwhichmayhaveassociatedGCformation. collapse process(whichwecallpre-galaxy,galaxyanddisk merging ofmanysmallsubunits(Searle&Zinn1978;Katz cooling athighermetallicities).BothfieldstarsandGCswill over starformationinGCs(possiblyduetomoreefficient undergone furthercollapse,fieldstarformationispreferred dispersion, aredistributedthroughoutthecloudvolumeand tationally boundclumps.TheseGCshaveahighvelocity GCs whichformedfromthechaoticmergingofdense,gravi- converted intostars,andmostofthesestarsarelocatedin galaxies, contrarytothepredictionsofAshman&Zepf merger modelbutinsteadhavemoremetal-poorones.This tail. Inparticular,theGCsystemsofhighSgalaxiesdonot match themergermodelexpectationswhenexaminedinde- Zepf 1992),althoughgenerallyconsistentwithcurrently radial distributionsthatmatchthestarlightandhighve- Evidence forthisincludescurrentlylowSvalues,steepGC population ofGCsthatmayhavebeenacquiredviatidal Thus thesmalldisks(e.g.,Scorza &Bender1996)andpeaky ally settleintoadisk-likestructure nearthegalaxycenter. of dissipationinthecollapse.Unconsumed gaswilleventu- are metal-poor.TheratioofstarsinGCstofieldislarge means thatGCmetallicitygradientsaresteeperinhighS have moremetal-richGCsaswouldbeexpectedinthe tional dataforGCsinthemassiveearly-typegalaxies.We merging galaxies,doesnotadequatelyexplaintheobserva- locity dispersionintheouterNGC1399GCs. tral cD(NGC1399)mayhavebeentidallystrippedofGCs. in somedetailandsuggestthatseveralgalaxiesnearthecen- velopment ofthecDenvelope.IncasesNGC1399 S valueandmayhaveplayedanimportantroleinthede- from GCsthathavebeentidallystrippedneighboring most GCswereformedinsitutwostarformationepi- early-type galaxies,wehavecometotheconclusionthat star formation.Onlyasmallfractionoftheavailablegasis find thatthebimodalGCmetallicitydistributionsdonot and M87wehaveidentifiedanintermediatemetallicity galaxies. Thisaccretedpopulationshouldhaveahighlocal stellar distributions(e.g.,Forbes etal.1995)seeninlow (i.e., thespecificfrequencyishigh).Therewillbelittleorno (see, forexample,Eggenetal.1962;Larson1975;Gott (1992). small galaxiesandtheirGCs.WediscusstheFornaxcluster stripping. Toamuchlesserextentallgalaxiesareaccreting sodes. 1992). Wespeculatethattherearethreemainphasesinthe 1977; Sandage1990)thatprobablyinvolvessomechaotic 1663 FORBESETAL.:THEORIGINOFGLOBULARCLUSTERS N N N N We favoranearlycollapsepictureforgalaxyformation The gaseousmergermodelforGCformation(Ashman& In cDgalaxiestheremaybeanadditionalcontribution After reviewingthecurrentideasonGCsinmassive © American Astronomical Society • Provided by theNASA Astrophysics Data System 5. SUMMARY,SPECULATIONS,ANDPREDICTIONS pend largelyontheinitialconditionsofprotogalactic properties fromlargetosmallellipticalsmayreflectase- luminosity ellipticalsthanthe giants. Sometentativeevi- be slowlyrotatingsystems,with morerotationseeninlow be nearsphericallydistributed. matics asafunctionofmetallicity. Themetal-poorGCswill large samples,willhopefullybeabletodetermineGCkine- their originiseitherfromthepre-galaxyphaseortidal form non-rotating,hotsystems(highvelocitydispersion)if the extremeendofthisprocess,andwhenprotoelliptical more metal-poorGCswithahighSvalueareproduced.At bient densityanddumpinessoftheprotoellipticalincreases, richment levelsofthesecondphasecollapsearereduced have virtuallynopre-galaxystarformation,sothattheen- with thisscenarioisthelackofdetailedmechanismforcre- quence ofdecreasinggas-to-starconversionefficiency(see metallicity fromcolumn4. in kpc;(4)AbsoluteVmagnitude;(5)Specificfrequency;(6)PredictedGC dence forGCrotationcomes from Huietal.(1995)who gests severaltestablepredictions.Theyinclude: of cDgalaxiesmayofferthe“bestbet”forsolving contribution fromaccretion/tidalstrippingintheouterparts is locatedclosetothegravitationalcenterofagalaxycluster, more clumpyandchaoticthanprotospirals.Astheam- gas cloud.Forexample,protoellipticalsmaybemoredense, of thefinalgalaxyproducedbyaboveprocesswillde- lipticals. Spiralsthengoontoformaprominentdisk(and though theyhavequitedifferentmetallicities,haloGCsin leading toGCsthatareingeneralmoremetal-poor(i.e., with veryinefficientconversionofgasintostars.Spiralswill collapse. also Benderetal.1992).Wenotethatthemaindifficulty collapse inthelessmassivesystems.Thevariationofgalaxy luminosity ellipticalsmayindicateaprolongeddissipative Galaxy and NGC4486(M87)havehighvelocitydispersions.Future stripping. Theouter(metal-poor)GCsinbothNGC1399 a cDgalaxyisformed. associated GCs)inthethirdphaseofcollapse.Thetype [Fe/H]~ —1.5)thanthesecondphaseGCsinellipticals.Al- ating twodistinctphasesofGCformationfromasinglehalo search” (McLaughlinetal.1994).ThisoriginforGCssug- spirals appeartobetheanalogmetal-richGCsinel- (1) Galaxyname;(2)type;(3)ProjectedseparationfromNGC1399 N1427 N1404 N1387 N1379 N1374 “most outstandingprobleminglobularclustersystemre- N (1) We concludethatepisodicinsituGCformationanda • Weexpectthemetal-richGCs inearly-typegalaxiesto • Weexpectthemetal-poorGCsinearly-typegalaxiesto Spiral galaxiesmayrepresenttheextremeofthisprocess Type E3 El E0 El (2) SO Table 4.Fornaxclustergalaxies. (AR) 215 190 140 (3) 45 85 Notes toTable4 -20.2 -21.1 -20.2 -20.3 -20.1 My (4) 4.2±0.8 2.3 ±0.4 2.4±0.5 3.7±0.8 3.2±0.9 (5) S N [Fe/H] — 0.9±0.3 — 0.7±0.3 — 0.9±0.3 — 0.8±0.3 — 0.9±0.3 predict (6) 1663 1997AJ 113.1652F Forte, J.C,Martinez, R.E.,&Muzzio,J.C.1982,AJ,87, 1465 Forbes, D.A.,etal.1997b,inpreparation Forbes, D.A.,Franx,M.,&Illingworth, G.D.1995,AJ,109,1988 Forbes, D.A.,Brodie,J.P.,&Huchra, J.1997a,AJ,(inpress) Forbes, D.A.,Brodie,J.P.,&Huchra, J.1996a,AJ,112,2448 Renting, D.E.B.,Harris,W.E.,Pritchet,C.J.,&Hanes,A.1995,AJ, Fall, S.M.,&Rees,M.J.1985,ApJ,298,18 Faber, S.M.,etal.1996,preprint Faber, S.M.,etal.1989,ApJS,69,763 Forbes, D.A.,Franx,M.,Illingworth,G. 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