197 6ApJ. . .206. .883V The AstrophysicalJournal,206:883-887,1976June15 for galaxieswithverydifferentphysicalcharacteristics. celebrations oftheRoyalGreenwich Observatory. flattest edge-ongalaxiesknown.Acounterexampleis National ScienceFoundationof theU.S.A. KPNO isoperatedbyAURA Inc. undercontractwiththe large nuclearbulges,andontheotheritcontains to NGC1302{HubbleAtlas,p.9),whichhavequite provided byNGC5866{HubbleAtlas,p.6)whichis The mainproblemareasappeartobethefollowing: is describedinTheHubbleAtlasofGalaxies(Sandage introduced amoreorlesshypotheticaltransitional the simplebeautyofclassificationschemepro- roots intheclassicalpaperofHubble(1926).Much an edge-onsystemoftypeS0which,intheabsence had thefeelingthatsometypesofgalaxiesdonotfit posed inthatpaperwaslostwhenHubble(1936) On theonehandthisclasscomprisesobjectssimilar of athinabsorptionlane,wouldhavebeenclassified SOi galaxyNGC4762{HubbleAtlas,p.8)isoneofthe naturally intotheframeworkprovidedbythisscheme. spiral galaxies.ThesubsequentevolutionofthisSO to theclassificationofgalaxiesIhave,formanyyears, classification typeinthehandsofHubbleandSandage stage, whichhecalledtypeSO,betweenellipticaland © 1976.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. E5orE6. 1961) isnotasequenceofincreasingflattening.The 1961). 3 The classificationsequencefromSOitoS0(Sandage t Guestobserver,1975,KittPeak NationalObservatory. The classificationtypeSahasbecomearepository In attemptingtoapplytheHubble-Sandagesystem * Based,inpart,onareviewpresented attheTercentenary Current thinkingongalaxyclassificationhasits 3 I. WHYANEWCLASSIFICATIONSYSTEMISNEEDED © American Astronomical Society • Provided by theNASA Astrophysics Data System parallel sequenceswithinwhich“early”and“late”systemsaredistinguishedbymeansoftheir thé evolutionofflattenedgalaxies. lenticulars (S0a-S0b-S0c)aretentativelyinterpretedintermsoftheinfluenceenvironmenton Subject heading:galaxies:structure disk-to-bulge ratios. have characteristicsthatareintermediatebetweenthoseofvigorousgas-richnormalspirals and gas-poorsystemsoftypeSO. 3) Thedifferencesbetweennormalspirals(Sa-Sb-Sc),anemic(Aa-Ab-Ac),and 2) Asequenceof“anemicspirals,”whichoccurmostfrequentlyinrichclusters,isfoundto 1) Anewgalaxyclassificationsystemisproposedinwhichnormalspiralsandlenticularsform b) GalaxiesoftypeSa a) Lenticulars A NEWCLASSIFICATIONSYSTEMFORGALAXIES* David DunlapObservatory,UniversityofToronto Sidney vandenBergh| Received 1975October6 ABSTRACT 883 that areintermediatebetween thoseofSOgalaxiesand flattened gas-richgalaxies suggests thattheremight exist aclassofflattenedobjects withcharacteristics of bothhighlyflattenedgas-free systemsandsimilarly primarily intheirtotalgascontentandhencethe parallel sequences.Thesetwosequencesdiffer exhibit astrongdisplayofPopulationI,andSO important resultsuggeststhatnormalspirals,which have thesamedistributionoftrueaxialratios.This galaxies ofsimilarHubbletype.Noaccountistaken mean ageoftheirstellarpopulations. Theexistence galaxies (whichcontainfewifanyyoungstars)form galaxies andofspiralsSandage,Freeman,Stokes cation schemesthatarepresentlyinuse. many galaxiesinrichclustersdiffersfromthatoffield graphs providedbytheglasscopiesofThePalomar (1970) concludethatspiralandlenticulargalaxies of suchdifferencesinanythemajorgalaxyclassifi- the effectsofdifferinggalaxyluminosity. galaxies suchasNGC4866{HubbleAtlas,p.11)which Sky Surveyhasmadeitclearthattheappearanceof the originalHubbleclassificationschemetoinclude come byvandenBergh(1960a,b,c),whoextended was definedintermsofsupergiantgalaxies.Thisbias common toallofthesediversetypesobjectsisthat have averysmallnuclearbulge.Theonlyfeature star formationisnotproceedingveryvigorously. of theHubblesystemhas,tosomeextent,beenover- From adetailedstudyoftheflatteningSO The largehomogeneoussampleofgalaxyphoto- The Hubble(1936)classificationsystemforspirals d) EffectsoftheClusterEnvironment II. PROPOSEDREVISIONOFTHE HUBBLE-SANDAGE SYSTEM c) EffectsofLuminosity 197 6ApJ. . .206. .883V incorporates asequenceofanemicspirals,isshownin referred toas“anemic”spirals. without adiskcomponentarecalledellipticals much starformationispresentlytakingplace,willbe normal spirals.Suchgas-poorobjects,inwhichnot (Sa-Sb-Sc), anemicspirals(Aa-Ab-Ac),andgas-freelentic- entirely ontheirdisk-to-bulgeratios.Thesedisk-to- (E0-E6). TheconventionalHubbleclassificationof ulars (S0a-S0b-S0c). proposed inthispaperwiththatofHubble(1936).Thenew relative importanceofthetwomaincomponentsin bulge ratiosare,inaverycrudeway,relatedtothe normal spiralsisreplacedbyasystemwhichbased Figures 1and2.Inthisprovisionalsystem,galaxies concentration oflight,whichMorgan(1958,1959) disk-to-bulge 'ratioisalsorelatedtothenuclear man 1970;Sándage,Freeman,andStokes1970).The galaxies, i.e.,theinnerspheroidalcomponentand system comprisesparallelsequencesofnormalspirals 884 outer exponentialdisk(deVaucouleurs1959;Free- has usedasaclassificationparameter. A newclassificationschemeforgalaxies,which Fig. 1.—Comparisonoftheprovisionalclassificationsystem © American Astronomical Society • Provided by theNASA Astrophysics Data System EO E3E6z'"nSaSbSc PROPOSED CLASSIFICATIONSYSTEM i 1T(so)—hHh HUBBLE CLASSIFICATIONSYSTEM Fig. 2.—Schematicrepresentation oftheproposednewgalaxyclassificationsystem VAN DENBERGH SOa SObSOc into typesSOa,SOb,andSOc.Itshouldbeemphasized Ab, andAc.SimilarlySOgalaxieswerealsodivided portance ofthecentralbulgeanddisksub- related totheHubble-SandageclassificationS0, that thissubdivisionofgalaxiesclassSOisinnoway tion wasusedtodefinetheanemicspiralsoftypeAa, very differentfromthatoriginallyenvisagedbyHubble type S0areprobablymorecloselyrelatedtothe 2. MostofthegalaxieswhichhaveHubble-Sandage and S0,whichdoesnotdependontherelativeim- bulge ratioslargerthan10asSc.Ananalogousdefini- type betweenellipticalsandspirals.Inthescheme (1936). HubblethoughtofSOgalaxiesasatransitional galaxies intheclassificationsystemoutlinedaboveis systems. ManyofthegalaxiesHubble-Sandagetype have beenclassifiedSa;thosewithdisk-to-bulgeratios have apparentdisk-to-bulgeratiosintherange1to3 in theirgascontentandhencerateofstar illustrated inFigures1,2,and3,lenticulargalaxies disk-to-bulge ratioareshowninFigure3. S0a-S0b-S0c sequencesinadiagramofcolorversus anemic spiralsthantheyaretotrueSOsystems. S0i fallontheS0a-S0b-S0csequenceinFigures1and of 3to10wereclassifiedSb;andthosewithdisk-to- formation. and