1993AJ 106.1354W 1/4 1 x/A Toomre’s (1977)listofelevencandidatedisk-diskmerg- NASA ContractNo.NAS5-26555. the AssociationofUniversitiesforResearch inAstronomy,Inc.,under obtained attheSpaceTelescopeScience Institute,whichisoperatedby as S82)findsthattheluminosityprofilefollowsarlaw, roidal distribution.Schweizer(1982a;hereafterreferredto recent interactioninvolvingtwodisks,theinnerregionof kpc). Whiletheouterregionsshowclearevidenceofa from thecenterofgalaxy,aswellbyacomplicated of whichextendsto~280"(130kpc)projecteddistance clearly revealedbythepresenceoftwolongtidaltails,one than anyothersuchmergingsystem.Itsmergerhistoryis ers, thisgalaxyhasprobablybeenstudiedinmoredetail merger betweentwodiskgalaxies.Originallyincludedin as dotheprofilesofellipticalgalaxies(deVaucouleurs NGC 7252appearstohaverelaxedintoarelativelysphe- set ofripplesandloopsatsmallerradii(15"-70";7-33 BasedonobservationswiththeNASA/ESA HubbleSpaceTelescope, 1354 Astron.J. 106 (4),October1993 0004-6256/93/106(4)/1354/17/$0.90 THE ASTRONOMICALJOURNAL 1953; Kormendy1977).Similar rlightdistributions NGC 7252isoneoftheprototypicalexamplesa © American Astronomical Society • Provided by theNASA Astrophysics Data System =-1 primarily ontheNEside;and(4)arippleisfoundwestside,5"0fromcenter. number ofglobularclustersmayincreaseduringthemergergas-richgalaxies.Thisweakensvanden center ofthegalaxy;(3)dustlanesandveryweakspiralstructureareseenouttoabout9"(4.2kpc), disk galaxieshavefewerglobularclustersperunitluminositythanellipticalsdo.Otherfindingsare:(1) Bergh’s objectionagainstellipticalsbeingformedthroughdiskmergers,basedmainlyonthefactthat collision oftwospiralgalaxies,andthattheyareyoungglobularclusters.Itthereforeappearsthe sizes areallcompatiblewiththehypothesisthattheseobjectsformedwithinlast1Gyrfollowing radius is10pc(fori/o50kmsMpc).Theluminosities,colors,projectedspatialdistribution,and magnitude oftheseobjectsisM=—13mag;themeancolorV—1=0.1andeffective is thediscoveryofapopulationabout40bluepointlikeobjectsinthisgalaxy.Themeanabsolute the HubbleSpaceTelescopearepresented.NGC7252(sometimesreferredtoas‘Atoms-for-Peace” NGC 7252showsasingle,semistellarnucleus;(2)relativelybrightspiralstructureisseenwithin3"5 Galaxy) isaprototypicalexampleofremnanttwomergeddiskgalaxies.Ourmoststrikingresult New, high-resolutionimagesofthecentralregionNGC7252obtainedwithPlanetaryCamera (1.6 kpc)ofthecenter,presumablyformedthroughcontinuedinfallgasintoadiskaround v HUBBLE SPACETELESCOPEDISCOVERYOFCANDIDATEYOUNGGLOBULAR Carnegie InstitutionofWashington,DepartmentTerrestrialMagnetism,5241BroadBranchRoad,NW, 1. INTRODUCTION Space TelescopeScienceInstitute,3700SanMartinDrive,Baltimore,Maryland21218 Space TelescopeScienceInstitute,3700SanMartinDrive,Baltimore,Maryland21218 1 Claus Leitherer,KirkBorne,andCarmelleRobert CLUSTERS INTHEMERGERREMNANTNGC7252 Electronic mail:[email protected],[email protected],[email protected] Received 1993June9;revisedJuly1 Electronic mail:[email protected] Electronic mail:[email protected] Bradley C.Whitmore VOLUME 106,NUMBER4 François Schweizer Washington, DC20015 ABSTRACT be createdduringthemerger of twogas-richspiralgalax- to haveal"-2"holeatitscenter. merger eventintherecentpast.Thisgaseousdiskappears 45% ofthemasswithin5"(2.3kpc)centerisin more detailtothispicture.Wangetal(1992)findthatthe discovery ofacounter-rotatingdiskionizedgaswithin elliptical galaxies.InNGC7252,S82alsoreportedthe that mergersbetweendiskgalaxiesmayeventuallyform with HSTobservationswaswhether globularclusterscan about 30"(14kpc)ofthecenter. Hence,coldgas(HIor H Itracesthetwotidaltailsquite well,butvanisheswithin form ofmoleculargas.Hibbardetal(1993)findthatthe a similar,thoughsmallerholeatitscenter.Anestimated associated withthecentralionized-gasdiskandmayshow CO) appearstobelackingbetween radiiofabout8"-30". CO emissiondiscoveredbyDuprazetal(1990)isclosely disk mergers(Wrightetal1990;Stanford&Bushouse have recentlybeenfoundinmany,thoughnotall,disk- 8" ofthecenter,providingakinematicsignaturemajor 1991) andprovidesomeofthebestobservationalevidence A particularlyimportantquestion wehopedtoaddress Several recentobservationsofNGC7252haveadded © 1993Am.Astron. Soc.1354 OCTOBER 1993 1993AJ 106.1354W 1 -1 induced increaseinthenumberofglobularclusters on ground-basedphotographstakeninexcellentseeing ies. Ahintofthispossibilitywasgivenbythediscovery Zepf &Ashman1993)inanefforttoanswervanden 6-7 unresolvedbluishknotsnearthecenterofNGC7252 cific globularclusterfrequenciescouldnotmergetoform Bergh’s contentionthattwospiralgalaxieswithlowspe- by Holtzmanetal(1992;hereafterreferredtoasH92) an ellipticalwithalargespecificglobularclusterfrequency observations ofNGC3597byLutz(1991)and1275 ground-based observationsofNGC3597haveinsufficient clusters maybeformedduringmergers.However,the have providedtantalizingevidencethatindeedglobular blue pointlikesourcesfoundinNGC1275maybepro- of globularclusters.InthecaseNGC1275,main resolution todeterminewhethertheobjectshavesizes distance of96.6MpcforaHubbleconstant this questionbystudyingtheprototypicalmergerremnant uncertainty iscausedbythepeculiarnatureofgalaxy 1355 WHITMOREETAL.:NGC7252 kms“Mpc“. Thecorrespondingdistancemodulusis this galaxy.Wehopedtomakeacleanerexaminationof duced bythecoolingflowwhichisbelievedtoexistaround For example,Richeretal(1993)havesuggestedthatthe and thefactthatitliesatcenterofPerseuscluster. Group is4828kms(S82),whichplacesthegalaxyata NGC 7252. (S82). Severalstudieshavesuggestedsuchamerger- and 1pixelofthePlanetaryCameracovers20.6pc. am—M= 34.92.Atthisdistance,thescaleis1"=468pc, (Schweizer 1987;BursteinAshman&Zepf1992; were takenthroughtheF555Wfilterandasimilarpair (see vandenBergh1990forarecentsummary).Recent through theF785LPfilter.Thenucleusofgalaxywas flash, anddarkcount;A-to-Dconversion;flatfielddi- centered inchipP6forbothobservations.Followingthe Camera on1992October8.Twoexposuresof700seach vision), theimagesweremultipliedby“deltaflats”tocor- standard pipelineprocessing(i.e.,correctionsforbias,pre- contamination and“measles”).Mostofthereductions rect forsensitivitydifferencesthatdevelopedsincethe were carriedoutusingtheImageReductionandAnalysis flatfield usedinthepipelineprocessingwastaken(e.g., by oneofus(F.S.)tocheckvariouspartstheprimary Facility (IRAF)andtheSpaceTelescopeScienceData performed usingtheVISTApackage(Laueretal1983) Analysis System(STSDAS).Independentanalyseswere by usingthecosmicraystask, followedbyamanualex- amination andthemaskingofa