3.2. Galaxian Contents of Clusters
As already stated, spiral and irregular galaxies appear to be rare or lacking in regular clusters, but are common in the irregular ones. The Local Group, although a poor irregular cluster, is the one for which the most complete census exists. It contains three spiral galaxies, with Mv in the range -19 to -21, four irregular galaxies (- 14 Mv - 18), (1) four intermediate ellipticals (- 14 Mv - 17), (2) and up to nine dwarf ellipticals of the Sculptor type (- 9 Mv - 14), including three suspected companions of M31 discovered by van den Bergh (1972). There are no giant ellipticals that are certain members of the Local Group, but evidence from the rich clusters suggests that the luminosity function of elliptical galaxies increases with increasing absolute magnitude and giant ellipticals are rare; perhaps we should not expect to find one in a sample of only 17 to 20 galaxies. Undiscovered members of the Local Group may well exist, especially in the direction of the Milky Way. In particular, there are probably galaxies of lower luminosity than the Ursa Minor system (the least luminous known member of the Local Group). Remote globular clusters are known to exist, for example, at distances up to 105 pc (Abell 1955); if such systems are distributed uniformly throughout the Local Group, and are not just outlying members of our own Galaxy, they can properly be classed as galaxies themselves, as Zwicky (1957) and others have proposed. There is no certainty that the Local Group does not contain ever larger numbers of objects of ever smaller mass, ranging down to individual stars.
The irregular Virgo cluster displays a wider range of galaxian types. The majority of the brightest galaxies are spirals; 205 Shapley-Ames galaxies lie in the region of the Virgo cluster: 68 percent are spirals, and only 19 percent are ellipticals, the remainder being irregular or unclassified. The incidence of ellipticals in the Virgo cluster, however, is still significantly higher than among the brightest galaxies in the field. Moreover, the fraction of cluster galaxies that are elliptical increases at fainter magnitudes; of the 272 brightest galaxies in a 6.6° × 6.6° region centered at (1950) = 12h25m, = + 13°, 53 percent have been classified by the writer as spirals and irregulars, and 47 percent ellipticals and S0's. According to Shapley (1950), just under half of the Virgo galaxies are spirals in the magnitude range 12-14, and only about a quarter in the range 14-16. Reaves (1956, 1967) has searched for faint galaxies in the Virgo cluster on plates taken with the Lick 20-inch astrograph. Of about 1000 objects that Reaves considers possible or probable cluster members, he describes 76; only 11 of these are definitely not of the elliptical type, and cluster membership of some of the 11 is in doubt. The majority of Reaves's objects appear to be similar to IC 3475. They are probably elliptical galaxies of intermediate to low luminosity, ranging from objects like NGC 221 to the brightest Sculptor-type systems in the Local Group; however, Reaves (1962) emphasizes that this interpretation must be viewed with caution until accurate colors are available. Faint dwarf ellipticals (like the Draco and Ursa Minor systems) might be expected to exist in still greater numbers in the Virgo cluster, but they would not have been detected in Reaves's survey. Elliptical galaxies slightly brighter than Sculptor-type dwarfs have also been observed in some other nearby clusters, in particular the M81 group and the Fornax cluster (Hodge 1959, 1960; Hodge, Pyper, and Webb 1965). The scanty evidence available, therefore, suggests that dwarf ellipticals are very common in nearby irregular clusters, and could well be the most numerous kind of galaxy in all clusters.
Whereas spirals are common among the brighter galaxies in irregular clusters, and are actually in the majority among the bright Virgo galaxies, the very brightest cluster members tend to be giant ellipticals - unless the cluster population is small. The four brightest members of the Virgo cluster, according to Holmberg (1958), are the giant ellipticals NGC 4472 (mpv = 8.5), NGC 4486 (mpv = 8.7), NGC 4649 (mpv = 9.0), and NGC 4406 (mpv = 9.2). If the distance modulus of the cluster is 31.1 (Sandage 1968), these galaxies have absolute visual magnitudes in the range -21.9 to -22.6. Similarly, Morgan (1961) found that the brightest members in each of the 20 nearest clusters in the Abell catalog tend to have stellar populations of the "evolved" type - yellow giant stars.
Frequently one or two particularly luminous giant elliptical galaxies will be found near the center of a regular cluster; sometimes these "supergiant" ellipticals will exceed the brightness of the next most luminous cluster members by more than a magnitude. The center of the Coma cluster, for example, lies roughly midway between the giant ellipticals NGC 4874 and NGC 4889 [= NGC 4884]. More than 30 clusters containing supergiant galaxies as their brightest members (Morgan cD galaxies) have been described by Matthews, Morgan, and Schmidt (1964) and by Morgan and Lesh (1965). Bautz and Morgan (1970; Bautz 1972) classify clusters I to III according to their brightest elliptical galaxy. A type I cluster contains an extraordinarily large luminous cD galaxy that dominates the cluster, as does NGC 6166 in Abell cluster 2199. The Virgo and Corona Borealis (A2065) clusters, on the other hand, represent type III, in that each has no member that stands out noticeably against the giant galaxy background.
