1978ApJ. . .226. .559B The AstrophysicalJournal,226:559-565,1978December1 trated clusterscontain,intheircores,onlyelliptical formless clustershaveagalaxypopulationsimilarto is correlatedwiththeirgalaxycontent.Irregular, trated clustershad,incontrasttonearbyobjects,a that ofthe“field,”whileregular,centrallyconcen- Oemler 1974)thatthestructureofclustersgalaxies large populationofgalaxieswiththecolorsspirals. (Butcher andOemler1978)wereportedthatthetwo and SOgalaxies.Inthefirstpaperofthisseries The simplestinterpretationofthisobservationisquite most prominentdistant(z>0.4)centrallyconcen- is equallyimportanttodemonstratethattherelation established. Insubsequentpapersweshallpresent all ofthestepsleadingtothisconclusionbewell about galaxyevolution,itisclearlyimportantthat startling: namely,thattherehasbeenstrongevolution © 1978.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. more dataonthepropertiesofdistantclusters;butit of theuniverse. of galaxypopulationsduringthelastthirdage centration ofaclusteranddetermineitsvaluefor and inthispaperwepresentamorequantitative work onthisphenomenonhasonlybeenqualitative, exact anduniversalamongnearbyclusters.Previous between structureandcontentdescribedaboveis in Astronomy,Inc.,undercontract withtheNationalScience moderately largesampleof clusters. In§IIIwedeter- demonstration. quantitative parameterdescribingthecentralcon- Foundation. It hasbeenknownformanyyears(Abell1965; Because thisresultissocontrarytopresentviews This paperisinthreeparts.In§IIwedevelopa * OperatedbytheAssociationof UniversitiesforResearch © American Astronomical Society • Provided by theNASA Astrophysics Data System nearby clustersateverystageofdynamicalevolution,theseresultsstronglysupportthehypothesis central concentrationofclusters,wefindittobestronglycorrelatedwiththespiralgalaxycontent new dataaswellthosealreadyintheliterature.Devisingasimplenumericalindicatorof in strongcontrasttothedistantclustersdescribedourpreviouspaper.//'weareobserving of anefficientstrippingmechanismincollapsedclusters. of theclusters.Onlyleast-concentratedclustersshowanappreciablecontentspiralgalaxies, Subject headings:galaxies:clustersof—evolutionstructure We havestudiedthestructureandgalaxycontentofmostnearbyrichclustersgalaxies,using I. INTRODUCTION THE EVOLUTIONOFGALAXIESINCLUSTERS.II. THE GALAXYCONTENTOFNEARBYCLUSTERS Received 1978May4;acceptedJune6 Kitt PeakNationalObservatory* Yale UniversityObservatory Augustus Oemler,Jr. Harvey Butcher ABSTRACT AND 559 § IVwediscussthesignificanceofourfindings. mine thegalaxycontentofthesesameclusters,andin unconcentrated orregularandcentrallycondensed. measure concentration,butanygoodmust degree ofregularity,anditisthatqualitywhichwe cluster dependsonwhethertheisirregularand ure tobequitesatisfactory.LetusdefineRthe tions arepossible,wehavefoundthefollowingmeas- and distance.Althoughmoresophisticatedformula- be independentofclustersize,richness,meandensity, Concentration isaneasierthingtoquantifythan distribution. Thenwedefinethecentralconcentration, radius containingnpercentoftheprojectedgalaxy shall investigate.Therearemanypossiblewaysto population convergestoafinitetotal.Workdonein to thecentralclusteritself,which,manystudieshave extends toradiiofseveraltensmegaparsecs.For- recent yearsonthecovariancefunctionofgalaxy tunately, thisveryextendedhaloisofnoconsequence