spiralsformparallelsequenceswhichdifferonly the photographsinHubbleAtlasandglasscopies cribed above.Theclassificationsweremadeusingboth galaxies inTheHubbleAtlasofGalaxies(Sandage to definetheRevisedDavidDunlapObservatory classifications, whicharelistedinTable1,willbetaken scheme proposedinthispaper,alloftheNGC (RDDO) system. of theblueplatesPalomarSkySurvey.These of thissystemarestillquiteprovisional.Inparticular 1961) havebeenreclassifiedinthenewsystemdes- l52 3 3 It shouldperhapsbeemphasizedthattheroleofSO The positionsoftheSa-Sb-Sc,Aa-Ab-Ac,and In ordertobetterdefinethenewclassification Objects which,ontheprintsofHubbleAtlas, It shouldprobablybeemphasizedthatsomefeatures Vol. 206 197 6ApJ. . .206. .883V unambiguously classified.InTable1allclassifications it isnotyetentirelyclearhowthesequenceofbarred theses. No. 3,1976 of SBOgalaxieshavethereforebeenplacedinparen- SO systemsofdifferingdisk-to-bulgeratiocanbe were assignedtotheAa-Ab-Acsequence. tion ofPopulationIhavebeenassignedtothe free galaxieswithoutevidenceforapatchydistribu- one. Forpurposesofclassificationbothdustyanddust- some youngPopulationIstarsseemedtobepresent, S0a-S0b-S0c sequence.Flattenedobjects,inwhich from thatoffieldgalaxies.ThepreponderanceE galaxian populationofrichclustersdifferssignificantly NGC RevisedType 1087 ScIII: 1068 SbpII:/Scp: 1084 ScI-II 1073 S(B)cII 2217 (SOBb) 1832 ScII 1398 S(B)bI 1300 SBbI 1097 S(B)bI-II 2366 IrIV-V 1302 A(B)bII: 1232 ScI 1201 SOa 1156 IrIV 1964 SbII 1637 SepII 404 EO 628 ScI 488 Sb"I 615 SbII-III 598 ScII-III 309 ScI-II 524 SOa 520 Pec 253 Sep 224 SbI-II 205 E6:(t?) 127 Aa: 925 S(B)cII-III 210 Sb"I 130 E5 128 SOa(t)p/SOb(t)p 972 Sbc/Abc? 751 E0 185 dEl 175 SBbI 891 Sb/Sc 750 Elt 718 Ab11: 157 ScI The transitionfromtypeSOtoAisacontinuous A numberoflinesevidencesuggestthatthe 45 SIV-V 23 Sbp © American Astronomical Society • Provided by theNASA Astrophysics Data System III. GASDEPLETIONINCLUSTERGALAXIES + + + 2950 (SOBa) 2903 ScI-II 2811 SbII-III 2775 SaorSb"11 2685 E6+Stt 2681 AbIII: 2523 SBbI 2403 ScIII 3115 SOa 3077 E2p 3065 Aa/Ab: 3031 SbI-II 2859 SBa: 2855 Aa 2525 S(B)cpII? 3359 S(B)cII 3329 Sa/Aa 3185 S(B)bIII: 3147 SbI-II: 3109 IrIV-V 3081 SbIII: 3034 Pec 3032 Ab 2841 Sb"I 3627 Sb(t?)II: 3351 S(B)bII 3245 E5/S0a 3145 SbI 3623 SbnII: 3521 SbII 3377 E5 3368 Sbp 4062 SbII:/Sc: 3810 ScI 3718 Abt? 3705 SbII-III 3556 Sc 3504 S(B)bII: 3367 ScI 3898 Sb"II 3672 ScII 3511 Sc NEW CLASSIFICATIONSYSTEMFORGALAXIES GALAXIES THATAREILLUSTRATEDINTHEHUBBLEATLAS REVISED DDOCLASSIFICATIONTYPESFORALLNGC TABLE 1 clusters, suchasVirgo,andthoseinthegeneralfield been knownformanyyears(cf.SpitzerandBaade and SOgalaxiesinverydenseclusterssuchasComahas + between spiralgalaxiesintheVirgoclusterandtheir are somewhatmoresubtle. + type (DaviesandLewis1973). times thatobservedinnonclusterspiralsofthesame gas inVirgospiralsofagiventypeisonly0.5±0.1 counterparts inthefield: galaxies inthefield(vandenBergh1975).Thefact systematically redderby0.04magthanaresimilar 1951). Thedifferencesbetweengalaxiesinmoreopen + NGC 4214 IrIII-IV4736Sb"pII: 4150 E2/S0a4725S(B)bI 4111 S0a/S0b4710Ab 4088 ScII:4699Ab11 4293 Ac:III:5055SbII 4254 ScI4800Sb:III: 4244 SIV:4793ScIII 4237 SbIII:4762SOb 4216 SbII4753E3p 4215 AbIII:4750Sb 4394 SBbII5204Sc/IrIV 4378 AbII:5195Pec(t) 4321 ScI5194Sc(t) 4314 (ABb)5128EOp 4303 ScI5101(ABb)? 