1989ApJ. . .338. .789C 4 Astronomy, Inc.,undercontractfrom theNationalScienceFoundation. Observatories, operatedbytheAssociation ofUniversitiesforResearchin (Shapley andAmes1932),anSOgalaxy(Sersic,Carranza, The AstrophysicalJournal,338:789-803,1989March15 (1936), HubbleandLundmark(1923),Evans(1952)con- (SandageandTammann1981).Fromthepointofview and Corwin1976),and,mostrecently,asanAmorphous an IrrII(Hodge1966),10(deVaucouleurs,de omers asaspiralgalaxy(Shapley1934),anelliptical © 1989.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. cerned therelationofgalaxytosupernovaZCen(SN able properties.TheearlyworkonthisobjectbyGaposchkin ately faintgalaxy(m=11,M=—16.8)hasmanyremark- of morphologyalone,itwouldappearthatthisnearby,moder- Pastoriza 1972),aMagellanicirregular(deVaucouleurs1963), work willbebrieflydiscussedintheAppendix.Asummaryof extended irregularemissionfeaturesinNGC5253,Sersic, luminosities (heexcludedOstarsfromconsideration,thinking nuclear emission-linespectrumofthegalaxywasthat an the modernstudiesfollows.Welch(1970)showedthat the that suchstarswouldhavebeenresolvedonhisplates). H iiregion,requiring10BOstarstomatchtheemission-line son betweentheactivelystar-forming galaxyM82andNGC chaotic motionsamongthecondensations ofthesameorderas dust andveryblueluminouscondensations(nowknownto be to thepresentoneinwhichtheyfound(1)anuclearcomplex of Carranza, andPastoriza(1972)wroteapapersimilarinscope the overallrotationofgalaxy. Theauthorsdrewacompari- shown tobemorefilament-likeinGraham1981),and (3) OB associations),(2)“jet-like”Haemissionfeatures(later Prodded byWelch’sworkandHodge’s(1966)paperdescribing 1895B) thatoccurredsome25"awayfromthenucleus.That B 1 NGC 5253hasatvarioustimesbeenclassifiedbyastron- CerroTololoInterAmericanObservatory, NationalOpticalAstronomy 89 -1_ falloflf, asidefromthenuclearstarformationsite.Themaximumstellarrotationrateisbarelydetectable,at7 ly throughoutthegalaxy.Thesurfacebrightnessprofileofgalaxyiswelldescribedbyanexponential range asthehalolight.Whilestarformationisnowseenprimarilyinnucleus,itoccurredrelativelyrecent- clustersoutsideofthenucleus,fewifanyareoldglobularclusters;mostappeartobeinsameage age; theyoungercomponentisprobablyolderthan10yrandyr.Of~100cataloged spectra. Thehaloofthegalaxyisincipientlyresolved,probablyintoredgiants.lightcompositein Our dataincludemulticolorCCDmeasurements,narrow-bandimagesinHa,andlong-slitCantriplet Type IsupernovathatoccurredinNGC5253. km s,whereasthestellarvelocitydispersionishigh,at50.Beforegalaxybeganactivelyforming Subject headings::individual(NGC5253)—nucleistellarcontent there isstillnoevidenceforexcitationofthegasbymeansotherthanhotstars.Anappendixreviewstwo , itwasprobablyadwarfelliptical.Thedistributionofionizedgasisshowntobeverycomplex,although © American Astronomical Society • Provided by the NASA Astrophysics Data System We presentnewobservationsofthenearbygalaxyNGC5253,whichisactivelyformingstarsinitsnucleus. I. INTRODUCTION stars: formation—supernovae 1 Cerro TololoInterAmericanObservatory,NationalOpticalAstronomyObservatories F. L.WhippleObservatory,SmithsonianInstitution STAR FORMATIONINNGC5253 Received 1988July18;acceptedAugust24 Nelson Caldwell M. Phillips ABSTRACT AND 789 9 from NGC5253canindeedbeaccountedforbytheHn reasons thatarenolongeracceptable(e.g.,theradioemission ing toaradiusof120"(2.2kpc).Althoughhehadnocolorsfor regions), vandenBergh(1972,1980)showedthatthenuclear NGC 5253canbeprovided bytheknownionizingstars formation regionthatisobscuredinthevisiblespectrumby 25 younger starclustersinstead,whichheregardedas“fossilevi- the clusters,hepresumedthemnottobeglobulars,but the galaxywassurroundedbyasystemofstarclustersextend- absorption-line spectrumwasduetoearly-typestarsandthat 5253, baseduponthecitedfindings,aswellafewother without recoursetosupernova remnants,andthe2gmemis- found broadÀ46S6emissioninthebrightestvisualnuclear mag ofextinction.Thisregionmustcontainorder500 04 Glass (1982)andAitkenetalrevealedanintense star which liesataprojecteddistanceofleast100kpc. more than10yragowiththespiralgalaxyNGC5236(M83), dence foractivestarformationinthepast,”remnantsof dust attemperaturesbetween100and700K.Thisparticular infrared emissionsisbestexplainedasthermalradiationfrom galaxy asistheK—10gmcolorof7.7mag.Thelargemid- condensation, whichtheyattributedtoWolf-Rayetstars. The stars toaccountfortheobservedstrengthof[Siv]emis- that theburstinNGC5253wastriggeredbyanencounternot star formationburstnowconfinedtothenucleus.Hesuggested observation isperplexinginthat,whilenumerousdustlanes K —Lnuclearcolorof2magisthelargestknownfor any sion lineat10.5gm.Campbell,Terlevich,andMelnick(1986) decrement inthevisualspectrum. 