distributions suggeststhattheinfluenceofacluster of thequantityR,whichassumesthatcluster distribution, withlimitingradiioftypically3-6Mpc demonstrated, hasadefinitelyboundedgalaxy background andcannotbe directly determined.One (see Bahcall1977). must thereforeextrapolate, for whichwehaveuseda of mostclustersarelostamong thefluctuationsof n n We wishtodemonstratethatthegalaxycontentofa One possibledifficultywiththisdefinitionistheuse A morepracticaldifficultyis thattheouterbounds II. CLUSTERSTRUCTURE C =log(R/iU.(1) 60 1978ApJ. . .226. .559B that radius. points, tothoseofWhite(1976).Projectedmass been onlydescribedbrieflyinOemler(1973);butthey the radiicontainingvariousfractionsoftotal cluster, hasbeendeterminedempiricallyinamanner suitable. Curvea,whichrepresentsanuncollapsed distributions forthesemodelsatvariousevolutionary are verycloselyrelated,exceptforthenumberofmass series ofnumericalmodelsclusterscalculatedby reliably classifiedonthephotographicmaterialto galaxies aspossible.Inordertobeincluded,acluster extrapolations ofthissize. typically requireextrapolationoutwardfromjRor cluster galaxycontent.Theprofilespresentedbelow to bedescribedlater.Alongeachcurveareindicated collapse, themassdistributioniscompletelydependent represent collapsedclusters.Beforethetimeof stages arepresentedascurvesb-eofFigure1.All Dr. SverraAarseth.Theseunpublishedmodelshave Three nearbyclusterswereexcludeddespitetheavail- class 1clusters,aswelleightmoredistantclusters. in nosenseacompletesample.However,ofthe their redshifts,arelistedinthefirsttwocolumnsof which wehadaccess.Theclustershavestudied,and must eitherhavemulticolorphotometryavailableforit by mistake.Neitherislargeenough orrichenoughtobe A2666 appeartohavebeen included inAbell’scatalog ability ofsuitablematerial.TheclusterA2634isinsuch Table 1.Becauseacluster’sinclusiondependedonthe R. ItisahappyaccidentthatCquiteinsensitiveto on initialconditionsandtheAarsethmodelsarenot 560 considered arichclusterof galaxies. Wehavelimited adequately determineitsstructure. BothA2162and of thedistanceclass0clusters,1119 availability ofsuitableobservationalmaterial,thisis objects inAbell’s(1958)catalog,itdoesincludeboth or mustbenearenoughtopermititsgalaxies a confusedregionofthesky thatitwasimpossibleto 65 80 Fig. 1.—Projectedgalaxydensityprofilesofmodelclusters.Indicatedalongeachprofilearethefractionalcontentswithin We havegathereddataonasmanyrichclustersof © American Astronomical Society • Provided by theNASA Astrophysics Data System BUTCHER ANDOEMLER from thedistributionofgalaxiesinSecondRefer- unambiguous evidencethatatleastsomeareconsider- background density,determinedfromtheouterrings, Vaucouleurs, andCorwin1976),withB<13.0 inch (1.2m)Schmidttelescope,whenavailable,or used plates(usuallyIIIa-J)takenwiththePalomar48 the otherclusters,galaxieswerecountedinrings which weshalldemonstratebelow. this surveytorichclusters—meaning,roughly,those ture andfromourcounts, it wasstrikinglyapparent that thishasnotgrosslyaffected ourdeterminationsof at theendofeachcluster’slisting. population containedwithinthatradius.Thegalaxy background, andthefraction/oftotalcluster are listedtheouterradiusofeachringinarcmin. nearer clusters,ascalewasusedtosetlimiting were counteddowntotheplate/printlimit;butin b >40°.Inafewofthemoredistantclusters,objects else theredprintsofPalomarObservatorySky about