198OApJ. . .236. .4300 9 The AstrophysicalJournal,236:430-440,1980March1 © 1980.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. in thenearbydwarfspheroidals(NorrisandZinn level indeterminingthehistoryofstarformation. in largeclustersmayhaveoccurredasrecently ago. Strongdynamicalinteractionbetweengalaxies, more complexthanwasenvisionedonlyafewyears ing toonlyafew10yearsago.Thisinterpretation gas, andsweepingofgasfromgalaxiesbysupernovae- rich spiralcompanions.Theconsequenttransferof be normalellipticalgalaxieswhichhaveprobably z ~0.4(ButcherandOemler1978). driven windsarealllikelytobeimportantatsome ablation oftheinterstellarmediumbyintergalactic is, ofcourse,contrarytotheclassicanalysesour formed overaverylongtimeinterval,perhapsextend- fresh gashasproducedburstsofstarformationinthe (Disney andPottash1977;Kinman1978)appearto ellipticals. Otherevidenceisnowaccumulatingthat galaxies. BothNGC205(Hodge1973)and1510 Major modificationsinthestellarcontentofgalaxies suffered recenttidalinteractionswithnearbyhydrogen- time scale. the globularclustersand the bulkofolddisk (Butcher 1977)the“old”stellarpopulationwas population formedrapidly on essentiallyacollapse Eggen andSandage1969), which suggestthatboth own galaxy(Eggen,Lynden-Bell, andSandage1962; 1975) andthebarofLargeMagellanicCloud The evolutionofgalaxiesnowappearstobemuch This complexityisevidentevenamongnearby © American Astronomical Society • Provided by theNASA Astrophysics Data System 3 -1 4-1 The main-sequenceturnoffisat{B—V)~0.5orF8equivalentspectraltype.Avarietyof nebulae. However,theupperlimitforstarformationduringpast1Gyris~3x10“M© yr. Thus,theavailableopticalobservationsdonotexcludecompleterecyclingofgaslost low-mass ellipticalgalaxiesareexpectedatlookbacktimesof~5Gyr.Theanomalouslyblue models forcoolerturnoffsareconsideredandfoundtobeexcludedbythedata.Themetallicity demonstrate theneedforgalacticwindsorotherspecialgasremovalmechanismsinelliptical from evolvingstarsis~8x10"M©yr,whichingoodagreementwithcountsofplanetary formation ratecouldhaveoccurredmuchearlier.However,significantchangesinthecolorsof haps 50%ofthegiantlightcouldariseinasignificantlyolderpopulation,andpeakstar continued inM32until~5Gyrago,or10aftertheoldestglobularclustersformed.Per- during stellarevolutionintonewgenerationsofstars.Ultravioletobservationsarerequiredto gE nucleiindicatesanupperlimitfortheturnoffageof6-8Gyr. galaxies nowbeingobservedindistantclustersmaybesuchobjects.Atentativeagedatingfor of M32issolarwithin~0.1dex.Takentogether,theseresultsimplythatmajorstarformation galaxies. Subject headings:galaxies:individual—nucleistellarcontent We performapopulationsynthesisonabsolutespectrophotometryforthecentral31"ofM32. The synthesismodelspredictthattherateofmassreturntointerstellarmediuminM32 GALAXY SPECTRALSYNTHESIS.ILM32ANDTHEAGESOFGALAXIES I. INTRODUCTION stars: evolution Received 1979June18;acceptedSeptember6 Department ofAstronomy,UniversityVirginia Robert W.O’Connell ABSTRACT 430 9 in asingleburst.Hereweperformsuchananalysis ted fromintegratedlightobservationsbyspectral three reasons,apartfromthefactthatitis type galaxieshaveassumedthatthestarswereformed tions. M32isthususefulasacontrolforstudiesof the effectsofmetallicityandyoungstellarpopula- the grosscolordifferentialisattributednottorecent are significantlybluerthanagiantelliptical.While brightest ellipticalgalaxy.First,M32hascolorswhich Virgo clusterwhichexhibitsdefiniteevidenceofrecent galaxies. Inprinciple,thisinformationcanbeextrac- tions isalreadyavailable, and morewillbeforth- that M32isinanorbitwhichpassesthroughthedisk (Faber 1973),nonethelessitprovidesatestofthe case ofNGC4459,ablue-nucleatedSOgalaxyinthe structure ofthestellarpopulationbrightE/S0 interaction hasaffectedstar formationinM32over arm patternofM31byByrd(1977,1978)suggest ability ofspectralsynthesistodiscriminatebetween star formation. on M32,andinacompanionpaperweexaminethe synthesis, althoughmostpreviousstudiesofearly- the last~10years. star formationbuttolowermetallicitythaningE’s