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197 6Apj. . .206. .883V the Astrophysical Journal, 206:883-887 .883V .206. The Astrophysical Journal, 206:883-887, 1976 June 15 . © 1976. The American Astronomical Society. All rights reserved. Printed in U.S.A. 6ApJ. 197 A NEW CLASSIFICATION SYSTEM FOR GALAXIES* Sidney van den Bergh| David Dunlap Observatory, University of Toronto Received 1975 October 6 ABSTRACT 1) A new galaxy classification system is proposed in which normal spirals and lenticulars form parallel sequences within which “early” and “late” systems are distinguished by means of their disk-to-bulge ratios. 2) A sequence of “anemic spirals,” which occur most frequently in rich clusters, is found to have characteristics that are intermediate between those of vigorous gas-rich normal spirals and gas-poor systems of type SO. 3) The differences between normal spirals (Sa-Sb-Sc), anemic spirals (Aa-Ab-Ac), and lenticulars (S0a-S0b-S0c) are tentatively interpreted in terms of the influence of environment on thé evolution of flattened galaxies. Subject heading: galaxies: structure I. WHY A NEW CLASSIFICATION SYSTEM IS NEEDED galaxies such as NGC 4866 {Hubble Atlas, p. 11) which have a very small nuclear bulge. The only feature Current thinking on galaxy classification has its common to all of these diverse types of objects is that roots in the classical paper of Hubble (1926). Much of the simple beauty of the classification scheme pro- star formation is not proceeding very vigorously. posed in that paper was lost when Hubble (1936) c) Effects of Luminosity introduced a more or less hypothetical transitional stage, which he called type SO, between elliptical and The Hubble (1936) classification system for spirals spiral galaxies. The subsequent evolution of this SO was defined in terms of supergiant galaxies. This bias classification type in the hands of Hubble and Sandage of the Hubble system has, to some extent, been over- is described in The Hubble Atlas of Galaxies (Sandage come by van den Bergh (1960a, b, c), who extended 1961). the original Hubble classification scheme to include In attempting to apply the Hubble-Sandage system the effects of differing galaxy luminosity. to the classification of galaxies I have, for many years, had the feeling that some types of galaxies do not fit d) Effects of the Cluster Environment naturally into the framework provided by this scheme. The large homogeneous sample of galaxy photo- The main problem areas appear to be the following: graphs provided by the glass copies of The Palomar Sky Survey has made it clear that the appearance of a) Lenticulars many galaxies in rich clusters differs from that of field The classification sequence from SOi to S03 (Sandage galaxies of similar Hubble type. No account is taken 1961) is not a sequence of increasing flattening. The of such differences in any of the major galaxy classifi- SOi galaxy NGC 4762 {Hubble Atlas, p. 8) is one of the cation schemes that are presently in use. flattest edge-on galaxies known. A counterexample is provided by NGC 5866 {Hubble Atlas, p. 6) which is II. PROPOSED REVISION OF THE HUBBLE-SANDAGE SYSTEM an edge-on system of type S03 which, in the absence of a thin absorption lane, would have been classified From a detailed study of the flattening of SO E5orE6. galaxies and of spirals Sandage, Freeman, and Stokes (1970) conclude that spiral and lenticular galaxies b) Galaxies of type Sa have the same distribution of true axial ratios. This The classification type Sa has become a repository important result suggests that normal spirals, which for galaxies with very different physical characteristics. exhibit a strong display of Population I, and SO On the one hand this class comprises objects similar galaxies (which contain few if any young stars) form to NGC 1302 {Hubble Atlas, p. 9), which have quite parallel sequences. These two sequences differ large nuclear bulges, and on the other it contains primarily in their total gas content and hence in the mean age of their stellar populations. The existence * Based, in part, on a review presented at the Tercentenary of both highly flattened gas-free systems and similarly celebrations of the Royal Greenwich Observatory. flattened gas-rich galaxies suggests that there might t Guest observer, 1975, Kitt Peak National Observatory. KPNO is operated by AURA Inc. under contract with the exist a class of flattened objects with characteristics National Science Foundation of the U.S.A. that are intermediate between those of SO galaxies and 883 © American Astronomical Society • Provided by the NASA Astrophysics Data System .883V .206. 