fewmorepixelswhereonly pass toremovethefewremaining coincidenceswasmade ferences incounts,thelower counts areused.Asecond image. Incaseswherecorresponding pixelsshowlargedif- the twoimagesobtainedineach filtertoformanaveraged using thecombinetaskinSTSDAS.Thiscompares analysis. Afirstpasstoremovecosmicrayswasmadeby The systemicvelocityofNGC7252relativetotheLocal NGC 7252wasobservedwithHSTandthePlanetary © American Astronomical Society • Provided by theNASA Astrophysics Data System 2. OBSERVATIONSANDREDUCTIONS were alsomaskedatthisstage,eitherbyinterpolating ishes suchasbadcolumns,spotsonthechips,and one imageappearedtobeenhanced.Othercosmeticblem- across thefeatureifitwasonlyoneortwopixelswide, regions partiallyobscuredbydustonthecamerapyramid from anearbyregion. else byreplacingvalueswithintheartifactwith Johnson passbandF,asevidencedbythefactthatHarris wavelength oftheF555Wfilterissimilartothat set byusingaperturephotometryfromS82.Theeffective 0.7 mag,withverylittledependenceoncolor.Wetherefore etal (1991)find(F555W-F)=0.04magataB—Vof pared withthesixaperturemeasurementsfromS82in integrated luminosityfromtheF555Wimageswascom- tied ourF555WimagesdirectlytotheFpassband.The Johnson passband/,sinceHarrisetal(1991)findthat the WF/PCFinalOrbital/ScienceVerificationReport range r=5"7-13"05.Theagreementwasexcellent,witha reasonably wellwiththepredictionof30.123magbasedon point forourFimagesis30.206mag,whichcompares scatter ofonly0.018magperaperture.Theresultantzero passband system.Insearchingtheliteratureforcalibration magnitudes quotedinthepresentpaperareonJohnson with onlyaweakcolordependence.Therefore,allFand/ more difficult.Wedecidedtotieourobservationsthe were convertedviatheequationF—/=(F—/)j values in/,basedontheKron-Cousinssystem The onlyavailableIphotometryforNGC7252isfrom mean colorforthegalaxyofF—/=1.268±0.072mag. range r=15"6-43'.'3.Thesefourmeasurementsyielda Poulain (1986),withfouraperturemeasurementsinthe values forthescatterindistributionwillbestatedex- plicitly. )Aslightcorrectionwasmadetothismeancolor since wewereabletointegratethefluxonlyoutabout value touseforourregionofintercomparisonisV—I of NGC7252areslightlyredder,whencethecorrected (Westphal 1991). us toexamineothermeansofdeterminingthezeropoint scatter inPoulain’s(1986)photometryofNGC7252led (F785LP—/) =—0.09magataB—Vof0.7mag,again mean colorbasedonPoulain’s photometryandthevalue mean valueofF—R=0.730±0.013magmeasuredby value ofF—/byextrapolatinginwavelengthfromthe for the/images.Onealternativemethodistoestimate couleurs &Longo(1988),we predictavalueofV—I axies ofvaryingcolorstakenfrom thecatalogbydeVau- = 1.30(F—/)—0.013(deVaucouleurs&Longo1988). Schweizer (1982b).Usingthemeanrelation 15" radiusbeforereachingtheedgeofchipP6.Basedon (Note: Throughoutthispaper,thenumberfollowing ± isthestandarddeviationofmean.Whenrelevant, = 1.289mag. V—R photometrybySchweizer(1982b),theinnerregions = 1.334±0.04magisthestraight averageofthecorrected = 1.379±0.044mag.Ourfinal, adoptedvalueofV—I (F—7)j= 1.983(F—R)j—0.069 establishedfrom20gal- KC The photometriczeropointfortheVphotometrywas Establishing azeropointfortheF785LPimageswas The relativelylargeuncertaintyresultingfromthe 1355 1993AJ 106.1354W lapping imagesunnecessary. X and0.26pixelsinT,withascatterofabout0.36 detailed modelingofthePSF and iterativefittingofover- the uncrowdednatureof pointlikeobjectsinNGC that itsticksupwellabovethe background.Inaddition, rated pointspreadfunction(PSF) hassuchasharpcore on pointlikesourcesworksfairlywellbecausetheaber- the V—Icolorswasjudgedtobebest.Despite tively uniform,sothemethodthatgaveleastscatterin VISTA system(objectapertureof2.6pixelsradius,back- the objectandanannulusbetweenradiiof510pixels way wefoundthatusinganapertureof2pixelsradiusfor with aboutadozendifferentaperturesetsinsystematic be critical.Intheinnerregionsstrongluminositygra- 7252 meansthataperturephotometry isadequate,making spherical aberrationofthetelescope,aperturephotometry assumption thatthecolorsofpointsourcesarerela- primary criterionforsuccessusedinthesetestswasthe ground annulusbetweenradiiof4.6to8.6pixels).The clusions werereachedindependentlybyF.S.usingthe as thebackgroundprovidedoptimalresults.Similarcon- counting statisticslimittheaccuracy.Afterexperimenting tures aretoolarge.Iftheaperturessmall,low dient andspiralarmstendtocauseproblemsiftheaper- ration ofthetelescopechoiceaperturesizewould diately apparentthatbecauseoftheseveresphericalaber- formed usingtheaphottaskinIRAF.Itbecameimme- throughput isafunctionoftimeduetochipcontamination this effect. to theImagnitudesofallpointlikeobjectscounteract enough toaffecttheresultingcolorindicesslightly.A cluster candidatesaremuchbrighterinallcases, were usedtodefineaperturecenterssincetheVimagesof combined withthefactthatpositionsfromVimage in eachcoordinate.However,thesesmallremainingshifts, the pointsourcesfortwocolorswereonly0.17pixelsin adequate job,sincethemeanresidualsinpositionsof peaks. Thissimpleconstantshiftappearstohavedonean avoid pixelinterpolationthatwouldhavesmoothedthe passbands. Aconstantintegershiftwasusedinorderto mates. and othereffects,wehavepreferredusingourownesti- report (e.g.,seeSec.12.6ofthereport)andsince parable uncertaintiesintheOrbital/ScienceVerification the valuespresentedhere.However,sincetherearecom- result inV—Icolorswhichwouldbe0.128magbluerthan values fromtheOrbital/ScienceVerificationreportwould small correctionof—0.025maghasthereforebeenapplied mag basedonWestphal(1991).Notethatadoptingthe compares reasonablywellwiththepredictionof28.746 zero pointforthe/imageis28.701mag,whichagain extrapolated fromSchweizers(V—R)j.Theresulting — 1pixelinYtoalignthenucleusofNGC7252both 1356 WHITMOREETAL.:NGC7252 The F785LPimagewasshiftedby+9pixelsinXand Aperture photometryofallpointlikeobjectswasper- 3. APERTUREPHOTOMETRYOFPOINTLIKEOBJECTS © American Astronomical Society • Provided by theNASA Astrophysics Data System jects introducesanegligibleeffect,whencewechosetouse Since thereisnobrightstarin thisfield,anartificialstar using 50iterationsoftheLucy-Richardson algorithm. images inVfromchipP6that havebeendeconvolved the Fimage(F555W)fromfourchipsofPlanetary had beendodgedtohighlightthefaintloopsandtailsin contrasts andslightlydifferent spatialscales.Figure4 gion ofthegalaxyfromP6alone, usingdifferentintensity Camera. ThecenterofthegalaxyisinchipP6.Figure3 observations isoutlinedbythewhitebox.Figure2shows outer region.TheareacoveredbyourPlanetaryCamera 7252 