objects suchasNGC4459. stellar contentindependentof integratedlightobserva- of M31.Itissomeinteresttoknowwhetherthis It isthereforeofsomeinteresttoexaminetheage Third, M32isnearenough that informationonits M32 isanexcellentcandidateforsuchastudy Second, studiesoftheHidistributionandspiral- 198OApJ. . .236. .4300 1 9 9 tion lines,wherevelocityand aperturesmoothingcan frame forM32isgiveninTable1.Ithasbeencorrected The uncorrectedzeropointof thes.e.d.isA£(À5050)= the standarderrorofmeanaveragedoverall lengths. Thenear-infraredspectralfeaturesofPaperI described inPaperI.Avelocity dispersionof83kms~ have significanteffects, alsobeencorrectedas 9.95 ±0.03.Fluxesattheposition ofstrongabsorp- for reddeningbythemethoddiscussedin§IVbelow. the 91cmCrossleyreflectoratLickObservatorywith important resultofthisworkisthatmajorstarforma- limit of-0.2%thegalaxy’smassinvolvedinstar wavelengths is0.015mag. were notincludedbecauseofbadweather.Thescans likely tobepossiblewithoutfar-ultravioletobservations The analysisiscompletelyindependentofearlier nebula productionrate,whichdeterminestherate were reducedtoabsolutefluxesasdescribedinPaperI. for 50503x10yrs),singlegenera- OurinterpolationoftheCDtracksfor1MandZ= ö © American Astronomical Society • Provided by theNASA Astrophysics Data System BL 3835.. H 4861.. H 4340.. CH 4305.. Nai 5892. CN 4200.. H 4101.. Ca ii3933. CN 3860.. feature isW=AA[1—dex(0.4/)],where TiO 7100.. tudes) aremeasuredwithrespecttoalocally Mg i5175. A ÀisthebandwidthandIindex. of absorptioninthebandpasscenteredon defined continuum.Thetotalequivalentwidth TiO 6180. a Absorptionindices(expressedinmagni- Spectral Index Absorption FeatureIndices TABLE 2 -0.02 M32 0.25 0.02 0.12 0.69 0.23 0.52 0.03 0.23 0.21 0.16 0.06 a Mean gE -0.04 0.27 0.08 0.10 0.77 0.61 0.04 0.26 0.33 0.09 0.44 0.10 O’CONNELL 69 limited toshortburstsandcontinuousstarformation tion likethesolarneighborhood’sandhavecalcu- D. ....G80.7391513 B F6-90.49465 A F0-50.353 the caseofcontinuousstarformation(CSF)witha created syntheticenergydistributionsforsixclusters bined semi-empiricalevolutionarymodelswithob- at aconstantrate.Fortheformercase,wehavecom- rich globularclusterssuchasNGC6356;however, nents: metalpoorandyoungstellarcomponents.The branch evolutionandwhichisthemostimportant mines therelativeratesofmain-sequenceandsubgiant- E K0-20.8618>20 C G0-50.636109 with agesbetween5x10yearsandyears.In sider twoextremecircumstances:starformation library (see§V). this doesnotappeartobeaseriousdeficiencyinour intergroup ratiosareallowedtovary±307. population. Asbefore,adoptedconstraintsforvarious constraint indeterminingtheageofasinglegeneration is aweightassignedonthebasisofobservationalerror tional fluxresidualbetweenthemodelandob- constant rate,wehaveassumedaninitialmassfunc- served compositeHRdiagramsforclustersandhave with anage~15Gyr(DemarqueandMcClure1977). metal-poor componentisthemeans.e.d.obtainedby lations describedinPaperI,ourstellarlibraryfor to includesuccessivelylarger contributionsfromit We donothaveobservationsforrelativelymetal- et al.1978),and,ofcourse,representsapopulation O’Connell (1973)fortheglobularclustersM5and weight 0.5. group ratioconstraints. Details aregiveninPaperIII(O’Connell1980)on lated synthetics.e.d.’sfortwoages:0.2and3.0Gyr. synthesis ofM32containstwospecialtypescompo- at 2.2/xm,whichisbasedontheobservednuclear at thatwavelength.Weassignunitweighttoall by e=1002^12Wt,whereistheabsolutefrac- mixed inanyproportionwith“SMR”Kgiants,and M15. Itcorrespondsto[M/H]~—1.5(Aaronson of agivenpopulationcomponent byforcingsolutions (V —K)=3.15(Frogelet al. 1978).Hereweassign points intheM32s.e.d.exceptanaddeddatapoint served galaxyspectrumattheithwavelength,andw* only thesumoftwotypesissubjecttointer- K giantsinthesolutions.Theseareallowedtobe NGC 4459. Model Group{B-V)Z=0.020.01Iben o To incorporateyoungstellarpopulationswecon- The goodnessoffitasynthesismodelisindicated One otherchangefromPaperIistoincludenormal Besides thestandardsolarneighborhoodpopu- We