884 VAN DEN BERGH Vol. 206 . Objects which, on the prints of the Hubble Atlas, 6ApJ. have apparent disk-to-bulge ratios in the range 1 to 3 have been classified Sa; those with disk-to-bulge ratios 197 of 3 to 10 were classified Sb; and those with disk-to- bulge ratios larger than 10 as Sc. An analogous defini- tion was used to define the anemic spirals of type Aa, Ab, and Ac. Similarly SO galaxies were also divided into types SOa, SOb, and SOc. It should be emphasized that this subdivision of galaxies of class SO is in no way PROPOSED CLASSIFICATION SYSTEM related to the Hubble-Sandage classification S0l5 S02, and S03, which does not depend on the relative im- portance of the central bulge and of the disk sub- systems. Many of the galaxies of Hubble-Sandage type EO E3 E6 z' "n Sa Sb Sc S0i fall on the S0a-S0b-S0c sequence in Figures 1 and i 1 T (so)—h H h 2. Most of the galaxies which have Hubble-Sandage type S03 are probably more closely related to the HUBBLE CLASSIFICATION SYSTEM anemic spirals than they are to true SO systems. Fig. 1.—Comparison of the provisional classification system The positions of the Sa-Sb-Sc, Aa-Ab-Ac, and proposed in this paper with that of Hubble (1936). The new S0a-S0b-S0c sequences in a diagram of color versus system comprises parallel sequences of normal spirals disk-to-bulge ratio are shown in Figure 3. (Sa-Sb-Sc), anemic spirals (Aa-Ab-Ac), and gas-free lentic- It should perhaps be emphasized that the role of SO ulars (S0a-S0b-S0c). galaxies in the classification system outlined above is very different from that originally envisaged by Hubble normal spirals. Such gas-poor objects, in which not (1936). Hubble thought of SO galaxies as a transitional much star formation is presently taking place, will be type between ellipticals and spirals. In the scheme referred to as “anemic” spirals. illustrated in Figures 1, 2, and 3, lenticular galaxies A new classification scheme for galaxies, which and spirals form parallel sequences which differ only incorporates a sequence of anemic spirals, is shown in in their gas content and hence in their rate of star Figures 1 and 2. In this provisional system, galaxies formation. without a disk component are called ellipticals In order to better define the new classification (E0-E6). The conventional Hubble classification of scheme proposed in this paper, all of the NGC normal spirals is replaced by a system which is based galaxies in The Hubble Atlas of Galaxies (Sandage entirely on their disk-to-bulge ratios. These disk-to- 1961) have been reclassified in the new system des- bulge ratios are, in a very crude way, related to the cribed above. The classifications were made using both relative importance of the two main components in the photographs in the Hubble Atlas and glass copies galaxies, i.e., the inner spheroidal component and the of the blue plates of the Palomar Sky Survey. These outer exponential disk (de Vaucouleurs 1959; Free- classifications, which are listed in Table 1, will be taken man 1970; Sándage, Freeman, and Stokes 1970). The to define the Revised David Dunlap Observatory disk-to-bulge ' ratio is also related to the nuclear (RDDO) system. concentration of light, which Morgan (1958, 1959) It should probably be emphasized that some features has used as a classification parameter. of this system are still quite provisional. In particular SOa SOb SOc Fig. 2.—Schematic representation of the proposed new galaxy classification system © American Astronomical Society • Provided by the NASA Astrophysics Data System .883V .206. No. 3, 1976 NEW CLASSIFICATION SYSTEM FOR GALAXIES 885 . it is not yet entirely clear how the sequence of barred and SO galaxies in very dense clusters such as Coma has 6ApJ. SO systems of differing disk-to-bulge ratio can be been known for many years (cf. Spitzer and Baade unambiguously classified. In Table 1 all classifications 1951). The differences between galaxies in more open 197 of SBO galaxies have therefore been placed in paren- clusters, such as Virgo, and those in the general field theses. are somewhat more subtle. The transition from type SO to type A is a continuous one. For purposes of classification both dusty and dust- a) Gas in Spirals free galaxies without evidence for a patchy distribu- tion of Population I have been assigned to the The following systematic differences are observed S0a-S0b-S0c sequence. Flattened objects, in which between spiral galaxies in the Virgo cluster and their some young Population I stars seemed to be present, counterparts in the field : were assigned to the Aa-Ab-Ac sequence. i) The average surface density of neutral hydrogen gas in Virgo spirals of a given type is only 0.5 ±0.1 III. GAS DEPLETION IN CLUSTER GALAXIES times that observed in noncluster spirals of the same type (Davies and Lewis 1973). A number of lines of evidence suggest that the ii) At a given disk-to-bulge ratio, Virgo spirals are galaxian population of rich clusters differs significantly systematically redder by 0.04 mag than are similar from that of field galaxies.
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