intheblue,takenatCTIO4mbyS82.Thisimage position-dependent corrections. tions is0.11maginV—I,wehavechosentoignoreany bands, whencethescatterincolorindexV—Iisonly bright starobservedat16differentlocationsonchipP6to 0.07 mag;theV—Icolorsshouldbegoodtoabout±0.05 mag. Theaperturecorrectionforthe/imageswasmore made usingObjects3,6,30,and32,doubleweighting rection isrequired.ThecorrectionfortheVimagewas only about0.05mag.Theeffectissimilarinbothpass- magnitudes measuredwithinther=2pixelsapertureis appear tobeinslightlybetterfocus,butthescatter ies acrosschipP6,theeffectonaperturephotometryis mag. total uncertaintyinthezeropointforVmagnitudesis the valueof3.122magasourbestestimate.Theestimated tions fromstarsratherthanslightlybroadenedob- alone. Experimentingwithartificiallybroadenedstars(see reassuringly closetothevaluedeterminedfromObject3 ratio yieldsanaperturecorrectionof3.122±0.07mag, rection of2.795magforVimagestoIwiththis ages shouldbe1.117.Extrapolatingfromouraperturecor- ratio betweentheaperturecorrectionsfor7andVim- determine thecolordependence.Thistestshowedthat ined highS/Ncalibrationobservationsofarelatively yield ausefulestimate(3.151mag).Ascheckweexam- difficult toestimate,sinceonlyObject3isbrightenough Object 3again.Theresultingcorrectionis2.795±0.063 aperture tothetotalmagnitude,asubstantialcor- for theVimage,andof0.163±0.02magIimage. has thebestS/N,wefindcorrectionsof0.093±0.01mag relatively insignificant.Objectsnearthecenterofchip Sec. 4.2.2)showedthatdeterminingtheaperturecorrec- clusters fartherthan5"fromthenucleus(i.e.,Objects3, PSF, abackgroundcorrectionisrequired.Usingsixbright (Plates 56and57)showstwo differentintensitycontrast (Plates 54and55)showstwoblowupsofthecentralre- 16 differentpositionsshowthatwhilethePSFclearlyvar- 10, 15,26,30,32)anddoubleweightingObject3which ±0.03 mag.Sincethetypicaluncertaintyofourobserva- Figure 1showsadeepground-basedimageofNGC To extrapolatefromthemagnitudewithinr=2pixel Further experimentswiththeimagesofsamestarat Because theannulusincludeslightfromextended 4.1 MorphologicalAppearance 4. RESULTS 1356 1993AJ 106.1354W the brightsemistellarnucleusshowsupveryprominently the colorindeximageshowsagreatdealoffinestructure, the colorindexV—I\brighterregionsarebluer.Notethat was usedforthedeconvolution.Figure5showsamap of The whiteboxshowsthepositionandorientationofPlanetaryCameraobservations. major NEloop. the mainbody,andaregion outtonearlytheedgeof nucleus region,thepartof northwesterntailclosestto cover adiamondshapedarea extendingfromabout15" image tobered.Figure7showsamosaicofdeconvolved V produced byassigningtheVimagetobeblueand / as ablueobject.Figure6(Plate58)showscolorimage even thoughnodeconvolutionisinvolved.Inparticular, Fig. 1.Adeepground-basedimageofNGC7252inthebluefromS82.Thehasbeendodgedtohighlightouterloops andtails. the fieldsof otherthreechips,littlecan beseenapart ered bychipP6 whichisnearlycenteredon thenucleus.In south ofthenucleusto—80" north.Thisareacoversthe images withidentificationnumbersfromTable1. generated withtheTinyTimsoftwaredevelopedatSTScl 1357 WHITMOREETAL.:NGC7252 As Fig.1shows,thefourchipsofPlanetaryCamera Most oftheobservablestructure occursinthefieldcov- © American Astronomical Society •Provided bythe NASAAstrophysics Data System can beseenweakly,butaretoofaintfordetectingany faint stars.TheabovementionedNEloopandtheNWtail the pointlikeobjects fartheroutinthegalaxy. These sourcesmaybethe brightest partsofmajorOB nearly unresolvedsourcesdescribed elsewhere(Sec.4.2). several appearcrossedbydust lanes,afewcontainbright, most ofthesesubcondensations areclearlyextendedand Fig. 5,andconsistsofmany subcondensations.Though or fourspiralarms,eachofwhichisbluish,asshown by ing outto~3"5(1.6kpc).Thisstructureconsistsofthree appears lacedwithrelativelybrightspiralstructureextend- discussed morefullyinothersections,thecentralregion objects andthesemistellarnucleus,bothofwhichwill be striking. Apartfromtheprominentscatteringofpointlike detailed structure. from thefewoutlying,bluepointlikesourcesanda associations or,whennearly unresolved,maybesimilarto The structureintheregioncoveredbychipP6ismost A majorpatch ofobscurationoccursabout 2"2NNEof 1357 1993AJ 106.1354W 2 gas distribution (Wangetal.1992). over 5"-7"(2.3-3.3kpc). Interestingly, theedgeofthis the planeofcentraldisk. exposure photographstakeninexcellentseeing(S82). scopically (S82)andwiththe —7"edgeofthemolecular- central spiral-structurearea coincidescloselywiththe8" shaped dustlanes,someof which extenduninterruptedly Both itsappearanceandcolorsuggestthatitmaylieabove kpc), andisjustbarelyvisibleonground-based,short- the center.Thispatchcoversmorethan1sqarcsec(0.22 1358 (3.7 kpc)edgeoftheionized-gas diskdetectedspectro- ~9" (4.2kpc).Muchofitappearstobedefinedbyspiral- At leastonesharp-edged, ripplelikefeature canbeseen Fainter spiral-shapedstructurecanbediscernedoutto © American Astronomical Society •Provided bythe NASAAstrophysics Data System WHITMORE ETAL.:NGC7252 Fig. 2.MosaicofV(F555W)imagesNGC7252fromallfourchips ofthePlanetaryCamera. been expectedinarecentmerger remnant,whereingen- likely tobeanewly formed,secondarydisk ofsettlinggas tation ofthecentral gasdiskalreadysuggested thatitis eral motionstendtobechaotic. However,theorderlyro- the centralionized-andmolecular-gas diskmightnothave weizer &Seitzer1988). ples observedinanumberofearly-typediskgalaxies(Sch- pearance andorientationarequitesimilartothoseofrip- projected lengthofatleast6"(2.8kpc),andhasapitch of atleast60°asseenfromthenucleus,correspondingto a angle oppositetothatofthemainspiralstructure.Itsap- at 4"6(2.2kpc)westofthenucleus[e.g.,betweenObjects 13 and38onFig.7(a)].Thisrippleextendsoveranangle Note thatthestrikingspiral structureassociatedwith 1358 1993AJ 106.1354W 1359 WHITMOREETAL.:NGC7252 ics (Bender&Surma1992, and referencestherein). progenitor ofacounterrotating stellarcore(Schweizer cause ofitshighmolecular-gascontentandstrongHa likely evidenceofcontinuedgaseousinfall(Toomre1990) ellipticals arenowknownto indeedshowdisklikekinemat- emission, thiscounterrotating gaseousdiskisthelikely and starformation.Ithasbeensuggestedbeforethatbe- nature oftherecent accretionofaclump of starsandisa weak counterpart ofthemuchlargerripples seenfurther (S82), andthespiralstructureitsbluecolorarenow 1990; Wangetal1992).The beststudiedsuchcoresin The ripple,ontheotherhand, isanotunexpectedsig- © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 5.AV—IimageofNGC7252fromchipP6.Brighterregions are bluer. by pre-existingellipticals(Quinn 1984;Hernquist&Quinn weizer &Seitzer1988).Suchripplesarenowknownto be galaxies (Hernquist&Spergel 1992). generated notonlyinaccretionsofminordiskcompanions elliptical (Malin&Carter1983)andspiralgalaxies(Sch- out inNGC7252itselfandmanyotherwisenormal Sec. 4.2.2)of thepointlikeobjectsinNGC 7252.These 1988), butalsoinfull-scale mergersofnearlyequaldisk Table 1gives the positions,magnitudes,and radii(see 4.2 PropertiesofthePointlike Objects 4.2.1 Photometry 1359 1993AJ 106.1354W Fig. 7.Identification chartforpointlikeobjectsinNGC 7252fromTable1.