acenterdeterminedfromstripcounts.Wehave existed foracluster,wehavenotrepeatedthem.In properties thanrichclustersandprobablywouldnot Poor groupsseemtohaveamuchwidervarietyof with atleastasmanygalaxiestheVirgocluster. and theestimatedvisuallimitingmagnitudearegiven density, />,persquaredegreeaftersubtractionof profiles arepresentedinTables2and3.Incolumn(1) angular sizeofthegalaxiescounted.Theresulting ence Catalog(hereafterSRC;deVaucouleurs, show thestrongcorrelationofstructureandcontent ably flattened.Wecanonly hope, probablyjustifiably, assumed thatallofourclustersarecircular,inspite Survey. TheprofileoftheVirgoclusterwasobtained Subsequent columnsgive,foreachcluster,thegalaxy C. T Where galaxycountsofsatisfactoryqualityalready As hasbeenthealmostuniversalpractice,wehave In examiningtheprofilesobtained fromthelitera- 1978ApJ. . .226. .559B background/V 20.0 40.0 25.0 15.0 10.0 80.0 50.0 30.0 90.0 60.00 35.0 70.0 2.5 5.0 7.5 © American Astronomical Society • Provided by theNASA Astrophysics Data System lim Hercules 603 176 272 368 256 356 310 <15 81 1.00 50 99 130/19.0 75 .78 .05 .02 .01 .23 .10 .64 .56 .37 .69 Abell 168 Abell 154 Virgo Cl. CLUSTERS Abell 400 Abell 194 Perseus Cl. Hercules Cl. Abell 1314 Abell 1228 Abell 1060 Abell 779 Abell 539 Abell 262 Coma Cl. Abell 1413 Abell 1367 Abell 1689 Abell 2197 Abell 2256 Abell 2255 Abell 2199 C10024+1654 4. Baier(1977) 6. RudnickiandBaranowska(1966) 5. Dressier(1976) 2. Oemler(1974) 1. Omer,PageandWilson(1965) 3C 295 3. Bahcall(1974) 170 .13 217 .08 204 .04 327 .01 26 .63 60 21 .30 12 .46 30 <2 71 .27 70 .19 40/18.2 A194 7 .47 .42 .70 0.0230 0.0360 0.0038 0.0183 0.0168 0.0181 0.0449 0.0652 0.0094 0.0267 0.0231 0.0200 0.1427 0.0205 0.0335 0.0334 0.0303 0.1747 0.394 0.0763 0.0312 0.4619 0.0594 102 .01 43 .34 92 .10 22 .72 92 .63 96 .29 95 .18 71 .04 73 .46 21 1.00 18 .89 <2 36/18.2 8 .77 Clusters Studied Cluster Profiles 0.59±.04 0,55±.02 0.30±.04 0.371.05 0.391.05 0.591.03 0.451.03 0.301.03 0.481.06 0.641.04 0.651.05 0.301.04 0.481.05 0.501.03 0.301.03 0.551.05 0.521.04 0.291.03 0.581.04 0.591.04 0.511.03 0.501.04 TABLE 1 TABLE 2 132 .05 232 219 145 .18 131 .10 High 42 .38 40 .49 46 .33 62 .22 27 .57 75 .29 11 .70 19 .65 25/17.8 <4 A400 .01 .03 10. B aier (1976) 9. Bahcall(1973) 8. AustinandPeach(1974) 11. B utcher andOemler(1978) 7. Noonan(1972) * Thispaper Source 193 .04 111 28 24 27 86 62 71 1,2 5 4.5 2,* * 2,* 3 * 2 * 2,* 2,* 2.6 2 2,7,8 A779 2,9 2,* 11 5 10 11 2 .82 7 .77 36/18.1 * .57 .29 .20 .08 .65 .50 .45 4715 19115 1013 4217 1514 15115 3419 5318 1214 1412 1613 5016 1113 1218 59115 50110 2015 2014 3115 %Sp 712 813 <15 916 268 .03 161 112 65 11 17 24 29 .39 11 .29 21 35 39 A1060 7/16.6 7 .07 .27 .18 .13 .69 .65 .49 .25 .21 .58 Types 119 .02 156 .13 180 .07 101 .35 139 .53 <10 64/18.6 A1228 43 .17 61 .98 23 .73 A2197 334 .02 132 150 89 .03 67 97 72 22 79 <5 38 33/18.1 .10 .07 .86 .75 .58 .42 .26 .94 1978ApJ. . .226. .559B 562 trated thaneventheearliestcollapsedAarsethmodel. that theprofilesofmostirregularappearing clusters wereallverysimilarandmuchlessconcen- A compositeprofile,constructedfromtheHercules cluster, A262,A1228,A1367,andA2197,ispresented as acurveofFigure1.Thisprofile,forwhich purposes, since,asmaybeseenfromFigure1,the ary. Thiscurveisincludedmerelyforcomparison sphere (C=0.27),exceptfora“soft”outerbound- mass distributionconvergessorapidlythatno