As stated above, the rich regular clusters, in contrast to the irregular ones, appear to be nearly or completely devoid of spiral and irregular galaxies. There are actually several spirals in the field of the Coma cluster, the nearest of the regular clusters. Whether or not they are cluster members, however, deserves discussion. The writer was able to find 47 galaxies that could definitely be classed as spirals on 48-inch Schmidt photographs covering a 70-square-degree region in the vicinity of and including the cluster. The radial velocities of some of these spirals are known, and most lie within 2000 km s-1 of the mean cluster velocity of 6866 km s-1. Rood, Page, Kintner, and King (1972) treat these objects as cluster members. Subsequently Rood (1974) has analyzed the radial distribution of 30 spirals in the Coma field and concludes that many are probably members of the cluster, and that spirals and irregulars make up 15 percent of the Coma galaxies in the interval of the brightest 2.7 mag. Rood similarly investigated spirals near cluster A2199, and finds that they are probably field galaxies. The detailed distribution of spirals in and around other more remote regular clusters has not yet been investigated. In Section 5.2 we review the strong evidence for superclustering. Quite possibly the spirals in and near a regular cluster (e.g., Coma) share membership in a larger cloud of galaxies but are not properly part of the main condensation of the cluster. The hypothesis cannot be ruled out that at least the inner regions of regular clusters are completely lacking in spirals.
Spitzer and Baade (1951) suggested that the absence of spiral galaxies in rich clusters may be a result of collisions between the cluster galaxies, and the consequent removal of interstellar matter from them. In the Coma cluster, for example, a typical galaxy moves with a speed of the order of 103 km s-1 with respect to the cluster center; in 5 × 109 years such a galaxy would traverse a distance of 5 × 106 pc. At the time of the Spitzer-Baade analysis it appeared that this was several times the diameter of the cluster, and that most galaxies in passing back and forth through the cluster would have suffered at least one collision since the cluster was formed. With modern estimates of the extragalactic distance scale, however, and the correspondingly larger cluster diameters, it is not certain that the Spitzer-Baade mechanism can have effectively removed interstellar matter from most or all galaxies in a typical rich cluster unless its age is much greater than that usually assumed for the Universe. It is possible, therefore, that spiral galaxies either were never formed in the regular clusters, or have disappeared through other evolutionary processes.
Rich clusters of galaxies also tend to contain strong radio sources. Mills (1960) compared the positions of 1159 sources found in the Sydney survey with those of the 877 rich clusters in the Abell catalog that appear in the same survey region. He found 55 coincidences, whereas only 16 coincidences would be expected by chance. Van den Bergh (1961a) similarly compared positions of 282 sources in the 3C catalog with galactic latitudes greater than 25° with positions of clusters in the Abell catalog and found 27 coincidences, whereas nine would have been expected by chance. Moreover, van den Bergh finds that the radio magnitudes of the sources are correlated with the distances of the clusters, further strengthening the significance of the association of sources and clusters. More recently, Pilkington (1964) has rediscussed the coincidences between positions of clusters and of sources in the Cambridge and Sydney catalogs, and has added a search for such coincidences among the sources in the partially complete 4C survey. He finds that the radio positions tend to lie near the projected centers of clusters, and he gives a table of 41 sources that lie (in projection) within about 1 Mpc of cluster centers. He estimates that about 8 of these are chance associations. Rogstad, Rougoor, and Whiteoak (1965) have searched for 21-cm continuum radiation from 39 nearby clusters with the Caltech radio interferometer at Owens Valley, and have detected significant flux from 25 of them. They estimate that nearly 60 percent of the clusters listed in the three nearest distance categories in the Abell catalog contain detectable sources. The small angular sizes of the sources observed suggest that the radiation comes from individual galaxies, and not from sources spread over large intracluster spaces.
According to Minkowski (1963) and Matthews, Morgan, and Schmidt (1964), more than a third of the individual galaxies identified with radio sources are in rich clusters in the Abell catalog. On the other hand, Minkowski estimates that only about 10 percent of all galaxies are in clusters as rich as those catalogued by Abell. Van den Bergh (1961b) independently arrived at a similar estimate. Both van den Bergh (1961a) and Pilkington find that the number of coincidences between source and cluster positions are not correlated with cluster richness in the manner expected if collisions between galaxies were responsible for the radio emission. On the other hand, Rogstad and Ekers (1969) find that E and S0 galaxies in the field are as likely to be strong radio sources as are those in clusters; their data suggest that the propensity of clusters to be radio sources may simply reflect the tendency for E and S0 galaxies to be in clusters.
Clusters also tend to be X-ray sources. Kellogg, Murray, Giacconi, Tananbaum, and Gursky (1973) give data for 20 clusters, of which 16 are in the Abell catalog. For six clusters with both velocity dispersion and X-ray luminosity known, they find the data consistent with Lx (V)3.9±0.8. The X-ray luminosities show a wide spread in clusters of all richnesses; some clusters of low richness are strong X-ray sources, which suggests that some unidentified high-latitude sources may be clusters too poor to have been catalogued.
1 Galaxies in the intermediate luminosity range (- 14 Mv - 18) - e.g., NGC 147, IC 1613, and NGC 205 - are often referred to as dwarfs; but since galaxies of lower luminosity appear to be even more common, here we shall use the term "dwarf" for the latter. Back.
2 Back. See n. 1.