9-10]), thereisnoevidence for anyreddeningintheBalmer are visibleintheopticalimage ofthegalaxy(Fig.1[Pis. Work intheinfraredonnucleusbyMoorwoodand Since theobservedX-radiation andtheradioemissionin 1989ApJ. . .338. .789C The imagewastakenwiththeCTIO 4 mtelescope,withatotalexposuretimeof2000s.(b)Sameas(a)forfield 2(NE). Caldwell andPhillips(see338,789) Fig. 1.—(a)BlueCCDimageofthe field 1(SW)ofNGC5253.Gridalongthepicturecanbeusedtolocate the starclustersofTable1.Scaleis0"3pergridunit. © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. \a PLATE 9 1989ApJ. . .338. .789C Caldwell andPhillips{see338,789) PLATE 10 © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. lb 198 9ApJ. . .338. .789C 7 89 79 10 7 (1979) andsyntheticapertures fromtheCCDdata.Theaper- larger thantheformatofCCD(5'x3'),galaxy’s significant differenceinthe U —Bcolors,butthatWegner’s difficulties duringthephotoelectric measurements.Wefindno We suspectthatthisdiscrepancy ismostlyduetocentering ture magnitudesagreetobetter than0.04magfor10''5and between thephotoelectricaperturephotometrybyWegner derived galaxyphotometryisprovidedwithacomparison 0"59. TheFWHMofthestellarimagesonframeswas0'.'9. field was2000s.Forthe900sUframe,galaxycentered (CTIO) 4mtelescopeonthenightof1984March27(UT) standard stars(Graham1982a)observedduringthenight. nated whitespotonthedome.ThepixelsizeCCD is and werethenflattenedusingexposuresofauniformlyillumi- BV exposures,andintheSEcornerforanothersetofexpo- nucleus wasputintheNWcomerofdetectorforoneset Standard extinctionvalueswereassumed.Acheckon the on thedetector.Therawdatahadadarkframesubtracted, surface photometry.Theexposuretimeineachfilterfor sures. Thisallowedaccurateskyvaluestobeobtained for using athinnedRCACCD.Sincethegalaxyissomewhat prime focusoftheCerroTololoInter-AmericanObservatory 14"3 apertures,butareoffby ~0.15forthelargerapertures. modern times(Barbonetal1984). as derivedfromSN1972EintheAppendix. Appendix, theybeingthethirdandfifthbrightestobservedin thought. gas distributiontobelargerandmorecomplexthanpreviously galaxy. AdeepHaimagetakenwithaCCDrevealstheionized data thatpointtoadwarfellipticalasthelikelyprogenitor recently (1x10yr). burst, thenitsageisbetween10andyr.Thusweconclude lation, andayoungerone.Iftheoneisresultof composite population,mostlikelyanoldunderlyingpopu- from 10toyr,butnoclearevidenceforasignificant what wenowseeintheLMC,thatthereareclustersofages cluster systemmaybeconsideredtoatanearlystageof throughout thegalaxy,butbecameviolentinnucleusonly that indeedtherecentepisodeofstarformationbegan number ofold(10yr)clusters.Thehalolightcomesfroma imaging, inwhichover100clustersthehaloarestudied.The provided fromthestarclustersanddiffusehalolightinan forms themajorpartofthisnewstudy.Weexamineevidence inward atamuchhigherrateremainstobeanswered,and attempt toanswerthisquestion.ThedataincludeUBVCCD began throughoutthegalaxyatalowrateandproceeded (1988). considered tobeinthefirststageofevolutionarysequence of starburstgalaxiesoutlinedinRieke,Lebofsky,andWalker the nucleusmorethan10yrago.NGC5253istherefore conclude thatmassivestarscouldnothavebeguntoformin tion fromredgiantsandsupergiants,MoorwoodGlass sion isweakcomparedtothatinM82,implyinglittlecontribu- 790 Calibration oftheUBVimageswasobtainedviaEregion Broad-band UBVimageswereobtainedofNGC5253atthe We adoptavalueof3.7MpcforthedistancetoNGC5253 The twosupernovaefoundinNGC5253arediscussedthe We alsopresentsurfacephotometryandstellarkinematic The questionofwhethertherecentepisodestarformation II. THECLUSTERSYSTEMANDHALOLIGHTOFNGC5253 © American Astronomical Society • Provided by the NASA Astrophysics Data System a) CCDObservations CALDWELL ANDPHILLIPS would arguethatthepresenceofsuchstarsfalloff giants atM=—4.However,thecolorgradientinhalo lation, andthatweareindeedresolvingtheredgiantsofan light iscontributedbythemainsequenceofayoungpopu- ently resolvedobjectsisaboutrightforredgiantsatadistance frames. Theapparentvisualmagnitudeof24to25theincipi- The incipientresolutionisevenmoreapparentontheVCCD stand outprominently,andtheincipientresolutionisapparent fields. rapidly withradius,andyettheincipientresolutionseenin assumed, sayat8Mpc,wecouldberesolvingyellowsuper- older population.Ifthegalaxyisfartherawaythanwehave giants alone(seebelow).Itisstillpossiblethatpartoftheblue