(a)Mosaic offourdeconvolvedVimages,(b) Blowupofcentralregion. make itveryunlikelythattherearemorecontaminating beyond adistanceof3"5fromthecenterwhichwillbe stars thanthat. stars amongtheobjectsofinnerand/oroutersamples parent sizes.Whiletheremaybeoneortwoadditional tified byboththeirveryredcolorsandsmallerap- from thecenterwhichwillbecalled“innersample,” and thelikelyfourstars.Thesestarshavebeeniden- called the“outersample,”nonstellarobjectswithin3"5 objects arebrokenintothreegroups:thenonstellar (e.g., Object7),thesizes,colors,andspatialdistribution 1360 WHITMOREETAL.:NGC7252 The nonstellarobjectsoftheinnersamplehavebeen © American Astronomical Society •Provided bythe NASAAstrophysics Data System $57 «36 ©37 .33 based onouroriginalvisualevaluationaswell center ofthegalaxy,takentolieatpixel(411,381)in structure ofthespiralarms. ground, thestrongluminositygradient,andsmall-scale ficult tomeasureaccuratecolorsduetheincreasedback- and (3),theirlocationnearthecentermakesitmoredif- later sizemeasurementsbasedonaperturephotometry; to bemorediffusethantheobjectsofoutersample, different typesofobjects;(2)manytheseobjectsappear objects arelocatedinthespiralarms,andhencemaybe considered separatelyforthreereasons:(1)mostofthese The objectpositionsgiveninTable1arerelativetothe »55 <:STAR> y54 (STAR) ■v 56 1360 1993AJ 106.1354W hms ble 1. magnitude of these objectsisveryhigh:M = —12.9mag. ters ofourown Galaxy(Harris1991).The meanabsolute of theoutersample,ascompared withthebluepointlike chip. Figure7showsthelocations oftheobjectsfromTa- icking ofthefourchipsand thedistortionswithineach termine thesecoordinates,takingintoaccountthemosa- nucleus areRA=222044.852andDec=-24°40'42r03 sources inNGC1275(H92), andwiththeglobularclus- Star Catalog).TheSTSDAStaskMETRICwasusedtode- 1361 WHITMOREETAL.:NGC7252 (J2000 coordinatesinthecoordinateframeofGuide v V image.Thecorrespondingequatorialcoordinatesofthe Figure 8showstheluminosity histogramfortheobjects © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 7.(continued) nonexploding starshaveM~—8mag.Thegreatprepon- has beenapplied inFig.8),andboth distributions are pointlike sourcesinNGC 1275 (Note:the+0.75mag in NGC7252isnearlyidentical tothedistributionof and theoutersamplesareforeground stars. makes itclearthatfew,ifany oftheobjectsinner chips, especiallyintheouter regionsofthegalaxy,also compared tothepaucityofobjectsinfieldsother derance ofpointlikeobjectsseeninthefieldchip P6 rather thanveryluminousstars,sincethebrightestknown, correction mentioned inthe“Noteadded in press”ofH92 This clearlyshowsthattheseobjectsmustbestarclusters v The luminositydistribution of theouter-sampleobjects 1361 1993AJ 106.1354W abc determined bythe equation 7?(pc)=28xAo_based on numericalexperiments(seetext); andCol.(11)istheobservationaluncertainty inÄ. galaxy; Col.(4)isthelengthofradius vectorfromthecenterofgalaxy;Col.(5)istotalFmagnitude; Col. (6)isthetotalImagnitude; magnitude withinther=0.5pixelradius aperture,correctedfortheeffectofPSFhaloinbackgroundannulus, and2.001istheaveragevalueof Col. (1)istheidentificationnumber; Col. (2)istherightascensionfromcenterofgalaxy;(3) the declinationfromcenterof On chipP7.P5. P8. (7) isthe(V—/)colorindex;Col.(8) istheobservationaluncertaintyin(F—/);Col.(9)A_=[F(0.5)— (3)]—2.001,whereF(0.5)istheF 1362 WHITMOREETAL.:NGC7252 F(0.5) —F(3)forthefourstars;Col. (10) istheeffectiveradiusdefinedasforwhichaGaussianprofile wouldcontainhalfthetotallight, eff53 eff 053 © American Astronomical Society • Provided by theNASA Astrophysics Data System Table 1.Positions,magnitudes,colors,andsizesofthepointlikeobjectsinNGC7252. a a a b 6 e c c 3 2 5 4 7 6 9 8 1° 10 (!) (2) 12 11 ID 13 22 21 20 23 17 16 15 14 25 24 27 26 19 18 29 28 30 31 33 32 34 35 37 36 51 49 48 47 46 45 44 43 42 41 50 40 53 52 39 38 57* 55 54 56 -11.37 -12.99 -14.56 -19.23 -23.92 -11.01 -10.40 -8.47 -8.96 -0.90 -1.14 -1.17 -3.14 -5.67 -6.24 -6.56 -7.29 -2.55 -4.54 -1.54 -0.52 -1.28 -0.23 23.19 14.53 RA 14.86 14.59 (") 22.78 11.72 17.38 12.07 0.99 2.54 2.36 2.31 0.67 0.53 1.84 1.65 1.57 1.38 1.04 1.01 0.03 2.04 2.32 2.06 4.52 3.01 2.70 6.92 7.31 6.98 9.08 7.53 9.81 9.40 -16.15 -15.65 40.12 42.7823.03 -11.35 35.86 36.98 -11.98 -13 85 -15.22 -0.27 -2.62 -2.69 -2.40 -2.19 -11.18 -0.93 -2.17 -1.36 -2.08 11.16 25.7320.57 -10.69 DEC -4.68 -0.03 -0.39 -8.56 -7.66 -4.38 -4.89 -6.57 -5.92 -7.71 -6.96 -6.67 -6.29 -7.69 n (3) 0.06 2.61 0.75 2.82 2.50 0.10 14.84 10.48 14.07 1.31 14.25 10.34 18.91 12.21 11.15 3.80 5.04 0.43 9.77 9.37 3.80 0.79 38.72 18.20 39.73 21.16 2.71 3.21 2.68 2.77 3.04 2.82 2.77 2.87 2.66 2.66 2.48 3.14 1.65 1.41 1.66 1.24 26.12 19.72 15.05 19.88 12.29 19.21 17.87 19.45 12.39 15.73 11.53 12.20 14.01 12.21 10.40 15.52 13.15 20.22 26.87 11.05 10.77 10.14 14.85 15.87 12.10 (") (mag)(pc) (4) (5)_ 7.29 8.48 9.88 7.88 5.69 9.78 6.70 5.51 5.92 8.17 9.90 9.85 r VI(V-I)±A.5-3Re// 21.73 21.64 20.24 20.37 20.73 21.46 22.32 21.67 20.50 21.58 22.25 21.83 21.65 20.87 20.47 21.64 Inner Sample:r<3.5" 21.19 22.91 22.31 Outer Sample:r>3.5" 22.77 22.83 21.96 22.92 17.86 22.24 22.76 22.33 19.43 20.69 22.69 22.70 21.99 22.45 23.09 19.81 22.60 21.58 22.50 23.49 22.58 21.97 20.30 22.25 22.94 23.09 20.91 21.33 22.74 22.08 23.12 19.39 23.39 23.22 Notes toTable1 MEAN: 20.76 21.08 20.79 20.72 20.10 20.18 21.18 19.97 19.68 17.86 16.01 17.80 19.13 MEAN: 22.07 21.90 20.41 21.97 21.76 21.99 17.02 21.41 21.75 22.50 21.30 21.23 18.57 22.88 21.90 21.66 18.98 21.25 21.80 21.78 19.74 22.33 20.62 Stars 21.55 21.40 21.19 22.17 22.25 20.30 20.05 19.30 22.43 22.45 21.77 21.18 18.86 ± (6) ± 0.35 0.71 0.94 0.27 0.69 0.71 0.93 0.77 0.28 2.71 2.20 3.36 0.47 1.37 1.96 0.84 0.84 0.41 0.80 0.79 0.86 0.94 0.83 0.55 0.26 0.95 0.94 1.07 0.81 0.73 0.79 0.22 0.70 0.82 1.01 0.97 1.02 0.71 0.84 0.77 0.53 0.76 1.16 (7) (JL 0.61 1.18 0.69 1.00 0.90 0.94 0.25 0.83 1.28 1.45 0.28 0.13 0.08 0.15 0.18 0.01 0.03 0.20 