extrapolation wasnecessaryinanyoftheclusters. C =0.30,isverysimilartothatofauniformdensity calculate theconcentrationvaluespresentedincolumn presented inTables2and3.Fromthesefromthe data, weobtainthecumulativeclustercontents,f(R) published datacitedincolumn(4)ofTable1,we true time-averagedclusterdistribution.Onecluster, calculated assumingPoissonfluctuationsaboutthe using ouravailablematerial.However,inspection A1689, wastoodistantforustodogalaxycounts (3) ofthesametable.Theuncertaintieshavebeen is typicaloftheconcentratedclusters. varies withradius,wemustbecarefultodetermineit and weshallhereafterassumethatitsconcentration shows thatitisobviouslyquitecentrallyconcentrated, in aconsistentmanner.Forthesamereasonsof ties; largersampleswouldincludesomeofthespiral- distance, richness,andsizeindependencewhichguided would dilutethecluster-to-clusterpopulationdiffer- rich envelopefoundeveninspiral-poorclusters,and R. The30%radius,,appearstobeagoodchoice: determine galaxycontentwithinafractionalradius asterisks incolumn(6),are closeenoughforusto 4 magfainterthanthebrightest clustermember. always determinethemtoa limitingmagnitudeabout ences. Sincepopulationsarealsoslightlydependent smaller sampleswouldhavelargestatisticaluncertain- our choiceofconcentrationparameter,weshallalways on theabsolutemagnitudes ofthegalaxies,weshall n30 Using theprofilesinFigure1toextrapolateour Because thegalaxycontentofspiral-poorclusters Fifteen oftheclustersinTable 1,thosemarkedwith © American Astronomical Society • Provided by theNASA Astrophysics Data System Background/ Fh, 10 12. 4 2 3 6 5 7 1 8 Virgo ClusterProfile III. GALAXYCONTENT (1) (2)(3) R° Pf TABLE 3 2.190 0.07 0.960 0.65 2.400 0.31 0.280 0.86 0.390 0.77 0.720 0.43 0.105 0.91 0.011 0.95 0.043 0.93 0.042/12.3 0.024 0.97 BUTCHER ANDOEMLER which includedKPNO4mplatesforfiveofthe classified galaxiesusingthebestavailablematerial, determine thegalaxycontentdirectly.Wehave clusters, Palomar48inch(1.2m)Schmidtplatesfor eight oftheclusters,andESO/SRCSouthernSky calculated assumingPoissonstatistics. percent spiralsineachcluster,withtheuncertainties are takenfromtheSRC.Column(5)presents Survey transparenciesforone.InVirgo,classifications classify theirgalaxiesbyinspection.Inthese,asinthe high-redshift clustersinourpreviouspaper,thetypes tions andthencomparingtheobservedclustercolor expected colordistributionsofvariousgalaxypopula- do thisweshallworkbackward,firstconstructingthe of galaxiescanonlybeinferredfromtheircolors.To rich andspiral-poor(seeOemler1974).Spiral-poor populations asbelongingtooneoftwotypes:spiral- distributions withthem. cores ofatleastsomeconcentratedclusters,while populations, aswehavesaidearlier,arefoundinthe the halosofsameclusters,inunconcentrated clusters, andinthe“field.”Typicalproportionsof spiral-rich populationsarefoundeverywhereelse:in from theSRC.AcomparablesampleinPerseushas field. ThereexistsalmostcompleteBVphotometryof photometry ofonlytheComaandPerseusclusters. spirals/SO’s/ellipticals are15/50/35intheformerand this fieldbySandage(1972),Weedman(1975),and galaxies inZwickyetal.’s(1961-1968)central0?5 brightness thatwhichweneedisrepresentedbythe photometry byWeedman(1975)andfromtheSRC. In Coma,asampleapproximatinginbothareaand 55/30/15 inthelatter. in shape,butthatofPerseusisshiftedtothered latitude (—13°),andwehavenoqualmsaboutcorrect- and