candetermine“maximum allowable”amounts Age CalibrationforSingleGenerationModels Turnoff CD, TABLE 3 Age (Gyr) Vol. 236 198OApJ. . .236. .4300 No. 2,1980 assumed foregroundreddening.Thethreecurvesarenormal- until eisincreasedby3e/2w*or0.12here.This reddening valuesadoptedfortheanalysis,whichM32is the equivalentinourlinearprogrammingfittingpro- ized atzeroreddening.Theminimainthecurvesyield E{B —V)=0.09. least-squares technique(cf.Bevington1969). cedure toa“3or”upperlimitintheconventional fact thatspectralfeaturesaresensitiveonlytothe using ageneralizedmodelwhichallowsburstsofany for M32’ss.e.d.dereddenedbyvariousamounts method usedinPaperI.Weobtainsynthesissolutions the smallesteisadopted.Themethodbasedon age >3Gyr.Theamountofdereddeningwhichyields stellar mix,whilethecontinuumisalsostrongly affected byreddening. obs that forthecentrallO"ofthree gEgalaxiesintheVirgocluster.TheplotisnormalizedatA5050 (□).Opencircles(O)indicate strong absorptionfeatures.Features whichappear“inemission”ontheplotareweakerinM32 thaninthegEnuclei. / Fig. 1.—Dependenceofthegoodnessfitparameteron Reddening hasbeenestimatedbythesynthesis Fig. 2.—ThedifferenceinAßmagnitude betweenthereddening-correctedenergydistribution for thecentral31ofM32and © American Astronomical Society • Provided by theNASA Astrophysics Data System IV. REDDENINGANDCOMPARISONOFENERGY DISTRIBUTIONS M32 ANDTHEAGESOFGALAXIES minimum eatE{B—V)=0.09whileNGC4459 galaxy isnotimprovedbydereddening.M32showsa exhibits alesswell-definedminimumcenteredat s.e.d. aregiveninFigure1.ThefitforthemeangE with valuesestimatedfrom21cmsurfacedensities foreground stars(McClureandRacine1969)also agreement withvaluesobtainedbyobservationsof E(B -V)~0.10.TheM32isingood which formthemeangEgroup,anditsreddening (D. Burstein,privatecommunication).Ithasbeen adopted forthesubsequentanalysis. excess thusappearstobeinternal.Thisisnotun- expected, sinceNGC4459possessesaconspicuous dust ringsurroundingitsnucleus.Thering,which produces amaximumdecreaseinsurfacebrightness ing to~0.4magaswellaconsiderablybluernear- reddening-corrected s.e.d.’sforM32andthemeangE measuring aperture,nonethelessdustclearlymust eter. Althoughitwasthereforeexcludedfromour10" peratures (cf.O’Connell19766). tures, whiletheinfrareddeficiencyisaconsequence infrared continuumthanthemeangE.Theoverall nucleus. M32exhibitsalargeultravioletexcessamount- NGC 4459observationsmaybefoundinPaperIII. extend throughoutthenuclearregion.Detailsof of A/x~0.3mag(Burstein1978),is~15"indiam- the central10"ofM31withM32.Thedifferenceis mainly oftheshiftgiantbranchtohighertem- excess resultsmainlyfromlowerlineblockinganda spectrum withtheexceptionofBalmerseries. Note thegenerallineweakeningthroughoutM32 undoubtedly aproductofthedifferentaperturesused. by OkeandSchwarzschild(1975)inacomparisonof shift ofthemain-sequenceturnofftohighertempera- color differenceacrosstheobservedbaselineis metallicity inM32,thenoneinfersthattheultraviolet s ~0.7 mag.Thisislargerthanthe0.35magobtained Results forM32,NGC4459,andthemeangE NGC 4459lieswithinafewdegreesofthegalaxies In Figure2weplotthedifferencebetween If theoveralldifferenceshereareattributedtolower 433 198OApJ. . .236. .4300 -1 434 distribution, withthefollowingpertinentresults:(1) The bulkofthestarsformedinaburstwhosemain- tioned above.Withr¡=0.05,theturnoffisat sequence turnoffdependsontheparametermen- from post-giant-branchstarsinarapidcontraction phase. However,themaximumallowablemeanrate 30,000 K)starlightispresent,probablyoriginating (B —V)~0.63.(2)Asmallamountofhot(T> in solarunits.