0.26 0.21 0.20 0.33 0.02 0.09 0.16 0.05 0.01 0.15 0.13 0.02 0.14 0.12 0.10 0.10 0.03 0.14 0.28 0.09 0.04 0.33 0.14 0.10 0.12 0.18 0.06 0.12 0.12 0.11 0.03 0.10 0.19 0.08 0.02 0.16 0.05 0.04 0.20 0.12 0.08 0.17 0.11 MEAN: -0.09 -0.04 -0.08 0.38 0.65 0.79 0.71 0.83 0.79 0.86 MEAN: 1.25 1.10 1.20 0.45 0.46 0.40 0.47 0.39 0.69 0.04 ± 0.04 0.35 0.22 0.29 0.29 0.25 0.17 0.15 0.28 0.24 0.36 0.39 0.17 0.26 0.51 0.50 0.30 0.28 0.44 0.48 0.76 (9) 0.27 0.35 0.45 0.03 0.46 0.34 0.31 0.73 ± -1.0 -2.1 30.7 6.0 33.7 23.2 34.9 22.1 24.1 23.9 22.1 10.7 5.0 19.8 18.2 -2.4 7.4 1.0 1.1 (10) (H) 12.8 11.3 12.7 19.4 13.1 11.1 10.9 14.3 10.0 21.2 _M 12.3 14.1 13.5 20.5 12.6 12.9 6.3 4.1 9.9 4.8 8.1 6.6 8.0 7.0 7.8 4.8 7.2 8.3 7.8 7.6 9.8 4.9 9.9 9.6 0.7 6.9 4.9 3.8 6.8 8.6 3.7 0.4 7.8 2.2 574 1.8 1.5 1.0 0.4 4.3 4.1 3.1 0.7 3.5 3.3 1.7 0.8 2.5 2.7 4.5 2.7 3.6 3.6 2.5 3.0 3.4 3.4 1.2 2.1 3.5 3.0 6.4 2.3 4.6 0.7 1.0 4.5 4£ 5.4 2.6 3.2 1.4 1.7 3.4 1362 1993AJ 106.1354W 1363 WHITMOREETAE:NGC7252 lower cutoffsintheextragalacticdistributionsarenotwell tion ofglobularclustersintheMilkyWay.However, butions yieldsadifferencebetweentheclustersofNGC determined, andcouldeasilybedueentirelytothedetec- tion threshold. shifted about6magbrighterthantheluminositydistribu- paring theabsolutemagnitudesofclustersat20thper- overestimate duetothecutoffofNGC7252luminosity in avarietyofways.Simplyusingthemeansdistri- distribution. Abetterestimatemaybeobtainedbycom- 7252 andoftheMilkyWay6.6mag.Thisisclearlyan number inadistributionisdefinedastwicethecumulative centile ofthecumulativedistributions,wheretotal number reachedatthepeakofdistribution(i.e.,this tion). Thisimprovedmethodyieldsaluminositydifference number doesnotusethefainterclustersincalcula- of 5.5mag.Theprincipalshortcomingthisapproachis the likelihoodthatpeakofdistributionforNGC minosity functionofclusters in NGC7252.Untilthisis fainter, makingitpossibleto detect aturnoverinthelu- possible todetectandmeasure objectsupto3-4mag The refurbishedHSTwiththe WFPC2shouldmakeit the estimatedluminositydifferenceisagainanupperlimit. sample ofNGC7252appear to beroughly5mag,ora done, thebestwecansayisthat theclustersofouter 7252 remainsaffectedbythedetectionthreshold,sothat ris 1991). applied basedon“noteaddedinproof”),andtheMilkyWay(Har- of NGC7252,1275(H92;correction0.75maghasbeen Fig. 8.Theluminosityhistogramfortheobjectsinoutersample One canestimatethedifferenceinabsolutemagnitudes © American Astronomical Society • Provided by theNASA Astrophysics Data System M (mag) v the MilkyWay. factor of100,moreluminousthantheglobularclusters the muchbrightercenterofgalaxy. meaningful sincefainterobjectsarehardertodetectnear outer sample.However,thiscomparisonmaynotbevery shown here)is0.7magbrighterthanthatofobjectsthe clusters inNGC7252and1275,forglobular reddening sincetheexternalisestimatedtobe NGC 7252.Mostofthisestimatedreddeningisinternal clusters intheMilkyWay.Areddeningcorrectionof only 0.02mag(Burstein&Heiles1984).Ourestimatefor the internalreddeningisbasedonanextrapolationfrom erence CatalogofBrightGalaxies(=RC3,deVaucouleurs the valueofE(2?—V)=0.06estimatedinThirdRef- bluer onaveragethantheglobular clustersoftheMilky the recentmergereventthat beganroughly1Gyrago et al.1991)usingthereddeninglawfromCardellial nosity suggestsstronglythatthey mayhaveformedduring Way. Theirbluecolorinconjunction withtheirhighlumi- spread incolorforNGC7252 objectsissignificantly —0.12 maginF—/hasbeenappliedfortheclustersof (1989). Asimilarreddeningcorrectionof—0.27magin (S82; Borne&Richstone1991). NoteinFig.9thatthe V—I hasbeenappliedtotheNGC1275datainFig.9. 0.12 and0.27maghavebeenmadetotheNGC72521275 ues), andtheMilkyWay(Harris1991).Reddeningcorrectionsof NGC 7252,1275(H92;extrapolatedfromtheirV—Rval- data, respectively(seetext). FIG. 9.TheV—Idistributionoftheobjectsinoutersample The meanluminosityofobjectstheinnersample(not Figure 9showsthecolordistributionsforcandidate The pointlikeobjectsofNGC7252are0.49±0.07mag 0 .511.52 V-I (mag) 1363 1993AJ 106.1354W with age. ulars, wewouldexpectthecolordistributiontotightenup intrinsic scatterof0.22maginV—LWhilethiscolor 0.11 maginV—Iquadrature;thisyieldsanestimated reddening withinNGC1275,itispossiblethattheobjects mate V—I\ormaybeduetoagreaterrangeofinternal which requiredanextrapolationbyafactorof2.8toesti- the smallerseparationbetweenpassbandsusedbyH92, much ofthelargecolorscatterinNGC1275maybedueto scatter isstillwiderthanobservedamongMilkyWayglob- of NGC1275haveanintrinsicallylargerspreadincolor. 7252 objects,wesubtractthemeanobservationalerrorof smaller thanthatforNGC1275sources(H92).While 1364 WHITMOREETAE:NGC7252 cö tio To estimatetheintrinsicscatterincolorofNGC © American Astronomical Society • Provided by theNASA Astrophysics Data System 20 — 24 16 18 Fig. 11.V—Ivsradiusfromthecenter ofthegalaxy. o 0 Fig. 10.Vvsradiusfromthecenterofgalaxy. " nO I -o % 0 outersample O innersample % • Î*1••* • %••* # • i«A 10 152025 I !1 r (arcsec) I 1 30 effective radiiofabout10pc (Binney &Tremaine1987). whether theseobjectsareglobular clusters,withtypical bright background.However,thetendencyforobjects Figure 13showsthatmostof these objectsareclearlyre- color-magnitude diagrams. age precludesanyeasydetectionofagingeffectsfromsuch of clusterluminositiesatanygivencolorand,presumably, tion histories(Faber1993)suggeststhatthelargespread vector” mightbetakentofavorarelativelynarrowage face value,theabsenceofanysuchcorrelationor“fading also foundbyH92forthesourcesofNGC1275.Takenat sample. Asimilarlackof(V—I)vsMcorrelationwas tween colorandmagnitudeforeithertheouterorinner probably moreseverelyaffectedbyinternalreddeningfrom the objectsinoutersample)differencemean the smallestRininnersampleandmaybesimilarto simulations ofclusterpopulationswithvariousstarforma- among theLMCclusters(Richeretal1993)andinmodel ever, thesimilarlackofapronouncedfadingvectorboth distribution amongthecandidateclusters(H92).How- dust. actually beconsiderablylarger,sincetheinnersampleis colors is—0.20±0.09mag.Thedifferenceincolormay using thefullsamples,ifweremoveObject45(whichhas difference isabarelysignificant—0.12±0.13magwhen of theinnersampletobebluerisprobablyreal.While tion ofthedifficultydetectingfaintobjectsagainsta appear tobeslightlymoreluminous,thisisalikelyreflec- trends foreitherMorV—Itovarywithrtheobjects of theoutersample.Whileobjectsinnersample distance rfromthecenterofgalaxy.Therearenoclear of pointlikeobjectsplottedasfunctionstheprojected F qS v The