SO’shaveidentical,verynarrowdistributions;the corrected byafurther—0.06mag,arepresentedin ing thePerseuscolorsbythisamount.Theresults,k amount ofinterstellarreddeningforPerseus’sgalactic Coma’s by0.18mag.Thisisaquitereasonable hatched area,andspiralstheremainder.Theellipticals tion fromphotometryoffieldgalaxies.Thedatawe There areveryfewspirals,andtheyspreadout,as field galaxies;however,also plottedforeachmorpho- have toworkwitharesummarizedinFigure3,which we willhavetoconstructtheexpectedcolordistribu- etry ofaspiral-richclusterlikethatinHercules,and expected, overawidecolorrangetotheblueof Figure 2,whereE’soccupytheshadedarea,SO’s the SRCwithtotalB—Vcolors.Thesearemostly presents thecolordistributionsof1083galaxiesin E’s andSO’s. one blueellipticalisNGC1275,averypeculiarobject. form ameaningfulaverage.) Thereappearstobené in A262,A1367,andtheHercules, Perseus,Virgo,and logical typeare<£—V}and a(B—V)forgalaxies Coma clusters.(Therewere insufficientirregularsto The remainingclustersaretoodistantforusto As afirstapproximationwemayconsiderallgalaxy Among spiral-poorpopulationsthereisadequate The colordistributionsinthetwoclustersaresimilar Unfortunately, wedonothavecomparablephotom- Vol. 226 1978ApJ. . .226. .559B h hm hm No. 2,1978 hatched area,SO’s;openspirals. clusters. galaxies ofagivenmorphologicaltypeinandout of theComaandPerseusclusters.Filledarea,ellipticals; cluster, webeginwithgalaxiesbrighterthanB= significant differenceinthecolordistributionof Virgo cluster.Ourpositionalcriteriaare12 To constructapopulationlikethatofspiral-rich Fig. 2.—CorrectedcolordistributionofgalaxieswithinR 30 Fig. 3.—Colordistributionofvarious morphologicaltypes © American Astronomical Society • Provided by theNASA Astrophysics Data System B-V GALAXY CONTENTOFCLUSTERS histogram, colordistributionofgalaxieswithinRthe Virgo cluster. population; filledarea,spirals;openE’sandSO’s.Line 23% S0/a-Sab,187Sb-Sbc,and197Sc-Irrgalaxies, This sampleof78galaxiescontains40%E+SO, necessary tomaintainthecorrectfractionofspirals. Virgo andthatfraction(£)oftheellipticalsSO’s mate anabsolutemagnitudelimitedsample.Tothis galaxies intheSRCdonothavemeasuredvelocities, group weaddallofthespiralsfrominnerpart members ofthelocalsupercluster)allowsustoapproxi- filled areaandE’sSO’stheremainder.Thisdis- is presentedinFigure4,wherespiralsoccupythe rich clusters.Thecolordistributionofthispopulation selecting objectsnearVirgo(mostofwhichwillbe a mixverysimilartothatinHerculesandotherspiral- the majorityofgalaxiesarespirals,distributionis tribution hastwonotablefeatures.First,eventhough colors asredorredderthantheE/S0’s. approximately one-thirdofthespiralgalaxieshave first chooseacolorsysteminwhichthecolorsof mind, wehaveadoptedthefollowingprocedurefor strongly peakedatthecolorofE’sandSO’s.Also, 30 estimating thespiralcontentofacluster.Onemust In practice,thismeansthatthecolor-magnitudeeffect E/S0’s aresymmetricallydistributedaboutthepeak. ç0 2 ofVisvanathanandSandage(1977).Next,wedeter- must beremoved,whichwehavedonebyusingFigure the peak;anyexcessnumbertobluearecon- mine theratioofgalaxiestoblueandred the colordistributionofspiralsislikethatin to thecontraryforourother clusters,thisapproxima- by 1.5.Thisfactor,ofcourse,willonlybecorrectif spirals, wemultiplythenumberofexcessblueobjects sidered spirals.Finally,totakeaccountofthered tion isthebestwecando.However, itisworthnoting that intheverydifferent spiral-poor populationin Figure 4.Sincewehavenoindependentinformation noticeably differentfromthat inFigure4. Figure 2,thecolordistribution ofthespiralsisnot Fig. 4.