of 3.7Mpc(seeGraham’s1982bstudyNGC55and as amottlingalongthemajoraxis,outtoradiusof~90". axis infield1,theotheralongSEminoraxis,seenboth Figures 1and2doesnotshowsuchadependence. clusters. Perhapssomeofthese objectsaresinglestars. candidates appearedonlyon theBandVframessome were inspected.Asmighthave beenexpected,manycluster mind, wenotedthepositionsofallobjectscloserthan images wehaveavailable.Bearingthispossibleconfusion in will lookthesameasasingleOBsupergiantonCCD cluster isdifficulttodistinguishfromasingleresolvedstar. In starting 30"fromthenucleusandrunningalongSWmajor 300), buttheobservedhalocolorsaretwobluetobeduered testament totheextremeblue colorsofsomethefainter ultraviolet andblueframes but notonthevisualframe,a nuclear bar(about20"x40"),thatwerebrighterthanabout 90" awayfromthenucleusyetoutsideofmainbulk the particular, averyyoungclusterwithanabsolutemagnitude of only ontheVframes.Surprisingly, afewappearedonthe My =—6suchasarefoundintheLMC(vandenBergh1981) 11-12) showtheresultantfilteredimages.Thestarclustersnow the galaxyimageshavebeenremoved.Figures2aand2b(Pis. displayed inFigure1bythesameimagesmoothedwitha initial estimateof50.Itisinterestingthatthisnumbercom- 7x7 boxmedianfilter.Thusalllow-frequencycomponentsin menon, animagewascreatedbydividingtheBimages which isofcomparableabsolutemagnitudetoNGC5253. parable tothenumberfoundinSmallMagellanicCloud, being equaltotwogridunits.Twothingsareimmediately A gridalongthebordersindicatesscale,oneCCDpixel quite large,infactmorethantwiceaslargevandenBergh’s apparent fromtheseimages.First,thestarclustersystemis ous starsorsmallstarclusters.Tobetterillustratethispheno- B V =23.5andfainterthanF19.Framesinallthreecolors error intheCCDB—Vcalibrationof0.05magwouldnot mag, wedonotregardthisdiscrepancyasmeaningful.An significantly changetheresultsofthispaperanyway. B—V colorsareredderby0.05magthantheCCDvalues. Since WegnerquotesanexternalB—Verrorestimateof0.05 Also apparentinFigures1and2aretwodustlanes,one At adistanceof3.7Mpc,where1"=18pc,smallstar Figures laandlbshowthetwofieldsimagedinBfilter. Second, thegalaxyisincipientlyresolved,eitherintolumin- c) ACatalogofClusterCandidates b) GeneralResults Vol. 338 1989ApJ. . .338. .789C Note themottledpatternatlargeradii alongthemajoraxis,thoughttobedueincipientresolutionofred giantstarsinNGC5253.(b)Sameas(u),forfield2. Caldwell andPhillips(see338,790) Fig. 2.—(a)Imagecreatedbysmoothing theimagepresentedinFig.\aanddividingitintorawimage, thus removingthesmoothcomponentofgalaxy. © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 2a PLATE 11 1989ApJ. . .338. .789C Caldwell andPhillips(see338,790) PLATE 12 © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 2b 1989ApJ. . .338. .789C No. 2,1989 cedure tomeasurethemagnitudesandcolorsofclusters. frame, andtheirrespectiveresolutionsweredegradedslightly eliminates centeringerrorscausedbyslopingbackgrounds. had beentreatedinthesamemannerasthatwhichproduced The centersoftheclusterimageswerefoundonVframesthat formation providedbystarsontheCCDframeswhosecoordi- the imagesshowninFigure2.Usingsuchfilteredframes to matchthatoftheFframe.Additionalclustersnotfoundon Absolute coordinateswerefoundfortheseclustersviaatrans- the Vframewereaddedtocoordinatelistatthispoint.An were shiftedtocorrespondthecoordinatesystemofV nates hadpreviouslybeendeterminedonplatestakenwiththe from thedatainanannuluswithinnerdiameter8pixelsand to extractmagnitudesforeachclusterincoloranaper- aperture photometrycodethenusedthederivedclustercenters Curtis SchmidtandYale1matCTIO.TheBUframes ture ofdiameter4pixels(2'.'3=41pc).Theskywasdetermined ticularly crowded,theskywasdeterminedfromanaperture outer diameter12pixels.Inthecaseswhereaclusterwaspar- mean erroroftheskydetermination,andreadoutnoisein photon statisticsintheapertureandskyannulus, of aparticularclusterwasusedforthedifferentcolorframes. offset fromthecluster.Thesameareaforskydetermination formal errorsbecauseofthedifficultyingettingappropriate Errors inthederivedmagnitudeswerecalculatedfrom the aperture.Thetrueerrorsareexpectedtobelargerthan were calculated,theclusterswithlargeerrorsorunrealistic sky values.Thatproblemcanonlybesolvedbymuchhigher colors wereinspectedontheframesunderhighcontrastagain. resolution imaging.Afterthecalibratedcolorsandmagnitudes color usingthatframewasdeleted.Inthisway,alargenumber In thecasesinwhichimageofobjecteithercouldnotbe for thefainterclusters.TheBaperturemagnitudeswerecor- seen orwasotherwisedefectiveinoneofthecolors,derived