radiiofthepointlikeobjects mayhelpusdetermine Figure 12showsthatthereisnoclearcorrelationbe- Figures 10and11showabsolutemagnitudescolors Fig. 12.F-/vsV. 4.2.2 Clusterradii V (mag) 1364 1993AJ 106.1354W judged fromthecurvesofgrowthatsmallradii,itisclear compares aperturesof2.0and 4.0 pixelradius,asH92did. tween starsandclustersisgenerally notapparentwhenone estimator. Ascanbeseenfrom Fig.11,thedifferencebe- been resampledbysubdividingeachpixelinto4x4pixels, the curvesofgrowthbyconvolvingGaussiandistributions these clustersmayberoughlyGaussian,wehavecalibrated ciations. egory suchas,e.g.,veryyoungandconcentratedOBasso- ing someevidencethattheymaybelongtoadifferentcat- extended thantheclustersfromoutersample,provid- other. Allclustersbutonefromtheinnersamplearemore the variouschipsarenotradicallydifferentfromeach within ther=3.0pixelaperture providedagoodradius the countswithinr=0.5 pixel aperturefromthose various aperturecombinations, wefoundthatsubtracting lution wasactuallydoneonanimageofthestarthathad try. SincethePSFisundersampledbyPC,convo- ing theconvolveddistributionthroughaperturephotome- of variouswidthswiththePSFStar2andthenmeasur- show similarcurvesofgrowth,suggestingthatthePSFon well beaforegroundstarthathappenstohaveV—I has thesmallest“size”ofallpointlikesourcesandmay to thefourstars.TheexceptionisObject7,whichwefind that allbutoneoftheclustersareresolvedwhencompared using theIRAFtaskblkrep.Afterexperimentingwith color similartotheclusters.Thesolidanddashedlines also randomlychosenclustersfromtheinnersample.As chip P6,andthedottedlinesshowcurvesofgrowthforsix at randomfromtheoutersampleandlyinginfieldof dashed linesshowcurvesofgrowthforsixclusterschosen curves ofgrowthforthesixclustersonouterchips, the fourobjectsjudgedtobestars.Thesolidlinesshow solved. Theasterisksdelineatethe“curveofgrowth”for 1365 objects (7)isclearlyresolvedbasedonther=0.5pixelaperture. outer samplelocatedonchipsP5andP7.Notethatallbutoneofthe typical objectsaredrawnfromtheinnersample,outerand Fig. 13.“Curveofgrowth”forfourstarsandselectedclusters.Six On theassumptionthatlightdistributionwithin © American Astronomical Society • Provided by theNASA Astrophysics Data System WHITMORE ETAL.:NGC7252 Aperture Radius(pixels) -1 similar totheluminosityprofile. Itisinterestingtonote it isclearthattheradialdistribution oftheclustersis of objectsineachbinprecludes averydetailedcomparison, luminosity profileinFfromS82. Whilethesmallnumber distribution oftheclustersinNGC 7252comparedtothe star formationwithinthespiralarms. tions or30Doradustypeknotsinmoreextendedregionsof effective radiusoftheobjectsininnersampleis23.9 outer sampleareindeedyoungglobularclusters.Themean radius providesstrongevidencethattheobjectsof light distribution(i.e.,M87andM49;Lauer&Kormendy clusters appeartobelesscentrallyconcentratedthanthe sample is9.9±0.8pc,withascatterof4.9pc.Thismean stars). Themeaneffectiveradiusoftheclustersouter generally similartotheunderlyinggalaxylightdistribution few objectsfromtheinnersamplethathaveRnear0pc mula, andFig.14showsthedata.Notethattherearevery yielded similarresults. obtained thedayfollowingNGC7252observations, checks usingthecalibrationimageof16starsonP6(Sec. good estimateoftheeffectiveradiiclustersinNGC 7252 (againforIl=50kmsMpc).Independent 3) todefinethePSF,andusingasinglecalibrationstar We findthattherelationR(pc)=28xA_3,where Ào.5-3 isthedifferencebetweenvalueof[F(0.5) 1986; Harris1991).Figure15 showstheprojectedradial (Harris 1991).Therearecases,though,wheretheglobular (i.e., thereislittleornocontaminationfromforeground ±2.3 pc.TheseobjectsmaybeeitheryoungOBassocia- — F(3)]fortheobjectcomparedtostar,providesa eff 0 eff05 stars isnotaproblem. The lackofmanyobjectsnear0pcshowsthatcontaminationby Fig. 14.Rvsradius.Theaverageoftheoutersampleis9.9pc. efreff The spatialdistributionofsystemsglobularclustersis Table 1includesourestimatesofjRbasedonthisfor- eff 4.2.3 Distributionofclusters r (arcsec) 1365 1993AJ 106.1354W just aslikelythatthedistributionforoutersample the PCfieldofview. with theouterloops,suchasmajorNEloopcoveredby that noneofthepointlikeobjectsappeartobeassociated be interpretedasevidencethatthedifferencesaresuperfi- population (i.e.,theyarefoundinthearms,havelarger in thisfiguresincetheyappeartobelongadifferent cial, andthattheseobjectswilleventuallybecomeglobular fact thatfainterobjectswouldbemissed).Whilethismight effective radii,andarebluer).However,ifincludedinFig. 1366 WHITMOREETAL.:NGC7252 the reddestcolorandmaybemostaffectedbydustobscu- actually fallsoffnearthecenter.Forexample,Figs.10and clusters likethoseseenintheoutersample,webelieveitis the luminosityprofile(aftermakingacorrectionfor range l"5-3"5and5objectsinthe5"5-7"5,thereare ration (Fig.5).Whilethereare14objectsintheradial around r=4"5,roughlywherethebackgroundlightshows would predictroughly9±3objects(i.e.,a327",althoughwe © American Astronomical Society • Provided by theNASA Astrophysics Data System r (arcsec) ber ofclusterswhichmightbeexpectedbeyondr=35" beyond 30"sincetherearenoclustersthisradius. yields onlythreeclusters,whenceatpresenttheapparent However, usingtheluminosityprofiletopredictnum- significant? Figure15doesnotincludeasingledatapoint the fieldofchipP8andonlyafewclustersalongedges deficit ofouterclustersdoesnotseemverysignificant. of thefieldschipsP5andP7.Isthisapparentdeficiency have beenobtainedwiththeDoubleSpectrographof striking whenlookingatFigs.2and7,withnoclustersin characteristic oflateA-typetomidF-typestarsatthe These spectrafeatureHa,Hß,andMgIabsorptionlines Palomar 5mtelescopebySchweizer&Seitzer(1993). spectra showunambiguouslythateventhetwobrightest redshift ofNGC7252,butnoemissionlines.Thus,these forming stars.Becauseoftheirextremeluminosities,they pointlike objectsarenotgiantHIIregions,buthaveceased eval stars. must berelativelyyoung(<1Gyr)clustersofnearlyco- be aconnectionbetweenmergersandthefeedingofnuclei in AGN’s,isthereanyevidenceforpastorpresentactivity cleus? Andgiventheoftenexpressedbeliefthattheremay galaxy, isthereanyevidenceforasurvivingdoublenu- Given thatamajormergerundoubtedlyoccurredinthis near-simultaneous observationsofstarstoyieldanaccu- aberration, themostseriousobstacletoanaccuratedeter- does itshowstrongnuclearemissionlines.Onthecon- observations, thenucleusappearstoberelativelyquiescent in thenucleusofthismergerremnant?Fromground-based niques ofanalysissuchasdeconvolutionwithsynthetic mination ofthecentralbrightnessprofileislack dress