(3)Thereisnoevidenceforasignifi- of starformationsincetheburstis~2x10" cant metal-poor(MP)population.Super-metal-rich Myr, whereListheF-luminosityofgalaxy the modelsbyasignificantmargin.However, (SMR) Kgiantsarepreferredovernormalin about 60%withrespecttothestarsinmodels. galaxies appeartobeenrichedinNaandMgby There islittleevidenceforanaccompanyingCaor the galacticclusterM67)or{B—V)=0.49.This cluster) light.Thereisasharpminimumineformodel were placedonthecontributionofMP(=globular Fe-peak enrichment. B, whichhasamain-sequenceturnoffatF6-9(like 0v and thelargestresidualotherthanatHß(see§VII) model andobservationsexhibitnosystematictrends, overall fit.ShortwardofÀ8400,residualsbetweenthe solution involvesnoMPlightandyieldsanexcellent standard modelslistedinTable4,noconstraints lines andcontinuumwell.SolutionsforCDin- is (model—observed)=0.036magatÀ3400. corporate MPlight(whichisbluerthanstarsatthe turnoff) inordertofitthebluecontinuum;MP contribution ismuchlargerherethanincomparable too redwithrespecttotheobservations. both exhibitsystematicresidualsforÀ>5050,being solutions forthegEnucleiinPaperI.Thesemodels limitations onMPlight,indicatethataturnofflater than F6-9isexcludedunlessalargeMPpopulation ferred foritsexcellentfit.Notethatsubstitutionof more metal-richglobularclusterssuchasNGC6356 is assumed.Evenso,theF6-9turnoffclearlypre- cannot improvetheCandDfitsbecausetheyare A. B. C. D. Model (Gyr)Group In PaperIweexaminedthemeangEnuclearenergy Here weconsideroldburstmodelsforM32.Inthe None oftheothersolutionsinTable4fitsboth Models ClandC2(seeTable5),withmorestringent © American Astronomical Society • Provided by theNASA Astrophysics Data System Age Turnoff Single GenerationSolutionsforM32 4 F6-91.76Excellentfit;noMP 3 FO-23.60Systematicresidualsin 9 G83.53Systematic residualsin 6 G0-52.28UVlinefitpoor;model Y. SINGLEGENERATIONMODELS TABLE 4 continuum. light. too redinIR;MP= continuum; poorK line fit;MP=307. 19%. o Comments O’CONNELL Cl. .. C3. .. C2. . G.... Model (Gyr)Group B1 . turnoff groupineithermodel(SprinradandTaylor main-sequence luminosityfunction. significantly redderthanthestarsinmain-sequence free parameter(seeC3solutioninTable5).Thefitis the choiceofturnoff,weranmodelCwithrjleftasa neither themain-sequenceturnoffnor77param- better thanforC,buteisstill17%higher on theparticulargiant/dwarfconstraintsapplied. F8 areincompatiblewiththedatadoesnotdepend model B.Thus,theconclusionthatturnoffslaterthan eter arespecifiedinadvance;itisa“minimum 1971). assumption” model.Theturnoffisdeducedfromthe off, wealsoranmodelG(seePaperI).Inthismodel, implied 77parameterwasthesameasthatassumedfor luminosity functionofthesolution.Notonlydid point wherealargejumpoccursinthemain-sequence identical toB. solution haveaturnoffatF6-9(seeTable5),butthe yielding goodfitsisnoteworthy.Althoughallmain- model B,andtheGsolutionwasotherwisevirtually none earlierthanF6contributedsignificantlytothe sequence typesupto30A/areallowedinmodelG, is onlyafactorof2smallerthanatG0-5;demanding solution. InmodelB,theluminosityfunctionatF6-9 there arenosignificantchangesbetweenF6andG8. a constantluminosityfunctionforF6-G8increasese by only0.03.Theimplicationisthatwhereasthere a largebreakintheluminosityfunctionnearF6, This suggeststhattheF6-9lightrequiredby quence andnotinsomeindependentcomponentwith observations doesindeedoriginateonthemainse- a similartemperature. in M32becauseonlyafewdatapointsareavailable Table 6.WehavelittleholdontheMstarpopulation 0 for A>7400,andtheseare oflowerprecisionthan the otherdata.ForTable6, wehaveconstrainedthe mise betweenthedynamical M/Ldeterminationsby solution toyieldM/L=5 (solar units)asacompro- Sargent etal.{1911)andFord, Jacoby,andJenner a v v Turnoffnotspecifiedinadvance.Inferredfromjump To testwhetherourchosenvaluefor77hasaffected To furthertestthemeaningfulnessofF6-9turn- The structureofthemainsequenceinmodels A componentbreakdownformodelBisgivenin Variations onSingleGenerationModels Age Turnoff 4a 4 a G0-5 G0-5 G0-5 F6-9 F6-9 TABLE 5 9.41 2.06 7.22 1.70 1.91 Require MP<5%. No constraintonrj. Require noMPlight. Excellent fit;noMP Require noSMRKill standard value. Selected rj~357oof B. light; verysimilarto larger. light; indexresiduals MP =167. 