4278 El5005Sb"II 4274 SbII-III4941Ab/AcIII: 4262 SB0a4866Ab111: 4258 Sb(t?)p4826AbII 4395 SIV-V5236ScI-II 4449 IrIII5364SbpI/ScpI 4433 Sb/Sc5273Aa/AbIII: 4406 E35248ScI 4457 A(B)b5457ScI 4450 AbII5383SBb 4565 SbI:5962Sell 4526 S0(B)b5866Aap 4486 El5614Sbt 4459 SOa:5566Abt 4579 A(B)bII/S(B)bII6384SbI 4569 AbI-II/AcI-II6181ScI 4548 A(B)bII5907Sc: 4593 SBbII6814SbI 4580 AcIII:6643ScT-II 4643 (ABa) 7392 SbII: 4612 Ep?7217Sb"II 4594 Sa6951S(B)bI-II: 4697 E4 7743 Ab 4691 (ABb)? 7741 SBcII 4684 SOa 7640 A(B)bII: 4656 Sett 7457 SOa 4636 E1/SOa 7331 SbI-II 4631 Sc7314II i) Theaveragesurfacedensityofneutralhydrogen ii) Atagivendisk-to-bulgeratio,Virgospiralsare The followingsystematicdifferencesareobserved Revised Type a) GasinSpirals NGC Revised Type 885 00> 00 KO CMO 886 VAN DEN BERGH Vol. 206 ^0 ft TABLE 2* KO r" Galaxies Showing [O ii] A3727 Emission SO Galaxies E Galaxies Type (percent) (percent) Noncluster galaxies 27 11 Virgo cluster 13 10 Dense clusters 0 0 * From Davies and Lewis 1973.

unusually low surface brightness and exhibits remark- ably diffuse spiral arms. The structural peculiarities of this galaxy are very reminiscent of those exhibited by NGC 4450, NGC 4569 (M90), and NGC 4579 (M58) in the Virgo cluster. In a color versus disk-to- bulge ratio diagram (van den Bergh 1976) the Coma Fig. 3.—Positions of the S0a-S0b-S0c, Aa-Ab-Ac, spiral NGC 4921 falls among the most extreme Sa-Sb-Sc sequences in a color versus disk-to-bulge ratio examples of the “anomalously reddened” Virgo diagram. This schematic figure is based on the classifications cluster galaxies. This suggests that objects in the dense listed in Table 1, and the intrinsic galaxy colors given by de core of the Coma cluster might suffer from even more Vaucouleurs and de Vaucouleurs (1964). severe gas depletion than do those in the Virgo cluster. that Virgo cluster galaxies of a given Hubble type are redder than their counterparts in the field has also b) Gas in Ellipticals been noted by Holmberg (1958) and by de Vaucouleurs, de Vaucouleurs, and Corwin (1972). The below-average gas abundance in cluster spirals iii) Inspection of photographs of Virgo cluster has its counterpart in the low gas abundance which spirals shows that many (but not all !) of them have a appears to be indicated for E and SO galaxies in rather anemic appearance which is, no doubt, due to clusters. According to Davies and Lewis (1973) the fact that they are forming stars less vigorously [O n] A3727 (see Table 2) occurs far less frequently than are typical galaxies of the same type in the field. in galaxies of types E and SO that are located in clusters Since the rate of star formation f cc pn (Schmidt 1959), than it does in galaxies of similar types that are situated the anemic appearance of many Virgo spirals is in the general field. Their data (which should be probably a result of the fact that they are gas-poor. checked by new homogeneous observations) suggest The low rate at which young blue stars are presently that ellipticals and lenticulars in the Virgo