thesequestions.Giventhesevereeffectsofspherical disk ofionizedgas(S82). otherwise strongHaand[Nll]lineemissionofthecentral trary, thereseemstobear'-2"“centralhole”inthe at present:NGC7252isneitherastrongradiosourcenor HST images(Figs.3and4)suggeststhatthecentrallight rate PSF.Nevertheless,experimentationwithvarioustech- PSFs, profilefitting,andconcentric-aperturephotometry suggests thatsomefirstanswersofinterestcanbeobtained. volved Iimageafterthathasbeensmoothedby nucleus. Figure16showsanisophotalmapofthedecon- distribution israthersymmetricandthatthereasingle isophotes appearroughlyelliptical withonlyslightboxi- the pixel-to-pixelnoise.Apartfromsomewigglesin convolution withaGaussianofFWHM=0"2todiminish ness beyondaboutr=2"5. innermost isophotesduetothe centralspiralstructure,the an improvedversionofthePROFILE commandinVISTA two differentmethods.First,we fitellipticalprofileswith For thetwobrightestclusters,Objects3and30,spectra The coreregionofNGC7252isinterestbyitself. The HSTdatapresentedaboveallowustofurtherad- Even cursoryinspectionoftherawanddeconvolved To derivebrightnessprofilesof thecoreregion,weused 4.2.5 Propertiesofthecore 4.2.4 Spectroscopy 1366 1993AJ 106.1354W judged tobeoptimal(soliddots). Thesedeconvolutions iV=30, 40,50,70,100,and130forI).Andsecond,we both ontherawimagesanddeconvolvedversionswith used throughoutthisanalysis becauseofthelacka involved 50Lucy-Richardson iterationsfortheVframe multiaperture photometry.Thelatterphotometryhasthe radii ofr<0"34(log/*<—0.47),thesurfacebrightnesses methods toderivethefinalsurface-brightnessprofiles.At reached atr~9"),weadoptedacombinationofthetwo ically, noboxinessinexcessof3%,thepeakvaluebeing from 6^0.1nearthecenterto0.25atr=10"-15"(specif- than agradualincreaseoftheellipticityisophotes tion atthecenter,butshowlittlenoteworthydetailother frames. Sincetheellipticalprofileshavebetterresolu- chip P6oftheVand/frames.Thesefitswereperformed and 100iterationsforthe/frame. SimulatedPSF’swere raw images(opensymbols)and fromdeconvolvedimages and structuresmayintroduceundesirablenoiseinthe over theinner“spiralstructure,”whereindividualknots advantage ofaveragingthelightdistributionazimuthally are thosemeasuredbyelliptical-profileanalysis,andat concentric aperturescenteredonthenucleusofsame also performedmultiaperturephotometrywithaseriesof Richardson method(TV=20,30,40,50,and70forF; different numbersofdeconvolutioniterationsbytheLucy- profiles intheVand/passbands, derivedbothfromthe the fivebrightestGCcandidates(Objects3,10,15,26,and elliptical-profile analysis.Tofurtherdiminishsuchnoise, r>0"34 thesurfacebrightnessesarethosemeasuredby all profilespresentedherearebasedonimagesfromwhich 30) wereremovedthroughbilinearinterpolation. (Lauer etal1983)tothelightdistributionrecordedon 1367 -2 the pronouncedspiralstructure. I. Thedeconvolved/imagehasbeensmoothedwithaGaussianof separated by0.5mag.Wiggleswithin5"fromthecenteraredueto isophote marks/=19.5magarcsec,andsuccessiveisophotesare FWHM=0.'2 todiminishpixel-to-pixelnoise.Theoutermost Fig. 16.IsophotalmapofcentralregionNGC7252inpassband Figure 17showstheresultingmeansurface-brightness © American Astronomical Society • Provided by theNASA Astrophysics Data System WHITMORE ETAL.:NGC7252 1/4 3 3 3 2 73 HST observations. features noapparentbreakthatmightsuggestanunre- law ofindex—1.3rightintothecenterandremains ness withdecreasingradius.Thedeconvolvedprofilessug- bright staronP6.However,experimentsusingthe case, thecoredoesnotappear to beresolvedbythepresent contributes significantlytothe totalcentralflux.Ineither half-intensity radiusofthedeconvolvedprofiles,or moment, thecorethenseemslikelytobesmallerthan low). Ignoringsuchapossibleweakpointsourceforthe is suggestedbytheobservedcolorsatcenter(seebe- unresolved evenat0"05resolution.Thelightdistribution gest thatthelightdistributionfollowsanearlypurepower completely, signifyingasteadyincreaseofsurfacebright- profiles inVand/northedeconvolvedflatten versus r(notshownhere)leadstothefollowingtwo tional brightnessprofilesplottedversuslinearradiusand at allradii.AdetailedinspectionofFig.17andaddi- star onP8forthePSFresultedindifferencesof<0.2mag ever, beanoverestimateifacentral pointsourceexistsand for normalellipticalgalaxies(i.e., 10Lqpc“,Laueretal the deconvolvedcentralsurfacebrightnessofF=12.37 solved centralpointsource,thoughaweaksource results. coincident withthespiralstructureobservedincentral~5". hump between/*~0.8"-4",alikelyreflectionofsmallexponentialdisk brightnesses measuredfromdeconvolvedimages.Thelarge,filledsquares brightnesses measuredfromrawimagesandfilleddotsmarksurface central volumeluminosityiswellwithintherangefound pc (see,e.g.,Schweizer1979,esp.AppendixA).This sual volumeluminosityinthecoreof(A)>4.3X10A) mag arcsec“thenleadstoalowerlimitforthemeanvi- r <0"053,correspondingto25pc.Takenatfacevalue, power-law natureofthedeconvolvedlightdistributionsandslight pair) havebeenshiftedupwardby2magtoavoidconfusion.Noticethe ground-based photoelectricphotometry(S82).TheIprofiles(upper along thelowerprofilesmarkVsurfacebrightnessesdeterminedfrom the logarithmofradius.Foreachpassband,opencirclesmarksurface Fig. 17.CentralbrightnessprofilesofNGC7252inFand/,plottedvs 1985, to10Z,0pc“Laueret al1992).Itcould,how- 0 FC c First, nearthecenterofNGC7252,neitherraw Log R 1367 1993AJ 106.1354W brighter portionsofthecentraldiskionizedgas(S82) ing deviationsfromaperfectpowerlaw,mostnotablythe be indicatedbythecentralbluingoflightdistribution. region ofobviousspiralstructure(r^3"5)andwiththe log r^0.45.Sincethisregioncoincidescloselywiththe one extendingfrom~0"8to4"radiusandpeakingnear both therawanddeconvolvedimages,itseemsunlikely This objectisclearlyseeninFig.5aswell.Notethatboth mation oraweakunresolvedcentralpointsourceseemsto from therawanddeconvolvedV/brightnessprofiles. portion ofthisgasdisk. with thepresenceofextrastarlightinstar-forming ing extendsoveratleast4pixelsinradiusandshows color profilesshownoticeablebluingfromaboutV—I Either asmallcentralregionofrelativelyrecentstarfor- an elliptical-likelightdistributionisprobablyassociated and moleculargas(Wangetal1992),thisdeviationfrom bands. Rather,itappearstoreflectarealbluingofthe to beduestrongPSFdifferencesbetweenthetwopass- be particularlyactiveatpresent, butdoesharborabluish cently discoveredweakcentralradio-continuumsource source ofsynchrotronradiationcorrespondingtothere- globular clustersdiscussedaboveand/oranunresolved ing areacentralsuperstarclustersimilartotheyoung central