found fromthegrowthcurvesofstarsinuncrowdedregions of theU—Bcolorswereexcisedfromcatalog,particularly rected tototalmagnitudesbysubtracting0.43mag,whichwas the CCDframes. were notobserved.Iftheclustersfollowdiffuselightdis- images wasnotcomplete:someplacesneartheminoraxis (~ 11innumber)oftheclustersgalaxyhavebeenmissed in thecatalog. tribution ofthegalaxyasseemstobecase,thenonly9% The firstcolumnisthenumberofcluster.clustershave been orderedbyrightascension,regardlessofwhichCCDfield absolute coordinates areneededforspace observationsof they werefoundon.Thesecondandthirdcolumnscontain the CCD X,Ycoordinatestransformedtothecoordinatesystem which fieldtheclusterswerefound,andhencetellswhether the twice whattheywereontheCCD.Thefourthcolumntells on of Figure1,meaningonlythatthequotedX,Ycoordinates are (F =2).Thecoordinatesforepoch1950arelistedincolumns cluster canbelocatedonFigurela(F=1)or lb (5) and(6).Column(7)liststhebluemagnitudestheir associated errorsobtainedthrough the4pixeldiameteraper- Y coordinatesaregoodto0"05. Inthecasethatmoreaccurate in the4pixelaperture,andtheir associatederrors. Columns (9)and(10)listtheB -FandU-Bcolorsmeasured ture. Column(8)liststhe extrapolated totalmagnitude. After sometrialanderror,wesettledonthefollowingpro- The arealcoverageofNGC5253bythelong-exposureCCD The absolutecoordinatesare good to1",buttherelativeX, Table 1containsthedataforstarclusterscatalogued. © American Astronomical Society • Provided by the NASA Astrophysics Data System STAR FORMATIONINNGC5253 hence wereverydifficulttomeasure.Themagnitudes,inpartic- ular, shouldbeconsideredonlyasguidelinestothetruelumin- are providedinTable4.Theseprovedtobeverycrowded,and still brighterthanmostoftheclustersinTable1),ascanbe are somewhatfainterthanthosemeasured(althoughthey osities. Inaddition,therearemanymorecondensationsthat coordinates ofthereferencestarslistedinTable2forfield1 seen fromtheB—Fcolormaps(seebelow). in Table1thathaveBFdata.Thepointsbeenplotted and Table3forfield2tobetterthan1". these clusters,theycanbeobtainedbymeasuringtheabsolute The gapinmagnitudebetweenthosecondensationsandthe assuming agalacticforegroundreddeningofF(B—F)=0.06 luminosity inthenucleus,butitprovedimpossibletomeasure and Heiles1984).Thenuclearpointsareplottedasplusmarks. and thecorrespondingvisualextinctionof=0.18(Burstein In general,weseethatmostoftheclustersarebluerthan them. WenowrestrictourdiscussiontotheclustersofTable1. outer clustersisartificial:thereareobjectsofintermediate colors, Nos.15and22.Bothoftheseareverysharplypeaked B —F=0.5,andarefainterthan20(M—7.8).There are onlytwoluminousobjectsthathaveoldglobularcluster objects haveB—F>0.5and~20(faintforanoldglobular on theCCDimages,andarethereforeprobablystars.Four Table 1. extinction correctionofA—0.18 has beenappliedtothedatalistedin v v Supplementary dataonsomeofthenuclearcondensations Figure 3showsthecolor-magnitudediagramforclusters Fig. 3.—Color-magnitudediagram for starclustersinNGC5253.An d) AnalysisoftheClusters 791 TABLE 1 Clusters in NGC 5253

# R.A. DEC. B B (B-V) (U-B) 2.3” Tot 2.3” 2.3”

1 76.00 174.00 1 13 36 58.19 -31 24 54.2 24.53 .32 24.10 1.206 .36 2 93.80 401.40 1 13 36 58.71 -31 23 47.0 23.01 .09 22.58 .497 .12 S 125.12 310.64 1 13 36 59.38 -31 24 14.0 23.14 .10 22.71 .537 .13 4 144.82 376.42 1 13 36 59.87 -31 23 54.6 22.93 .07 21.98 .639 .11 5 145.12 432.28 1 13 36 59.90 -31 23 38.1 22.12 .04 21.40 6 154.00 247.16 1 13 37 .02 -31 24 32.9 24.15 .24 28.72 .772 .29 —1.005 .42 7 166.32 473.50 1 IS 37 .41 -31 23 26.0 24.48 .32 24.05 1.431 .35 8 170.60 393.70 1 13 37 .47 -31 23 49.6 23.36 .13 22.93 9 176.48 313.32 1 IS 37 .56 -31 24 13.4 24.85 .44 24.42 10 179.04 237.12 1 IS 37 .59 -31 24 36.0 22.53 .06 22.10 .691 .07 —.061 .22 11 202.18 188.14 1 IS 37 1.10 -81 24 50.5 23.77 .17 23.34 .832 .21 12 204.52 409.00 1 IS 37 1.26 -31 28 45.2 23.16 .12 22.78 .339 .16 13 206.42 397.76 1 IS 37 1.29 -81 23 48.5 22.35 .06 21.92 .254 .08 —.478 .14 14 224.00 346.00 1 13 37 1.67 -31 24 8.9 23.26 .13 22.83 — 1.373 .17 IS 228.00 466.22 1 IS 37 1.82 -31 23 28.4 19.75 .01 19.32 .588 .01 — .026 .02 16 235.38 340.84 1 IS 37 1.93 -31 24 5.5 23.65 .18 23.22 17 235.18 387.90 1 13 37 1.95 -31 28 51.6 18 239.00 500.00 1 13 37 2.09 -31 23 18.4 22.73 .08 22.30 —.305 .14 — .935 .14 19 246.50 369.46 1 IS 37 2.20 -31 23 57.1 22.64 .08 22.21 .215 .12 20 258.12 340.42 1 IS 37 2.45 -81 24 5.7 22.30 .06 21.87 1.619 .06 21 261.96 377.00 1 13 87 2.56 -31 23 54.9 22.24 .06 21.81 .588 .07 — .244 .16 22 262.00 