0 Comments Vol. 236 198OApJ. . .236. .4300 No. 2,1980 F6-9 V.... G8 V GO-5 Y.... K5-M0 V.. KO-4 V.... G0-K1 IV. F5-8 IV.. Ml-4 V.... G8-K5 IIP M7 V M5-6 V.... M8 V total luminosityofthemodelisL=138^(0),andits M6 III MO-5 III... M¡L =5(solarunits). (1977). ApartfromtheMstarpopulation,itisessen- near-infrared linestrengthsfortheinner4"ofM32. tially identicaltothestandardBsolutionofTable4. Faber andFrench(1979)estimateM¡L~3from v the dataarecompatiblewithaburstturnofflaterthan V The answerisno,asillustratedinTable7.Herewe population. Thefirstquestiontoaddressiswhether populations (<3Gyrold)areaddedtoanoldburst have allowedtheCSFpopulations,withagesofeither F8 ifyoungpopulationsareallowedinthemodels. V lation light.Noneofthesemodels,however,fitas model C(turnoffatG2)withoutconstraints.Each 3 or0.2Gyr,youngclusterstobeincorporatedin well asmodelB,andallhave2-3aresidualsinthe solution containssignificantamountsofyoungpopu- the datasowellthatadditionaldegreesoffreedom with youngstellarpopulationsadded.ModelBfits and therestofthisdiscussionwillbeformodelB CN 3860andCan3933indices. 3 GyrCSF2.14 0.2 GyrCSF2.26 All youngclusters....1.99 a b Normalizedto10starsintheF6-9Vturnoffgroup.The SMRonly. Here weconsidermodelsinwhichyoungstellar Thus, wefindagainthattheF8turnoffispreferred, © American Astronomical Society • Provided by theNASA Astrophysics Data System Group Components e Added VI. LIMITSONRECENTSTARFORMATION Model C(Age6Gyr)withAddedYoung Solution forModelB(Age4Gyr) a 9.92E +00 4.64E +02 3.72E +01 3.23E +01 1.74E +01 1.00E +01 4.80E +00 8.38E +02 9.60E-01 5.59E +02 1.58E +03 2.52E-03 1.63E +00 Number 1.47E-02 TABLE 6 Populations TABLE 7 6 CSF =5.1%,MP10.3%; CSF =1.8%,MP=14%; 5 x10yrcluster=1.4%, A3620 A5050A8400 systematic residuals systematic residuals A >5800. A >5300. MP =0%. 1 Gyrcluster=12.27, 27.7 0 17.9 Percentage Light 18.7 18.0 4.3 0.9 0.7 4.9 0.1 0.0 0.0 6.1 0.6 0.1 M32 ANDTHEAGESOFGALAXIES Comments 15.6 12.2 35.3 17.2 4.0 0.0 0.3 2.0 0.0 5.7 1.2 0.3 3.4 2.8 35.4 12.6 10.8 4.6 2.7 7.2 2.3 7.0 7.9 0.5 0.7 2.2 4.5 1.6 6 78 2%. introduced byallowingallyoungclustersinthesolu- 7%. 5%. 8 contribution at 78 tion donotresultinasignificantlyimprovedfit.In 56 fact, onlythe5x10yroldclusterisincorporated young clusters,andeisreducedonly2%.Likewise,if Minimum CSF the CSFpopulationsareplacedinstellarlibrary, (0.47 atA5050)ifnoconstraintsareplacedonthe to doso.Qualitatively,thereisthereforenoevidence the solutionsdonotincorporatethemunlessforced _21 that starformationhasoccurredinM32sincethe burst. tion oflightfromit.Typicalresultsareillustratedin amounts ofagivenyoungpopulationcomponent,we 3_1 constrain successivesolutionstoincludealargerfrac- star formation?Toderive“maximumallowable” residuals begintoappearintheUVlineindiceswhen Table 8forthe3GyrCSFcomponent.Significant o component. Systematiccontinuumresidualsappear 27,• Itis37,foreitheroftheyoungclusterswith upper limittothe3GyrCSFcomponentis37,of 37 oftheÀ5050lightiscontributedbyyoung at 57).Usingthecriterionin§IIIabove,3a light ratiosoftheyoungcomponentsandmustscale the resultstoentiregalaxy.Forthispurposewe maximal youngpopulations,werequirethemass-to- ages of2x10oryrs. À5050 light.Forthe0.2Gyrcomponent,limitis Tinsley (1978),whichapplyforupperandlowermass adopt theM/L’sgivenforclustersbyLarsonand included thefactorof2increasewhichtheyfoundto limits of30M©and0.1M,respectively.Wehave 0 be necessarytocorrecttheirtheoreticalratiosob- Vaucouleurs, deandCorwin1977), which impliesatotalVluminositycorrectedfor served values.WetakeVforM32tobe8.21(de two CSFupperlimitsarequitesimilarandaverage extinction of=2.5x10(solarunits). the massofstarsformedinbursts2x10or v years agoare6x10Mand2M©,respec- 0 tively, implyingalessstringent upperlimitforrecent bursts ofstarformation of ~2x10Myr (where therateiscalculated bydividingtheage T has evidentlynotresultedin theformationofmore of theburst).Theinteraction betweenM32andM31 0 ~3 x10~MoyrinM32.Theupperlimitsfor o What quantitativelimitscanbeplacedonrecent To estimatetheamountofmasscontainedinthese The meanratesofstarformationinferredfromthe A5050 Model B(Age4Gyr)with3GyrCSF 2.86 107oKlineresidual;systematic 2.07 7%Klineresidual;systematic 1.81 27oKlineresidual. 