—Solidhistogram,colordistributionofaspiral-rich With thecolordistributionsofFigures2and4in 563 1978ApJ. . .226. .559B A1413 andA1689isourownvidiconphotometry from severalsourcesandisofvariablequality.That photometry ofA154,A168,andA2256wasvery mine thespiralcontentofoursampleclusterscomes etry ofA2255byBorngenandKalloglyan(1977).As kindly madeavailableinadvanceofpublicationby will bedescribedinthenextpaperthisseries.The correction forthisitappearsreasonablygood.Follow- published, thisphotometrycontainsagross(~0.3 quite good.Ofsomewhatlowerqualityisthephotom- mag/mag) magnitude-dependentcolorerror,butafter Dr. AlanDressier.Thedataonthesefiveclustersare fractions needsomecomment.Inadditiontothe we obtainspiralcontentswhichareincludedincolumn color distributionsobtainedfromthisphotometry, ing theproceduresdescribedabovetoanalyze errors introducedbysmallnumberstatistics,thereis 564 the peakwelldetermined.If,however,color elliptical andSOgalaxiesifthephotometryhas in thecolordistribution.Ifaclustercontainsonly (5) ofTable1. some errorduetomisdeterminationoftheE/SOpeak determined. InA154andA168itwasdifficultto distribution iswide,dueeithertoalargespiralcontent small errors,thetotalcolorspreadwillbeand large uncertaintiesattachedtothespiralfractionsin photometry wasnotaccuratelyknown,northere decide howmuchofthecolorspreadwasduetoeach these twoclustersreflecttheuncertaininterpretation or tolargephotometryerrors,thepeakwillbepoorly are gratifyinglyunambiguous.Nonearbyclusterof a well-determinedzeropointforthecolorsystem.The of thesetwofactors,especiallyastheaccuracy of thecolordistributionsclusters. sample clusters. The BVphotometrywhichwehaveusedtodeter- The uncertaintiesassignedtotheselatterspiral The resultsfromTable1areplottedinFigure5and Fig. 5.—Spiralcontentversus centralconcentrationin © American Astronomical Society • Provided by theNASA Astrophysics Data System IV. DISCUSSION BUTCHER ANDOEMLER tion. Thusthelong-heldqualitativeimpressionof in thoseirregularclusterswithnocentralconcentra- two verydistantclustersdiscussedinourprevious relation betweenclusterstructureandgalaxycontent Virgo cluster,spiralsarefoundinlargenumbersonly high centralconcentrationcontainsalargepopulation clusters differfromthenearbyones. Table 1.Thereaderisinvitedtoaddthesepoints paper, andtheseresultsareincludedattheendof appears tobealmostuniversallytrue.Wehavealso have beenfoundbyobservationsinpassbandswhich determined theconcentrationandspiralcontentof of spiralsinitscore.Infact,withtheexception containing bluegalaxies.Itispossible,thoughnot regard thisastotallyestablished.Thedistantclusters represented byour21nearbyclusters,wedonot Figure 5inordertoseehowstrikinglythedistant than onein21beforethecontrastwithdistant concentrated clustersmustbeloweredtomuchless established, thatthelimitsonnearbyspiral-rich produces astrongselectioneffectinfavorofclusters are quitefarintheblueemittedframe,which distant clustersaredrawnfromaparentpopulation cluster populationwillbecomesignificant. most straightforwardinterpretationofthesedata concentration isanindicatorofwhetherornota in Figure5areinterestingfortheirownsake.The fact