cluster have being formed also explains why Virgo spirals are, in characteristics that are intermediate between those of the mean, redder than are field galaxies of similar galaxies in very dense clusters and galaxies in the field. types. A number of other lines of evidence, which are dis- Two of the most extreme examples of Virgo cluster cussed in van den Bergh (1975), also support the view galaxies that are too red for their disk-to-bulge ratios that elliptical galaxies outside rich clusters contain are NGC 4941 and NGC 4866, which are illustrated more gas than do those in clusters. on pages 10 and 11, respectively, of The Hubble Atlas of Galaxies (Sandage 1961). These objects are good IV. DISCUSSION examples of members of the anemic spiral sequence. NGC 4941 is classified Ab/Ac III: and NGC 4866 as On the basis of the evidence presented in the pre- Ab III : on the Revised DDO System. vious section one might expect gas-poor lenticulars It should perhaps be emphasized that not all spirals and anemic spirals to be more common in the Virgo in the Virgo cluster are anemic. The Sb+ I galaxy cluster than they are in the field. This suspicion is NGC 4501 is a good example of a perfectly normal vigorous spiral in (or projected on) the core of the TABLE 3*t Virgo cluster. Its neighbor NGC 4548, which is of Relative Frequencies of Lenticulars, Anemic Spirals, and type A(B)b II, is a fine example of an anemic spiral. Normal Spirals of Similar Disk-to-Bulge Ratio This juxtaposition of anemic and vigorous spirals might be due to projection effects. Alternatively it may SOb Ab Sb be assumed that some of the (relatively rare) normal (%) (%) (%) «.a spirals in the cores of rich clusters are on orbits that Field 0 20 80 56 have only recently penetrated into the dense central Virgo 14 57 29 14 regions of these clusters. Figure 4 (Plate 12) shows a photograph of the * From van den Bergh 1976. anemic spiral NGC 4921, which is the brightest spiral t Data for normal systems and for barred systems of a given galaxy in the Coma cluster. This object has an classification type have been combined.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 197 6ApJ. . .206. .883V 2-3 -48 confirmed byananalysisofthefrequencyclassifica- No. 3,1976 collected inTable3showthat,atagivendisk-to-bulge tion typeslistedinTable1.Theseresults,whichare whereas only20percentofthegalaxiesthatarenot ratio, 71percentofallVirgoclustergalaxiesinThe Hubble Atlasareofthegas-poortypesSObandAb, in gascontentbetweengalaxiesofthesamedisk-to- be accountedforintermsofmasslossbyasupernova- bulge ratioinsideandoutsidetheVirgoclustercannot situated intheVirgoclusterbelongtothesetypes. is sobecausethestrengthofsuchawindshouldbe independent ofthelocationagalaxyinspace. driven stellarwind(MathewsandBaker1971).This that suchgaslosswilltakeplaceprimarilyfromthe (Gunn andGott1972).Zasov(1975)haspointedout for theexcessrednessofgalaxiesinVirgocluster might possiblybeunderstoodintermsofthesweeping clusters waslostduringtidalencounters.Furthermore, course, beassumedthatsomegasfromtheouterparts outer regionsofgalaxies.Alternativelyitmight, out ofinterstellargasinVirgospiralsbyrampressure by SpitzerandBaade(1951),removetheinterstellar