1.76 CSFnotselected;standardModel Component Added TABLE 8 B solution;0.5%Klineresidual. residuals A>5500. residuals throughout. Comments 435 198OApJ. . .236. .4300 6 4 9 -1 41 than ~3x10Minstarsor~0.2%ofthetotal 436 identifications ofplanetariesinM32tobe6x10“ production fromplanetarynebulaeinM32?Ford mass ofM32overthelast10yr. and Jenner(1975)haveestimatedthisratefromtheir leave themainsequence.Thisrateis~(j)(clMldt), from thesynthesismodelsbyassumingthatrate M© yr.Thisvalueinvolvesacorrectionfortheun- where isthemain-sequenceluminosityfunctionand observable planetariesinthecentralregionsand of planetaryproductionequalstherateatwhichstars assumes that0.2M©isejectedperplanetary. tary hasamassof0.2M©,thetotalproduction 0 tion isseveraltimeshigherthantherateofmassre- at aget.UsingtheCDisochrones,wefindthat M isthemagnitudeoftopmainsequence turn totheinterstellarmediumfromplanetary the FordandJennercensusofplanetariesisreasonably the main-sequenceturnoff,leadingtoanestimated rate is~0.2<£/i. viz., thebestavailableopticalobservationsdonot nebulae. ThisconclusionwasalsoreachedinPaperI: that moststarsproduceplanetarynebulaeand galaxy. TheexcellentagreementwithFordandJenner’s production rateof8x10“M©yr“forthewhole exclude completerecyclingofgaslostduringstellar value isundoubtedlyaccidental;butitdoessuggest galaxies. Thus,galacticwinds,starformationwitha evolution intonewstellarpopulationsinelliptical complete. tions inM32domarginallyexcludecompleterecycling necessarily havetobeinvokedexplaintheabsence peculiar IMF,orotherspecialmechanismsdonot t AMi/Alog/ ~2.4.Then,assumingthateachplane- (iii) largeresidualsforMg iandNalineswhich the meanmetallicityofgEnucleiishigherthansolar: the galaxyactuallyobservedhere,sincemaximum if allejectedgasaccumulatesinthecentralregionsof of interstellargasinellipticalgalaxies(cf.Bregman tor of4smallerthanquotedabove. is operatingontheinterstellarmediumofearly-type tions ofE/S0galaxiesforunambiguousevidencethat mean rateofstarformationinthoseregionsisafac- (i) thefactthatbestsolutions yieldedeabout25% violet observationsofM32indicatesthatmaximal galaxies. Apreliminaryanalysisofunpublishedultra- higher thane;(ii)adistinct preferenceforSMR some removalmechanismotherthanstarformation t Faber, privatecommunication). smaller thanestimatedfromouropticaldata(S.M. star formationratesmaybeanorderofmagnitude K giantsovernormal ;and,mostimportantly, could notbeeliminatedby any plausiblemodel. 1978 andreferencestherein).However,theobserva- obs 71 The rateofproductioncanalsobeestimateddirectly How dotheselimitscomparewiththerateofgas The maximumallowablemeanrateofstarforma- From modelB,wehave~2x10mag“at Three piecesofevidenceinPaperIsuggestedthat It appearsthatwemustawaitultravioletobserva- © American Astronomical Society • Provided by theNASA Astrophysics Data System VII. THEMETALLICITYOFM32 O’CONNELL BL 3835... HS 4101... Ca ii3933. CN 3860.. CH 4305.. CN 4200.. HjS 4861... Hy 4340... Na i5892. TiO 6180. TiO 7100. Mg i5175. tional error.Hßis3 the observationsis~437atÀ5050.Allowingforun- maximum GKgiantlightcontributionallowableby tied upinolderobjectsiftheinitialmassfunctionat maximum fractionofthegiantpopulationwhichcould earlier timesfavoredtheproductionofverylowmass be significantlyolderthantheF8turnoffstarsis insensitive tothemain-sequenceluminosityfunction to yieldmuchinformationonthehistoryofstar formation priortotheturnoffage.Thisisbecause tually impossibletodistinguishgiantsofdifferent the historyofstarformationinearly-typegalaxies (see, e.g.,BurbidgeandSandage1958)makesitvir- indeed beentidallystrippedofasignificantamount probably requiresdirectobservationsofhigh-redshift masses inintegratedlight.Adecentunderstandingof observations (includingultravioletphotometry)are Unfortunately, synthesisofnearbygalaxiesisunlikely stars. by interactionwithM31.WhileM32hasapparently anomaly, anditsformationwasstronglyinfluenced objects. 