418.02 1 IS 37 2.58 -31 23 42.8 19.70 .01 19.27 .742 .01 .238 .03 23 262.76 450.10 1 IS 37 2.61 -31 23 33.3 23.16 .13 22.73 .460 .17 24 280.00 284.44 1 13 37 2.93 -31 24 22.4 22.47 .06 22.04 .609 .07 .412 .30 25 282.04 422.94 1 IS 37 3.04 -31 23 41.4 21.17 .03 20.74 .194 .04 —.486 .05 26 283.70 447.36 1 13 37 3.09 -31 23 34.2 21.10 .02 20.67 .221 .03 —.024 .07 27 288.30 371.84 1 13 37 3.16 -31 23 56.5 23.25 .14 22.82 28 287.60 469.40 1 IS 37 3.19 -31 23 27.7 22.57 .09 22.14 29 300.74 384.94 1 13 37 3.45 -31 23 52.7 23.06 .13 22.63 — .892 .21 30 307.26 409.72 1 IS 37 3.62 -31 23 45.4 21.66 .04 21.23 —.326 .07 81 11.34 133.92 2 IS 37 3.80 -31 22 53.3 32 318.08 406.92 1 13 37 3.86 -31 23 46.3 20.98 .02 20.55 .058 .03 —.634 .04 33 13.90 318.12 2 13 37 3.93 -31 21 59.2 34 19.82 242.60 2 13 37 4.03 -31 22 21.5 22.53 .06 22.10 — .214 .10 —.052 .21 35 325.02 423.44 1 13 37 4.03 -31 23 41.4 20.81 .02 20.38 .309 .03 — .238 .05 36 330.22 359.52 1 IS 37 4.12 -31 24 .3 22.68 .08 22.25 .749 .09 —.626 .17 37 26.74 110.60 2 IS 37 4.14 -31 23 .3 28.17 .13 22.74 —.338 .28 — 1.569 .16 38 332.08 350.00 1 13 37 4.16 -31 24 3.2 23.22 .12 22.79 .834 .17 39 30.00 126.00 2 IS 37 4.22 -31 22 55.8 21.67 .03 21.24 .081 .05 —.485 .08 40 36.84 123.04 2 IS 37 4.37 -31 22 56.7 21.10 .02 20.67 — .146 .03 —.824 .04 41 343.94 287.68 1 13 37 4.40 -31 24 21.7 21.85 .03 21.42 .913 .04 —.303 .10 42 355.94 393.46 1 IS 37 4.73 -31 23 50.4 22.22 .06 21.79 .482 .07 43 53.46 119.18 2 13 37 4.75 -31 22 58.0 44 362.44 448.92 1 13 37 4.90 -31 23 34.0 21.79 .06 21.36 45 61.62 297.42 2 IS 37 5.01 -31 22 5.7 23.88 .19 23.45 .353 .25 — .409 .47 46 372.16 340.12 1 IS 37 5.07 -31 24 6.3 23.18 .11 22.75 .521 .14 47 371.12 410.96 1 IS 37 5.09 -31 23 45.3 21.33 .03 20.90 —.068 .04 —.709 .05 48 69.28 202.24 2 13 37 5.14 -31 22 33.8 23.83 .19 23.40 1.702 .20 49 378.60 306.88 1 18 37 5.21 -81 24 16.1 22.09 .04 21.66 .697 .05 —.001 .15 50 378.80 346.18 1 IS 37 5.23 -31 24 4.5 22.60 .07 22.17 —.073 .11 — .945 .12 792

© American Astronomical Society • Provided by the NASA Astrophysics Data System TABLE 1—Continued

# R.A. DEC. B B (B-V) (U B) 2.3” Tot 2.3” 2.3” 51 377.58 450.44 1 13 37 5.25 —31 23 33.6 22.05 .07 21.62 52 75.60 206.22 2 IS 37 5.29 —81 22 32.6 23.48 .14 23.05 1.084 .16 53 384.00 393.28 1 13 37 5.37 —31 28 50.6 21.58 .03 21.15 1.234 .03 —.136 .09 54 80.52 154.00 2 IS 87 5.38 —31 22 48.0 23.29 .14 22.86 —.440 .26 —1.155 .20 55 390.34 381.00 1 13 37 5.51 —31 23 54.2 21.93 .04 21.50 .503 .05 .169 .16 56 394.00 406.00 1 IS 37 5.61 —31 23 46.8 23.02 .11 22.59 -1.195 .31 —1.169 .16 57 396.42 474.92 1 13 37 5.69 -31 23 26.5 19.79 .01 19.36 58 398.30 445.28 1 13 37 5.72 —31 23 35.2 23.82 .25 23.39 —1.964 .29 59 101.30 164.18 2 13 37 5.86 —31 22 45.2 22.52 .07 22.09 .138 .10 60 108.70 109.26 2 13 37 6.00 —31 28 1.4 20.59 .02 20.16 —.275 .03 — 1.108 .02 61 112.66 13.38 2 13 37 6.05 —81 23 29.6 23.01 .12 22.58 —.100 .19 —2.097 .13 62 114.34 165.70 2 13 37 6.16 —31 22 44.9 21.33 .02 20.90 .691 .03 .016 .08 63 116.92 139.64 2 13 37 6.20 —31 22 52.6 20.87 .02 20.44 .286 .03 .191 .07 64 420.76 387.68 1 IS 37 6.21 —31 28 52.4 22.66 .07 22.23 —.251 .12 —.907 .13 65 425.48 475.74 1 13 37 6.36 —31 23 26.3 20.48 .01 20.05 —.039 .02 —1.150 .02 66 430.44 306.52 1 13 87 6.40 —31 24 16.4 24.13 .24 23.70 .886 .28 67 123.92 262.10 2 IS 37 6.42 -31 22 16.7 22.93 .08 22.50 1.255 .09 68 432.00 400.66 1 13 37 6.48 -31 23 48.6 21.90 .04 21.47 —.108 .06 —.785 .07 69 432.06 433.70 1 13 37 6.49 —31 23 38.8 23.92 .23 23.49 70 129.76 158.34 2 IS 37 6.50 —31 22 47.2 23.00 .11 22.57 .247 .16 71 130.32 116.02 2 IS 37 6.50 —31 22 59.6 21.90 .05 21.47 — .117 .08 —.824 .08 72 437.00 455.00 1 IS 37 6.62 -31 23 32.5 23.12 .12 22.69 —.069 .19 -1.241 .17 73 133.58 243.48 2 IS 37 6.63 —31 22 22.2 23.41 .13 22.98 1.315 .14 74 136.88 64.68 2 IS 37 6.63 —81 23 14.8 20.25 .01 19.82 —.044 .02 —.630 .02 75 441.84 296.52 1 IS 37 6.65 —31 24 19.4 23.11 .10 22.68 — .400 .18 —.864 .19 76 137.28 169.36 2 13 37 6.68 —81 22 44.0 22.61 .08 22.18 .355 .11 —.090 .23 77 448.62 337.64 1 IS 87 6.83 —31 24 7.3 23.46 .13 23.03 —.553 .27 —1.090 .23 78 143.48 228.70 2 13 37 6.85 —31 22 26.7 22.93 .09 22.50 .234 .13 79 146.56 130.18 2 13 37 6.87 —81 22 55.6 24.62 .46 24.19 80 147.36 69.36 2 13 37 6.87 —31 23 13.5 22.49 .09 22.06 —1.717 .08 81 145.88 213.48 2 13 37 6.89 —31 22 31.1 24.81 .44 24.38 2.430 .45 82 148.00 143.10 2 18 37 6.91 —31 22 51.8 21.67 .04 21.24 .038 .06 — .271 .09 83 153.50 26.08 2 13 37 6.99 —31 23 26.3 23.64 .18 23.21 -1.894 .21 84 152.38 241.72 2 13 37 7.06 —31 22 22.9 23.81 .19 23.38 —.120 .30 85 152.44 276.36 2 13 37 7.07 —31 22 12.7 24.28 .27 23.85 1.355 .29 86 159.38 145.04 2 13 37 7.17 —31 22 51.4 22.54 .08 22.11 —.682 .14 87 162.16 18.48 2 18 37 7.18 —31 23 28.6 27.02 1.73 26.59 88 464.60 395.12 1 13 37 7.22 —31 23 50.4 21.41 .02 23.98 .621 .03 .134 .10 89 164.42 267.22 2 IS 37 7.34 —31 22 15.5 22.25 .05 21.82 —.183 .08 —.751 .10 90 170.00 111.74 2 IS 87 7.40 —31 23 1.2 22.18 .05 21.75 —.188 .09 .173 .20 91 171.56 18.10 2 IS 37 7.40 —31 23 28.8 24.06 .25 23.63 1.300 .27 92 173.26 84.36 2 13 37 7.46 -31 23 9.3 20.42 .01 19.99 —.133 .02 —.626 .02 93 176.00 9.80 2 IS 37 7.50 —31 23 81.2 22.41 .06 21.98 .843 .07 —1.132 .09 94 173.54 190.48 2 IS 37 7.52 —31 22 38.1 24.96 .51 24.53 95 174.88 175.26 2 13 37 7.54 —31 22 42.6 23.24 .13 22.81 .106 .19 96 180.56 104.12 2 IS 37 7.64 —31 28 3.6 21.79 .04 21.36 — .620 .08 -.837 .07 97 181.38 158.74 2 IS 37 7.68 —31 22 47.5 27.84 2.49 27.41 98 191.78 197.30 2 IS 37 7.93 —31 22 36.3 24.09 .25 23.66 —.052 .40 99 194.18 87.98 2 IS 87 7.94 —31 22 8.4 22.72 .08 22.29 — .223 .14 —.780 .15 100 195.70 120.88 2 13 37 7.99 —31 22 58.8 24.34 .32 23.91 — .587 .63 — 1.985 .37

793

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1989ApJ. . .338. .789C Christian andSchommer1988).Ifthenuclearcondensations of galaxies thathavesubstantialclusterpopulationsofallages: M33, theLMC,andSMC(seevandenBergh1981; the NGC5253clusterswiththoseofthreeothernearby are soredthatwewerenotabletodetectthemontheUframe; which arefainterthanF=21.5(M-6.3).Manyofthese stars aswell.TherearemanyobjectswithB—V>0.5but but wecannotruleoutthepossibilitythattheyareforeground galaxy. Objects62and53mayindeed,befaintoldglobulars, the color-magnitudediagram. foreground stars.Thusthereislittleevidenceforasignificant old globularclusterpopulationfromtheevidencepresentedin flattened object,andisthereforeprobablyabackground hence, therearenoU—Bcolors.Thesealsosuspectedtobe at 3.7Mpc).Ofthese20iscertainlyastar,41theonlyhighly v 794 It isinterestingtocomparethecolor-magnitudediagramfor hmS 11 311.39241.36133659.279-312349.05 10 349.73234.7613371.041-312353.33 9 340.21275.54 13 370.636-312329.09 Number XYR.A.Decl. 7 367.09277.1113371.872-312328.37 6 402.56285.0713373.509-312323.95 4 414.01207.0713373.959-312410.19 5 384.12253.0813372.634-312342.74 3 479.29171.7313376929-31°24'31?67 and object93issoextendedthatitsphotometrylikelytobeinerror. ,107 121 120 119 118 117 116 115 114 113 112 111 110 109 108 106 105 104 102 103 101 Notes.—Objects 20,22,48,53,67,73,102,and109maybeforegroundstars.Object41isprobablyabackgroundgalaxy.62double, © American Astronomical Society • Provided by the NASA Astrophysics Data System # 371.26 335.26 335.64 332.18 330.44 315.54 306.80 293.96 287.76 260.80 256.14 252.46 252.58 245.02 229.38 210.96 209.54 203.50 204.98 208.16 199.60 Stars inField1 248.44 186.78 207.18 237.60 224.82 302.80 120.00 244.60 139.68 231.74 190.50 264.22 200.98 234.54 80.60 63.60 97.80 94.84 75.88 79.32 13.22 TABLE 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 IS 37 IS 37 IS 37 IS 37 13 37 IS 37 13 37 13 37 IS 37 IS 37 13 37 13 37 IS 37 13 37 13 37 13 37 13 37 13 37 13 37 13 37 13 37 R.A. 12.02 11.23 11.22 11.15 11.11 10.71 10.50 10.23 10.08 9.55 9.38 9.34 9.27 9.16 8.79 8.33 8.29 8.26 8.23 8.20 8.13 CALDWELL ANDPHILLIPS -31 23 -31 23 -31 22 -31 23 -31 22 -31 22 -31 22 -31 23 -31 22 -31 22 -31 23 -31 23 -31 22 -31 23 -31 22 -31 22 -31 22 -31 22 -31 22 -31 22 -31 22 TABLE 1—Continued DEC. 41.0 22.5 34.7 25.7 29.4 59.9 11.7 16.6 53.8 22.9 26.7 38.6 12.5 35.3 25.4 30.5 11.1 16.8 6.4 5.9 6.6 rather thanalackofclustersinthosegalaxies. luminosity functionoftheM33clusterspeak.However, the lack ofinterestorthedifficultyinmeasuringsuchfaintclusters, at F=22.25(M—5.6),whichisalsowherethetotal seems likelythattheyareforegroundstars.Thefunctionpeaks Those withB—V>1.2areshownasdottedlines,since it of thethreeothergalaxies.Thisisapparentlyduetoeither a nearly ahalfmagnitudefainterthanhasbeenmeasuredinany (of orderzero)redclusterscomparedtotheothertwo.Note that wehavemeasuredobjectsdowntoM=—4.3,whichis NGC 5253,butthatagain,thelattergalaxyhasrelativelyfew pared totheLMC,and(2)thatblueclusterpopulationof the SMCandM33occupysameluminosityrangeasthatof are fewerbrightclustersinNGC5253atanycolorwhencom- condensations fromconsideration,oneconcludes(1)thatthere relative toredonesthanintheothergalaxies.Excludingthose NGC 5253areconsideredtobeclustersinthecomparison, then obviouslythatgalaxyhasmanymorebluestarclusters v v lm Figure 4showstheVluminosityfunctionforallclusters. 