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Regular clusters of galaxies are the largest organized structures in the universe. They typically contain hundreds of galaxies, spread over a region of whose size is roughly 1025 cm. Their total masses exceed 1048 gm. They were first studied in detail by Wolf (1906), although the tendency for galaxies to cluster on the sky had been noted long before this. A great advance in the systematic study of the properties of clusters occurred when Abell compiled an extensive, statistically complete catalog of rich clusters of galaxies (Abell, 1958). For the last quarter century, this catalog has been the most important resource in the study of galaxy clusters. Optical photographs of several of the best studied clusters of galaxies are shown in Figure 1. 1 The Virgo cluster (Figure 1a) is the nearest rich cluster to our own galaxy; the Coma cluster (Figure 1b) is the nearest very regular cluster.

Figure 1a
Figure 1b
Figure 1c
Figure 1d

Figure 1. Optical photographs of clusters of galaxies. (a) An optical photograph of the Virgo cluster of galaxies, an irregular cluster that is the nearest cluster to our galaxy. The galaxy M87, on which the X-ray emission is centered, is marked, as are the two bright galaxies M84 and M86. Photograph from the Palomar Observatory Sky Survey (Minkowski and Abell, 1963). (b) The Coma cluster of galaxies (Abell 1656), showing the two dominant D galaxies. Coma is one of the nearest rich, regular clusters. Photograph copyright 1973, AURA, Inc., the National Optical Astronomy Observatories, Kitt Peak. (c) The Perseus cluster of galaxies (Abell 476), showing the line of bright galaxies. NGC1275 is the brightest galaxy, on the east (left) end of the chain. NGC1265 is a head-tail radio galaxy. Photograph from Strom and Strom (1978c). (d) The irregular cluster Abell 1367. Photograph from Strom and Strom (1978c).

In 1966, X-ray emission was detected from the region around the galaxy M87 in the center of the Virgo cluster (Byram et al., 1966; Bradt et al., 1967; Figure 1a). In fact, M87 was the first object outside of our galaxy to be identified as a source of astronomical X-ray emission. Five years later, X-ray sources were also detected in the directions of the Coma (Figure 1b) and Perseus (Figure 1c) clusters (Fritz et al., 1971; Gursky et al., 1971a, b; Meekins et al., 1971). Since these are three of the nearest rich clusters, it was suggested that clusters of galaxies might generally be X-ray sources (Cavaliere et al., 1971). The launch of the Uhuru X-ray astronomy satellite permitted a survey of the entire sky for X-ray emission (Giacconi et al., 1972) and established that this was indeed the case. These early Uhuru observations indicated that many clusters were bright X-ray sources, with luminosities typically in the range of 1043-45 erg/s. The X-ray sources associated with clusters were found to be spatially extended; their sizes were comparable to the size of the galaxy distribution in the clusters (Kellogg et al., 1972; Forman et al., 1972). Unlike other bright X-ray sources but consistent with their spatial extents, cluster X-ray sources did not vary temporally in their brightness (Elvis, 1976). Although several emission mechanisms were proposed, the X-ray spectra of clusters were most consistent with thermal bremsstrahlung from hot gas.

This interpretation requires that the space between galaxies in clusters be filled with very hot (approx 108 K), low density (approx 10-3 atoms/ cm3) gas. Remarkably, the total mass in this intracluster medium is comparable to the total mass in all the stars in all the galaxies in the cluster. As to the origin of this gas, it was widely assumed that it had simply fallen into the clusters from the great volumes of space between them, where it had been stored since the formation of the universe (Gunn and Gott, 1972).

In 1976, X-ray line emission from iron was detected from the Perseus cluster of galaxies (Mitchell et al., 1976), and shortly thereafter from Coma and Virgo as well (Serlemitsos et al., 1977). The emission mechanism for this line is thermal, and its detection confirmed the thermal interpretation of cluster X-ray sources. However, the only known sources for significant quantities of iron or any other heavy element in astronomy are nuclear reactions in stars, and no significant population of stars has been observed which do not reside in galaxies. Since the abundance of iron in the intracluster gas was observed to be similar to its abundance in stars, a substantial portion of this gas must have been ejected from stars in galaxies in the cluster (Bahcall and Sarazin, 1977). This is despite the fact that the total mass of intracluster gas is on the same order as the total mass of stars presently observed in the clusters. Obviously, these X-ray observations suggest that galaxies in clusters have had more interesting histories than might otherwise have been assumed.

In this paper, the X-ray observations of clusters of galaxies and the theories for the intracluster gas will be reviewed. Because clusters are still largely defined by their optical properties, I shall first review the optical observations of clusters (Chapter 2). I shall particularly emphasize information on their dynamical state, and the possibility that the galaxy population has been affected by the intracluster gas. Radio observations of clusters also provide information on the intracluster gas, which is summarized in Chapter 3. For example, certain distortions seen in radio sources in clusters are most naturally explained as arising from interactions with this gas. Moreover, extensive searches have been made for 'shadows' in the cosmic microwave radiation due to electron scattering by intracluster gas. Then the X-ray observations will be reviewed (Chapter 4), including the recent results of X-ray imaging and spectroscopy from the Einstein X-ray satellite. In Chapter 5 theories for the X-ray emission mechanism, the physical state, the distribution, the origin, and the history of the intracluster medium will be reviewed. Finally, I shall comment briefly on the prospects for further observations of X-ray clusters, particularly with the AXAF satellite, in Chapter 6.

Review articles on clusters of galaxies emphasizing their optical properties include Abell (1965, 1975), van den Bergh (1977b), Bahcall (1977a), Rood (1981), White (1982), and particularly Dressler (1984). Superclusters of galaxies are reviewed by Oort (1983). Some recent reviews which include the X-ray properties of clusters are Gursky and Schwartz (1977), Binney (1980), Cavaliere (1980), Cowie (1981), Canizares (1981), and Holt and McCray (1982). Fabian et al. (1984b) give an excellent review of cooling flows in X-ray clusters (see Section 5.7). Up-to-date reviews of the spectroscopic properties of X-ray clusters are given by Mushotzky (1984, 1985). Forman and Jones (1982) give a comprehensive review of the X-ray images of clusters. This book is based in large part on my review paper on X-ray clusters (Sarazin, 1986a).

1 Unless otherwise indicated, all the figures in this book showing optical, X-ray, or radio brightness on the sky have north at the top and east at the left. When coordinates are given, the east-west coordinate is right ascension (in hours, minutes, and seconds) and the north-south coordinate is declination (in degrees, minutes, and seconds). Back.

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