0 the outerpartsofmoreluminousspheroidalgalaxies pletely normalforitsinferredoriginalmass(Faber mass, nonetheless,itsenergydistributioniscom- which didnotapplytootherlow-luminosityE/S0 ~50%. Amuchlargerfractionofthemasscouldbe special conditionsprevailedduringM32’sformation mality ofM32’scolors.Thereisthusnoevidencethat galaxies. Itshouldbenotedinthisconnectionthat Sandage andVisvanathan(1978)confirmsthenor- et al1976). also tendtohavebroad-bandcolorsM32’s(Strom bility thatstarformationactuallybeganmuchearlier, this doesnotimplythatmaximal starformationrates star formationcontinuedinellipticalgalaxiesof 1973). Themorerecent,veryextensivesurveyby however, thatsignificantmodifications tothecolors “primeval galaxies”(see,e.g., Meier1976)atlook- occurred thisrecentlyorthat weshouldbedetecting back timesofonly5Gyr (z ~0.4).Itdoesimply, 10-10 Muntil~5Gyrago.Becauseofthepossi- 0 5) StarformationinM32actuallybegan15Gyr This importantissuedeservesamorecarefulstudy. 6) TheageforM32iscorrect,butthisobjectan Thus, itisdifficulttoavoidtheconclusionthatmajor Vol. 236 198OApJ. . .236. .4300 in whichstarformationisjustbeingcompleted. in unexpectedlylargenumbersrelaxedclustersat tinued starformationinspheroidalsystems,and, than atpresent.Thebluegalaxiesnowbeingdetected z ~0.4-1(ButcherandOemler1977;Krön,Spinrad, No. 2,1980 novae-driven galacticwindsultimatelyterminatethis formation predictedcontinuinggasinfallwhichwould and King1977)maybelowermassspheroidalgalaxies and thattheywouldbeseveralmagnitudesbrighter of low-massellipticalsweretakingplaceatz^0.5 produce newstarsevenafter10Gyr.Probably,super- fact, Larson’s(1974a)dissipationalmodelsforgalaxy not becomeeffectiveincompletelysweepinggasout process (Larson19746).Itappearsthatsuchwindsdid than theoldestglobularclusters.Itshouldbestressed younger theturnoff.Thereareotherconsiderable that theageinferredisstronglydependenton of low-massellipticalsuntil~5Gyrago. uncertainties inthisestimateaswell.Aproperanalysis assumed metallicity:thehighermetallicity, thetic stellarspectraaswellmoredetailedabsolute off inthegEnucleiis6-8Gyror2-3timesyounger ply thatmaximalratesofstarformationoccurredfor near (B—V)~0.5orF8equivalentspectraltype, tion continuedinellipticalsuntil7Gyragobutnot ticals (Sandage1973;Crane1975)outtoz~0.5. spectrophotometry. Again,theevidencedoesnotim- of thegE’srequiresgenerationmetal-richsyn- tions, youngstellarpopulations,andarbitrarydwarf/ like thegalacticclusterM67.Modelsforcoolerturn- redshift hasbeendifferentfromthatofthemore nuclei. Itisalsopossiblethattheevolutionaryhistory giant ratios)donotfitaswell.Thisresultisinagree- gE” energydistribution. commonplace gEsystemswhichconstituteour“mean necessarily ifthisformationwereconfinedtothe d{B —V)¡d\ogt<0.2forthemostluminousellip- Larger coloreffectswouldbeexpectedifstarforma- Bell, R.A.1971,M.N.R.A.S.,154,343. Baade, W.1944,Ap./.,100,137. of theverymassivegalaxiesselectedforstudyathigh Bregman, J.N.1978,/l/?./.,224,768. Bevington, P.R.1969,DataReductionandErrorAnalysisfor Allen, C.W.1973,AstrophysicalQuantities(London:Athlone Aaronson, M.,Cohen,J.G.,Mould,J.,andMalkan,M.1978, offs (includingcaseswithlargemetal-poorpopula- gE’s only7Gyr(z~0.6)ago. Butcher, H.,andOemler,A.1978, Ap./.,219,18. Burstein, D.1978,Ph.D.thesis, UniversityofCalifornia, Burbidge, E.M.,andSandage, A.1958,Ap.128, of M32indicatesthatitsmain-sequenceturnoffis Butcher, H.1977,Ap./.,216,372. Theoretically, thereisnostrongobjectiontocon- Our preliminaryupperlimitontheageofturn- The bestobservationalevidenceindicatesthat Press). the PhysicalSciences(NewYork:McGraw-Hill). Santa Cruz. Ap. J.,223,824. 