lightdistribution.Candidatesforcausingthisblu- 1368 solved attheresolutionprovided bythePlanetaryCamera central component.Thiscore appearssingleandunre- core luminositydensitysuggest thatitisrelativelyhigh, ^ 1.4at/*~0"5toF—/~0.9thecenter.Sincethisblu- ( Hibbardetal1993). (i.e., smallerthanabout25pc). Estimatesofthemean Figure 18showsprofilesofthecolorindexV—Iderived Second, thelightdistributionshowssignificantundulat- In summary,thecoreofNGC 7252doesnotappearto sured fromdeconvolvedimages.Noticethestrongbluingob- sured fromrawimages,whilefilleddotsmarkcolorindicesmea- central 15"ofNGC7252.Opencirclesmarkcolorindicesmea- unresolved sourceofsynchrotronradiation. and [2''8].Thelatterregionscorrespondtopronouncedsegments served inthecentral0"2andlesserbluingobservedat1"2 Fig. 18.ColorindexF—/plottedasfunctionoflinearradiusin ative ofarelativelyyoungnuclearsuperstarclusterand/oran of thecentralspiralstructure,whereasformermaybeindic- © American Astronomical Society • Provided by theNASA Astrophysics Data System WHITMORE ETAL.:NGC7252 the merger,wefirstbrieflyreviewmergerageofNGC ferent ways.Toputtheseageestimatesinthecontextof tail lengthsandvelocitiesis1.0Gyrwithafactorof2 7252. giant ellipticals. but wellintherangeofluminositydensitiesobserved developed computersimulationsthatindicatethestar of NGC7252,Borne&Richstone(1991)deriveamerger uncertainty eitherway(S82).Fromdynamicalmodeling the starformationoccurringroughly200-500Myrafter prograde versusretrogradeencounters),withthepeakof formation ratedependsonavarietyofparameters(e.g., age intherange0.8-1.0Gyr.Mihosetal(1992)have luminosity resultingfromthedetectionthresholdmakesa merger-age estimates,wemightthereforeexpecttheages roughly anequalamountoftime.Fromthesevarious the closestapproachandstarformationcontinuingfor of theclusterstolieinrange0-800Myr. based ontheextinction-correctedV—Icolors,assuming brighter clusterswhichtendtobeyounger. distribution. AsdiscussedinSec.4.2.1,thecutoffof that eachclusterformedinasingleburst.Usingthelatest This isalowerlimitsinceweareonlyabletodetectthe yields alowerlimitforthemeanclusteragesof60Myr. the meanluminositiesofclusterinNGC7252and ever, assuminganupperlimitforthedifferencebetween determination ofthemeanluminosityproblematic.How- bluest 20thpercentileoftheircolordistribution(seeFig. Milky Wayof5.5mag,andusingaBruzual&Chariot color-evolution modelsbyBruzual&Chariot(1993)fora clusters willfadeinMby5.6and3.3mag,respectively. their ageisabout500Myr.Overthenext14Gyr,such age ofabout34Myr,whereasatthereddest20thpercentile 9) theclustersofoutersamplehaveacolor-evolution Salpeter (1955)initialmassfunction,wefindthatatthe ing continuouslysincethefinalphasesofmergerbegan, observed clustersinNGC7252formedduringtherecent case, theseagesareconsistentwiththehypothesisthatall For aScalo(1986)initialmassfunction,theagesareabout the extinctioncorrection[weadopted E(F—/)=0.12,see more peakedperiodofintenseclusterformation.Notethat whereas asmallerintrinsiccolorspreadwouldsuggest color spreadisintrinsic,theclustersmayhavebeenform- merger oftwospiralgalaxiesthere.Ifmosttheobserved disk galaxiesroughly0.8-1.0Gyr ago,andthattheclusters hypothesis thatNGC7252resulted fromthemergeroftwo these resultsarerelativelyinsensitive totheexactvalueof 30% higherandthefadingis0.8-1.5magless.Ineither Sec. 4.2.1]. ( 1993)modelwithaSalpeter(1955)initialmassfunction v The beginningofthemergerinferredfromcurrent We haveestimatedtheagesofclustersintwodif- The first,roughageestimatecomesfromtheluminosity Our secondandmorereliableestimateofclusteragesis Both methodsofdatingyield ages compatiblewiththe 5.1 ClusterAges 5. DISCUSSION 1368 1993AJ 106.1354W n ter population. interaction andaretheprecursorofanormalglobularclus- frequency forNGC7252,sinceonlythe—40brightest mate oftheenhancementspecificglobularcluster we seeintheoutersamplewereformedshortlyafterthis intrinsic cutoff.However,observationswiththeWideField truncated bythedetectionthresholdratherthanan clusters havebeendetectedandtheluminosityfunctionis sible tomakethisimportantmeasurementinthenearfu- detection thresholdbythreemagnitudes,soitmaybepos- and PlanetaryCamera2oftherepairedHSTwilllower ture. main bodybeyondthisradius(Hibbardetal.1993).Fi- ionized andmoleculargaswithinabout8"ofthecenter stellar diskinitsplace.SimulationsbyMihosetal.(1992) to theouterionizedgas.Fromthesevariousproperties,it nally, thecentralgasdiskiscounter-rotatingwithrespect spiral structurecoincidescloselywiththedistributionof the galaxy,extendingtoonly~0.05R5-Iaddition, note thatthespiralstructureoccursonlynearcenterof Will itbeaspiralorellipticalgalaxy?Itisimportantto elliptical galaxyintheprocessofformingwefindspiral major diskorbeanormalspiralgalaxy. is clearthattheremnantgalaxynotgoingtofeatureany question: Whatisthefinalfateofthismergerremnant? structure coincidentwithagaseousdisk.Thisbegsthe 1369 WHITMOREETAL.:NGC7252 the gastobedepletedinnextfewGyr,leavingonlya praz etal.1990;Wang1992)andofpronounced like featuresmayhaveformedatthecentersofothergal- may beevenless.This,then,explainhowsmalldisk- suggest thatthetimescaleforcompletionofstarformation a rapidrateinthiscentraldisk.Wemightthereforeexpect spiral structuresuggestthatstarformationisoccurringat ney 1993)andellipticalgalaxies(e.g.,deZeeuw&Franx axies, andwhyroughly1/3ofbothspiral(Rubin&Ken- (S82; Wangetal.1992),andthereisnocoldgasinthe Cardelli, J.A.,Clayton,G.C,&Mathis, J.S.1989,ApJ,345,245 Burstein, D.,&Heiles,C.1984,ApJS, 54,33 Ashman, K.M.,&Zepf,S.E.1992,ApJ,384,50 Burstein, D.1987,inNearlyNormal Galaxies,editedbyS.Faber Bruzual, A.G.,&Chariot,S.1993,ApJ, 405,538 Binney, J.,&Tremaine,S.1987,inGalacticDynamics(PrincetonUni- Bender, R,&Surma,P.1992,A&A,258,250 Borne, K.D.,&Richstone,D.1991, ApJ, 369,111 2 1991) featurekinematicsubsystemsneartheircenters. versity Press,Princeton) 9 (Springer, NewYork),p.47 At present,itisnotpossibletomakeareasonableesti- It isironicthatatthecenterofwhatbelievedtobean The presenceof—8X10^#qmoleculargas(Du- © American Astronomical Society • Provided by theNASA Astrophysics Data System 5.2 TheSpecificGlobularClusterFrequency 5.3 TheFateoftheInnerSpiralDisk REFERENCES Hubble SpaceTelescope.Theseobservationshaveresulted in thediscoveryofapopulationabout40bluepointlike that theyareyoungglobularclusterswhichformedfollow- sizes oftheseobjectsareallconsistentwiththehypothesis ing thecollisionoftwospiralgalaxiesroughly1Gyrago. objects. 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