17 71.8353.4313376.743 -31232.86 16 144.37150.42133710.187 -31226.91 15 203.49136.60133712.878 -312215.61 14 252.6879.31133715.062 -312250.65 13 220.9445.21 13‘3713’593 -3r23'9"77 Number XY R.A. Decl. 24.30 .26 22.81 .08 24.03 .21 22.83 .08 22.47 .06 22.03 .04 24.31 .27 23.15 .10 24.55 .33 23.73 .17 24.88 .44 23.61 .16 24.39 .30 23.65 .17 21.73 .03 20.74 .01 24.91 .47 24.22 .26 23.09 .15 23.64 .17 24.71 .40 2.3” B 23.87 22.38 23.60 22.40 22.04 21.60 23.88 22.72 24.12 23.30 24.45 23.18 23.96 23.33 21.30 20.31 24.48 23.79 22.66 23.21 24.28 Tot B Stars inField2 —.866 .24 —.024 .02 1.026 .25 1.227 .30 1.823 .85 1.621 .47 TABLE 3 1.916 .31 1.356 .19 1.350 .29 .711 .09 .515 .10 .459 .07 .350 .05 .236 .24 .292 .22 .121 .25 .221 .05 .151 .19 2.3” (B V) —1.286 .19 —.026 .14 —.987 .18 —.449 .03 .292 .26 .214 .13 2.3” (U B) Vol. 338 O'!u 00r" No. 2, 1989 STAR FORMATION IN NGC 5253 oo 795 oo TABLE 4 ^0 Nuclear Knot Colors a (T) B B-V U-B (T)00 Number X Y F R.A. Deel. (2:3) (2:3) (2:3) 1 70.00 58.00 2 13h37m5?10 -31°23T6:2 16.84 0.01 -0 80 2 74.00 66.00 2 13 37 5.19 -31 23 13.9 16.52 0.03 -0.73 3 72.00 44.00 2 13 37 5.14 -31 2320.3 17.41 0.01 -0.74 4 74.00 66.00 2 13 37 4.73 -31 23 17.8 17.04 0.15 -0.54 5 74.00 66.00 2 13 37 5.06 -31 23 12.7 16.56 0.22 -0 54

luminosity function for clusters with B—V <03 has a peak very youngest (with ages of 107 yr), those of intermediate color nearly a magnitude fainter than that for similar color clusters and hence age, and a few red ones as well. The NGC 5253 in M33 (Fig. 5), whose luminosity function is itself depleted in clusters with UBV colors that are similar to those of galactic bright clusters when compared to the LMC. The luminosity globular clusters, at least to within the errors here, are Nos. 10, functions for the other color bins in NGC 5253 are also dis- 15,21,22, 24,49, 62, and 88. We have already identified 15 and played in Figure 5. There are not many objects in those bins, 22 as foreground stars; 24 is probably one also. The other six but it still is clear that the redder the bin, the fainter the peak of may well be clusters, but they are not necessarily old globulars. the luminosity function. The peak for the reddest bin occurs at Given their observed colors of Æ—F ~ 0.6 and l/ —£ ~ 0.1 such a faint magnitude (B = 24), that it is safe to say that most and the large measuring errors, they could have ages anywhere of the objects detected are in fact foreground stars, rather than from 1 to 15 billion years. In fact, since Mould (1988) and clusters. Olszewski (1984) have found that such Magellanic Cloud clus- The two-color diagram provides further proof that the color ters as NGC 1754 and 1978 (SWB Type VI clusters; Searle, range occupied by the clusters in NGC 5253 includes examples Wilkinson, and Bagnuolo 1980; Elson and Fall 1985) have of the very bluest and hence youngest clusters known, but few ages less than two billion years, and are in fact somewhat red clusters that might be identified as old. Figure 6 is the redder than the colors just quoted, we feel that the data are two-color plot for clusters in NGC 5253, shown with the locus consistent with no clusters being older than 1 billion years in of LMC clusters plotted as a line. Colors of clusters from NGC 5253. both galaxies have been corrected for a reddening of The cluster system in this galaxy is clearly unique among E(B—V) = 0.06. Clearly, in NGC 5253 there are representa- nearby galaxies, and at the risk of sounding hackneyed, the tives of clusters at every color found in the LMC, including the space telescope’s resolution will allow these clusters to be

Fig. 4.—V luminosity function for all clusters in NGC 5253. Those clusters with B-V > 12 are plotted as a dashed line since it seems likely that they are foreground dwarf stars.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1989ApJ. . .338. .789C © American Astronomical Society • Provided by the NASA Astrophysics Data System sja^sn^) jojaqmnj^; CV2 <0 CM CM CM CM CM CO CM O CM Ci 796 CM CO CM CM CM CM CM O CM Ci CM <0