1) Populationsynthesisofabsolutespectrometry 174. © American Astronomical Society • Provided by theNASA Astrophysics Data System x. SUMMARY M32 ANDTHEAGESOFGALAXIES 93-1 _41 pear tobeviablealternativesfortheFstarcomponent, ment withmostearlierspectralsynthesesofM32. the Ca/Mgabundanceratiodoesnotremainconstant (within ~0.1dex).Thereispreliminaryevidencethat horizontal-branch distribution. although wecannotdefinitelyexcludeapeculiar Horizontal-branch starsorbluestragglersdonotap- in asignificantlyolderpopulation,andthepeakstar therefore threetimesyoungerthantheoldestglobular until ~5Gyrago.ThestarsattheturnoffinM32are we findthatmajorstarformationcontinuedinM32 galaxies. as galaxyluminosityincreasesfromM32tothegE formation ratecouldhaveoccurredmuchearlier. clusters. Perhaps50%ofthegiantlightcouldarise elliptical galaxiesareexpectedatlookbacktimesof However, majorchangesinthecolorsoflow-mass turnoff ageof~6-8Gyr.Thisresultislesssecure than forM32andissensitivetothemetallicity analyzed inPaperIindicatesanupperlimitforthe observed indistantclusterscouldbesuchobjects. .1979,Ap./.,228,405. tions, totheDirectorofLickObservatoryforgenerous to theinterstellarmediumbyevolvingstarsis~8x assumed forgEnuclei. winds orotherspecialgasremovalmechanisms. are requiredtodemonstratetheneedforgalactic tions inellipticalgalaxies.Ultravioletobservations Byrd, G.1978,Ap./.,226,70. Byrd, G.1977,Ap./.,218,86. This researchwassupportedinpartbyNSFand helpful commentsonthefirstversionofthispaper. lost duringstellarevolutionintonewpopula- ary nebulaecounts.Thus,theavailableopticalob- rate estimatedbyFordandJenner(1975)fromplanet- models predictthattheamountofmassbeingreturned past 10yrinM32is~3x10“Moratotal Ciardullo, R.B.,andDemarque,P.1977,Trans.Astr.Obs. servations donotexcludeacompleterecyclingofgas of only0.2%themass.However,synthesis Demarque, P.,andMcClure,R. D.1977,inTheEvolutionof Crane, P.1975,Ap.J.{Letters),198,L9. Cohen, J.G.1978,Ap./.,221,780. Christensen, C.G.1972,Ph.D.thesis,CaliforniaInstituteof NASA grants. amounts ofobservingtime,andtoS.M.Faberfor de Vaucouleurs,G.,and A.1972,Mem. ^5 Gyr.Theanomalouslybluegalaxiesnowbeing 10Moyr~. Thisisingoodagreementwiththe 0 2) ThemetallicityofM32isapproximatelysolar 3) UsingtheCDisochronesforsolarabundance, 4) Anextensionoftheage-datingtogEnuclei I amgratefultoR.T.Roodforusefulconversa- 5) Theupperlimitforstarformationduringthe Technology. R.A.S., 11,Part1. p. 199. R. B.Larson(NewHaven:Yale UniversityObservatory), Galaxies andStellarPopulations, ed.B.M.Tinsleyand Yale Univ.,35(CD). 439 198OApJ. . .236. .4300 440 — .1973,Ap.J.,179,731. Disney, M.J.,andPottasch,S.R.1977,Astr.Ap.,60,43. de Vaucouleurs,G.,A.,andCorwin,H.G. Eggen, O.J.,andSandage,A.R.1969,Ap./.,158,669. Eggen, O.J.,Lynden-Bell,D.,andSandage,A.R.1962, Ford, H.C.,Jacoby,G.,andJenner,D.C.1977,Ap.J.,213, Faber, S.M.,andFrench,H.1979,preprint. Faber, S.M.1972,Astr.Ap.,28,109. Ford, H.C.,andJenner,D.C.1975,Ap.J.,202,365. Faber, S.M.,Gaskeil,C.andBurstein,D.1979,Bull. Johnson, H.L.1966,Ann.Rev.Astr.Ap.,4,193. Iben, I.1967,Ap.J.,147,624. Frogel, J.A.,Persson,S.E.,Aaronson,M.,andMatthews,K. .19746,M.N.R.A.S.,169,229. Hodge, P.W.1973,Ap.J.,182,671. Kron, R.G.,Spinrad,H.,andKing,I.1977,Ap.J.,217, Kinman, T.D.1978,A.J.,83,764. Larson, R.B.1974ö,M.N.R.A.S.,166,585. Station, Charlottesville,VA22903 Robert W.O’Connell:DepartmentofAstronomy,UniversityVirginia,PostOfficeBox3818, (Austin: UniversityofTexasPress). Ap. J.,136,748. AAS, 11,431. 1976, SecondReferenceCatalogueofBrightGalaxies 18. 951. 1978, Ap.J.,220,75. © American Astronomical Society • Provided by theNASA Astrophysics Data System O’CONNELL .1978,Ap.J.,219,46. .19766,Ap.J.{Letters),203,LI. .1976a,Ap.J.,206,370(PaperI). .1980,inpreparation(PaperIII). Larson, R.B.,andTinsley,B.M.1974,Ap.J.,192,293. Norris, J.,andZinn,R.1975,Ap.202,335. Pritchet, C.1977,Ap.J.Suppl.,35,397. O’Connell, R.W.1973,A.J.,78,1074. Sandage, A.R.,andEggen,O.J.1969,Ap.J.,158,685. Sandage, A.R.1973,Ap.J.,183,711. Rood, R.T.1973,Ap.J.,184,815. Oke, J.B.,andSchwarzschild,M.1975,Ap.J.,198,63. McClure, R.D.,andRacine,1969,A.J.,74,1000. Sandage, A.R.,andVisvanathan,N.1978,Ap.J.,225,742. Mould, J.R.1978,Ap.J.,220,434. Meier, D.L.1976,Ap.J.,207,343. Sargent, W.L.W.,Schechter,P.L.,Boksenberg,A.,and Spinrad, H.,andTaylor,B.J.1971,Ap.Suppl.,22,445. Strom, K.M.,andS.E.1970,Ap.J.,162,523. van denBergh,S.1975,Ann.Rev.Astr.Ap.,13,217. Strom, S.E.,K.M.,Goad,J.W.,Vrba,F.J.,and Williams, T.B.1976,Ap.J.,209,716. Shortridge, K.1977,Ap.J.,212,326. Rice, W.1976,Ap.J.,204,684.