collisions betweengalaxiescould,aswasfirstsuggested of thehydrogendisksspiralgalaxiesindense gas fromafewgalaxieswhichhaveorbitsthatpass pressure differencebetweenthecoolgasindisksof through thedensecoresofrichclusters.Finally cluster spirals(«#10cm,T£K)andthe cantly. hot intragalactic{næ10,TäK)gasinclusters might affecttheevolutionofclustergalaxiessignifi- .1936,TheRealmoftheNebulae(NewHaven:Yale Freeman, K.C.1970,Ap./.,160,811. Davies, R.D.,andLewis,B.M.1973,M.N.R.A.S.,165,231. Hubble, E.P.1926,Ap./.,64,321. Holmberg, E.1958,Medd.LundAstr.Obs.,Ser.2,No.136. de Vaucouleurs,G.,A.,andCorwin,H.G. de Vaucouleurs,G.,andA.1964,Reference Gunn, J.E.,andGott,R.1972,Ap./.,176,1. de Vaucouleurs,G.1959,HandbuchderPhysik,53,311. Sidney vandenBergh:DavidDunlapObservatory,Box 360,RichmondHill,Ontario,L4C4Y6,Canada Mathews, W.G.,andBaker,J.C.1971,Ap./.,170,241. It shouldperhapsbeemphasizedthatthedifference A hydrogendeficiencyofthetyperequiredtoaccount Press). University Press),p.45. 1972, AJ„77,285. Catalogue ofBrightGalaxies(Austin:UniversityTexas © American Astronomical Society • Provided by theNASA Astrophysics Data System NEW CLASSIFICATIONSYSTEMFORGALAXIES REFERENCES some fieldEandSOgalaxiescouldbeduetoasuper- nova-driven stellarwindortooccasionalexplosions which ejectgasandtriggerburstsofstarformation. it mightpossiblybekeptgas-freebysupernova-driven Once mostofthegashasbeenejectedfromagalaxy, stellar winds. the geneticheritageprovidedbyinitialmassand the presentmorphologyofgalaxiesispredestinedby angular momentum.Theresultsdiscussedabovesug- gest thattheevolutionofgalaxiesisalsosubstantially initial collapseiscompleted,mosthighlyflattened conditions determinethepresentdisk-to-bulgeratio affected byenvironmentalfactors.Probablyinitial their disks.Ifallofthisgasisretained,itwillbecomea of spiralsandtheflatteningellipticals.After probably becomeananemicspiral.Finallyaflattened normal spiral.Ifsomegasissweptout,thegalaxywill galaxies willhaveasignificantamountofgasleftin galaxy, fromwhichallofthegashasbeenremoved, will becomealenticular. by hotintergalacticgas.Thisresearchwassupported evolution ofclusterspiralswillprobablybeaffected by agrantfromtheNationalResearchCouncilof .1976,inpress.* .1960c,Pub.DavidDunlapObs.,2,159. Canada. .19606,ibid.,p.558. .1959,ibid.,71,394. van denBergh,S.1960«,Ap.J.,131,215. Spitzer, L.,andBaade,W.1951,Ap.J.,113,413. Schmidt, M.1959,Ap.J.,129,243. Sandage, A.R.,Freeman,K.C.,andStokes,N.R.1970, Zasov, A.V.1975,SovietAstr.—AJ,18,426. Sandage, A.R.1961,TheHubbleAtlasofGalaxies(Washing- Morgan, W.1958,Pub.A.S.P.,70,364. The relativelylowgasdensitywhichisobservedin Canonical viewsongalaxyevolutionsuggestthat I thankJimGunnforpointingouttomethatthe Ap.J., 160,831. ton: CarnegieInstituteofWashington). V. CONCLUSIONS 887 197 6ApJ. . .206. .883V is classifiedA(B)bIontheproposed newclassificationsystem. van denBergh(seepage886) NGC 4921intheComacluster.Thisgalaxy,whichisintermediate betweenanormalanemicspiralandbarredspiral, Fig. 4.—45min(IIIaJ+GG385)exposure(northattop)obtained withthe4mMayalltelescopeofanemicspiralgalaxy © American Astronomical Society •Provided bythe NASAAstrophysics Data System • '# PLATE 12 . «