whichisbynomeanscertain,thenthefractionof tion ofCwithspiralcontentthensupportstheidea cluster hascollapsedandrelaxed.Thestrongcorrela- assumes thatthepresenceorabsenceofcentral mechanism. have foundonlyonetransitioncluster,Virgo,which that strippingisresponsibleforthelackofspiralsin trated clusters.Also,itsspiralcontent,althoughhigh, radial populationgradientfoundonlyintheconcen- low, Virgodisplaysthesmoothmassdistributionand spiral-rich collapsedclustersisaninversefunction clusters ofeverystagetheirdynamicalevolution,a has apparentlyrelaxedbutwhichnotyetlostall the highCclusters.Itisinterestingthatweseemto object. Eventhoughitscentralconcentrationisquite of theefficiencyandspeedstripping of itsspirals.Ifweareseeing,amongourobjects, within RoinVirgo.TheVirgocolorsaremuchless tion inFigure4isthecolordistributionofgalaxies clusters. Superposedonthespiral-richcolordistribu- is significantlydifferentfromthatofthespiral-rich from vandenBergh’s(1976)discussionofthe“ane- population, aresultwhichcouldhavebeeninferred losing itsspirals.Itmayalso benocoincidencethat with auniqueexampleofclusterintheprocess skewed towardthebluethanthoseofspiral-rich mic” spiralsinVirgo. it isthenearestmoderately richclusterofgalaxies. ping, andarethereforeof lower meandensity.Itis slowly thanthosewhichhave completedtheirstrip- Clusters inthisevolutionary statehaveevolvedmore 3 Despite theseemingimprobabilitythatourtwo Evolutionary questionsaside,theresultspresented We feelquiteconfidentthatVirgoisatransition The Virgoclustermay,therefore,beprovidingus Vol. 226 1978ApJ. . .226. .559B .1977,Astr.Nach.,298,151. .1977,Ann.Rev.Astr.Ap.,15,505. .1974,Ap./.,187,439. .1965,Ann.Rev.Astr.Ap.,3,1. would bedeficientinthesenewlyformedobjects,and are ascloseVirgo.Inthiscase,ourclustersample possible thattheyarenoticeableasclustersonlyif Baier, F.W.1976,Astr.Nach.,297,295. Bahcall, N.A.1973,Ap./.,186,1179. Austin, T.B.,andPeach,J.V.1974,M.N.R.A.S.,168,591. Abell, G.O.1958,Ap.J.Supply,211. stripping maynotbenearlyaseffectivewewould Butcher, H.,andOemler,A.1978,Ap.J.,219,18. Borngen, F.,andKalloglyan,A.R.1977,Astrophysics,1$,1. otherwise conclude. Dressier, A.M.1976,Ph.D.thesis,UniversityofCalifornia. de Vaucouleurs,G.,A.,andCorwin,H.G. No. 2,1978 Noonan, T.W.1972,A.J.,77,9. Augustus Oemler,Jr.:YaleUniversityObservatory,Box2023Station,NewHaven,CT06520 Harvey Butcher:KittPeakNationalObservatory,P.O.Box26732,Tucson,AZ85726 (Austin: UniversityofTexasPress)(SRC). 1976, SecondReferenceCatalogueofBrightGalaxies © American Astronomical Society • Provided by theNASA Astrophysics Data System GALAXY CONTENTOFCLUSTERS REFERENCES .1974,Ap.J.,194,1. Aarseth fortheuseofhisnumericalclustermodels,to the selectioneffectdiscussedabove. photometry, andtoDr.JerryOstrikerforpointingout van denBergh,S.1976,Ap./.,206,883. Oemler, A.1973,Ph.D.thesis,CaliforniaInstituteofTech- Dr. AlanDressierforsendingushisunpublished Visvanathan, N.,andSandage,A.1977,Ap.J.,216,214. Rudnicki, K.,andBaranowska,M.1966,ActaAstr.,16,55. Sandage, A.R.1972,Ap.J.,176,21. Orner, G.C.,Page,T.L.,andWilson,A.1965,A.J.,70, Zwicky, F.,Herzog,E.,Wild,P.,Karpowicz,M.,andKowal, White, S.D.M.1976,M.N.R.A.S.,177,717. Weedman, D.W.1975,Ap.J.,195,587. We wishtoexpressourgratitudeDr.Sverra nology. 440. nology). C. T.1961-1968,CatalogueofGalaxiesandClusters Galaxies, 6vols.(Pasadena:CaliforniaInstituteofTech- 565