V Magnitude Fig. 5.—V luminosity function for clusters in NGC 5253 broken up into color bins 1989ApJ. . .338. .789C negative colorgradientinthe haloimpliesthepresenceofa followed byashallowergradient intheouterpart.Sucha maps, thereisastronggradient intheinnerpart(R<40"), rection hasbeenappliedhere. Asisobviousfromthecolor plotted againstdistancefrom thenucleus.Noreddeningcor- B-V colorgradientforthe major axisoffield2,withcolor radius alongthemajoraxiswasextracted.Figure8shows the tion (see§IV).Fromthesecolormaps,therunofwith distribution, ifliketheHaemission,isfairlycircularinprojec- frame, almostcertainlybecausethelargecontributionof the 00 Areenngcorrction than theisophotes.ThisflatteningisnotobservedinU —B phrase popularagenerationago,theisochromesareflatter flatter thanthecontoursofconstantintensity,or,touse a [O it]3727emissionlinetothelightinUband,whose true thatthecontoursofconstantcolorinB—Vframe are may notbeapparentinthereproducedfigures,butitis also very blueclusterscanbeseenfarfromthenucleusaswell. It nuclear regionisapparentfromthesefigures,althougha few displayed. Theconcentrationofyoungblueclustersaroundthe reference, acodeoneachframeindicatestherangeofcolors frames usedinobtainingtheclusterphotometry.Theseare recently fromthegroundforclustersinMagellanicClouds. studied withthesameamountofdetailashasbeenobtained shown inFiguresla-ld(Plates13-16),forbothfields.Asa looked attheopticalandinfraredcolorsofdiffuselight. First, wemadeU—BandB—VcolormapsfromtheCCD the diffusehalolighttoshowevidenceoffact.Wetherefore began throughoutNGC5253sometimeago,wemightexpect described bjMtheLMC^chisters^inthis^agram.^^‘eofE(B—V)=0.°6bäshcenäppliedtothedatainTable1.Solidline isthemeanline If thestarformationcomplexwenowseeinnucleus © American Astronomical Society • Provided by the NASA Astrophysics Data System e) TheDiffuseLightoftheHalo STAR FORMATIONINNGC5253 field 2. Fig. 8.RunofB—Vwithradiusalong themajoraxis.Datatakenfrom 797 1989ApJ. . .338. .789C of field2.(d)U—Bmap2. (/ —ßmapoffield1.Thegrayscaleindicates thecodingforfollowingC/—ßcolorsinorderdarktolight: —0.8, —0.6,—0.4,—0.2,0.0,0.2,and0.4.(c)B—Vmap Caldwell andPhillips{see338,797) Fig. 7.—(a)B—Vmapoffield1.The grayscaleindicatesthecodingforfollowingB—Vcolorsinorderdark tolight:0.0,0.20,0.40,0.60,0.80,1.0,and1.2.{b) © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. la PLATE 13 1989ApJ. . .338. .789C Caldwell andPhillips{see338,797) PLATE 14 © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. lb 1989ApJ. . .338. .789C Caldwell andPhillips(see338,797) © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 1c PLATE 15 1989ApJ. . .338. .789C Caldwell andPhillips(see338,797) PLATE 16 © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. Id 1989ApJ. . .338. .789C 7 9 7 8 9 young stellarcomponent,sincemetallicity-drivencolorgra- in thehalo(45"SWofnucleus),alongwithJHKcolors dients arepositive(redderatsmallradii). densations inNGC5253arelistedTable4.Whereasthe the sameregionkindlyobtainedforusbyJayFrogelon must thereforebeayounger,bluerpopulation.Ifwethen elliptical galaxy-like,bothofwhichhaveredderopticalcolors CTIO 4mtelescope.Thecolorsofthebrightestnuclearcon- 798 than theobservedcolorsofhalo.Thesecondpopulation clearly indicateacompositepopulation.Wepresumethatlike accounted forbyasinglepopulatonof10year-oldstarssuch optical colorsofthenuclearcondensationsareeasily tion, thequestionwethenseektoansweriswhatspecific assume thattheyoungerpopulationhasasmallagedistribu- every othergalaxythathasbeenstudiedsofar,NGC5253 as theSWBTypeIclustersinLMC,colorsofhalo we knowtheyoungerpopulationmustbesignificantlybluer age ofthatyoungerpopulation.Fromtheopticaldataalone, an oldPopulationIIcomponent,eitherglobularcluster-likeor redder thantheobservedcolorsofhalo.Theseclusters than theSWBTypeVIclusters,sincethoseclustersarealso population mustbeyoungerthanthat. have agesaround1x10yr(Mould1988;Hodge1983),sothe NGC 5253halo 4 E 2 SWB3 (0.44)4 +(0.56)2 (0.58)4 +(0.42)1 (0.83)5 +(0.17)1 (0.39)5 +(0.42)2(0.37)3 (0.14)4 +(0.43)2(0.43)3 (0.24)5 +(0.38)2(0.38)3 (0.63)5 +(0.37)2 3 SWB4 to accountfortheobservedcolorsofhalo.Acombination 5MPGC blue. Suchaclusterwouldalsomostlikelyhaveanassociated ruled outimmediately:thederivedopticalcolorsarejust too of SWBclusterTypeIandeithertheoldpopulationcan be H iiregion;noneareseenatsuchlargeradii(see§IV).Thus 1 SWB because thepossibilityofcontinuousstarformationcannot difficult toputanupperagelimitonthesecondpopulation, the secondpopulationmustbeolderthan10yr.Itismore demand somecontributionfromSWBclusterTypesIV(which have luminousredstarsandagesaroundlx10yr), the really beruledoutcompletely.However,theredIRcolors population suchastheTypeIIIclusters.Thelastentryin the table comesclosesttoreproducingtheobservedcolors,given optical colorsrequiresomecontributionfromayetyounger 0.60