Annu. Rev. Astron. Astrophys. 1982. 20: 547-85
Copyright © 1982 by . All rights reserved

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3.1 Irregular, Cool X-Ray Clusters

One of the remarkable results from the X-ray studies of clusters of galaxies is that most clusters are not symmetric, high-luminosity, relaxed systems like the Coma cluster. Rather, most clusters have low X-ray luminosities, low velocity dispersions, low gas densities, low X-ray temperatures, low central galaxy densities, and (usually) high spiral fractions. In addition, the X-ray observations of Abell 1367, Abell 194, and the Virgo cluster (see Section 4.1) show that hot X-ray coronae are associated with some galaxies in dynamically unevolved clusters.

3.1.1 ABELL 1367     Observations suggest that Abell 1367 (z = 0.0214) is a young, dynamically unevolved cluster. Carter & Metcalfe (1980) described Abell 1367 as an irregular cluster, which Bautz & Morgan (1970) classified as type II-III. The galaxy content is 43% spiral, 40% S0, and 17% elliptical (Oemler 1974, Bahcall 1977b). Its irregular appearance and low central concentration, observed both optically and in X-rays, indicate that it has not fully relaxed. Its low X-ray luminosity (L = 3.6 x 1043 erg s-1) and low temperature (2.8 ± 1.0 keV; Mushotzky & Smith 1980) imply a shallow cluster potential. Strom & Strom (1978) found that elliptical galaxies in Abell 1367 were larger than those of the same absolute magnitude in denser clusters like Coma. Their interpretation, that galaxies in Abell 1367 have undergone fewer collisions and little tidal stripping, agrees with the classification of Abell 1367 as an unevolved cluster.

Abell 1367 (see Figure 1) has an elongated central region of X-ray emission with a nuclear X-ray source in NGC 3862 (= 3C264) to the southeast (Elvis et al. 1981) and an unresolved source associated with a blue stellar object to the northwest (Bechtold et al. 1983). Assuming a hydrostatic-isothermal gas model with beta = 1 (Section 2.2), Bechtold et al. obtained X-ray core radii of 0.8 Mpc and 0.42 Mpc along the long and the short axes. From optical galaxy counts. Carter & Metcalfe (1980) measured a cluster ellipticity of 0.5 ± 0.1 with a position angle similar to that of the X-ray elongation.

Einstein IPC observations of Abell 1367 were reported by Jones et al. (1979) to show evidence for clumps in the X-ray emission. In the high-resolution images of Abell 1367, Bechtold et al. (1983) detected thirteen X-ray enhancements with characteristic sizes of one arcmin. Eight of these sources are associated with cluster galaxies and have X-ray luminosities in the range 1041 to 6 x 1041 erg s-1. Bechtold et al. argued that these sources are hot gaseous coronae and not integrated emission from galactic point sources, since their luminosities and extents are larger than found in the discrete source component in normal galaxies. If these galaxies reside in the cluster core and have velocities characterized by the cluster velocity dispersion, then mass loss rates from ram-pressure stripping and thermal evaporation of these galactic coronae would require that the galaxies replenish the gaseous halos at a rate higher than that predicted for normal galaxies. If conduction is suppressed by galactic magnetic fields, then slow-moving galaxies could retain their halos. Alternatively, Bechtold et al. suggested that if these galaxies have dark, massive halos (i.e. GM/R > 3/2 kT/µmH), the hot coronal gas would be gravitationally confined. Ram-pressure stripping will be ineffective, except for the high-velocity galaxies, and while conduction may heat the gas to the ambient cluster temperature, the gas will not escape from the galaxies. Bechtold et al. also suggested that all the X-ray-emitting galaxies may not be permanent residents of the cluster core, and thus they maintain their gaseous halos until they enter the core.

3.1.2 ABELL 194     The cluster Abell 194 (z = 0.0178) was described by Zwicky & Humason (1964) as a medium-compact cluster containing a central concentration of about two dozen galaxies. The bright galaxies, most of which are S0s, form a chain, giving the L classification (Rood & Sastry 1971). The cluster has a low velocity dispersion (396-35+45 km s-1; Danese et al. 1980) and a central galaxy density comparable to that of Abell 1367 (Bahcall 1981). Although these are characteristic of a weak cluster potential and therefore a dynamically young system, the galactic content is more typical of an evolved cluster with 18% ellipticals, 56% S0s, and only 26% spirals (Oemler 1974; ~ 28% spirals from Dressler 1980b).

In X-rays, Abell 194 is one of the lowest luminosity clusters (2.3 x 1042 erg s-1 within a 0.25 Mpc radius) for which extended emission has been observed. The 0.5-3 keV X-ray luminosities of enhancements around the double galaxies NGC 545 /547 and around NGC 541 are each about 1041 erg s-1, similar to those in Abell 1367.

Although Abell 194 was classed as spiral-poor by Oemler (1974), the central gas density is only ~ 4 x 10-4 cm-3, which is similar to the cluster component in Virgo, where the galaxy spiral fraction is 55% (Bahcall 1977b). Melnick & Sargent (1977) noted that in clusters like Abell 194 with low velocity dispersions, the S0 galaxies could not be produced by ram-pressure stripping since the ram pressure of the intergalactic medium would not exceed the binding force of the material in the disk (Gunn & Gott 1972). The ram pressure felt by a typical large spiral galaxy in the core of Abell 194 is five times less than the force binding the gas. Another mechanism for removing gas from spirals is thermal evaporation by the hot intracluster medium (Cowie & Songaila 1977). However, the similarity of the cluster gas in Abell 194 and Virgo suggests that evaporation in Abell 194 should be no more effective than in Virgo.

From a sample of clusters including Abell 194, Dressler (1980a) concluded that the relation between galaxy density and population holds within individual clusters. Thus the Abell 194 chain of bright, predominantly S0 galaxies corresponds to a region of high local galaxy density, although the more global galaxy density determined within a 0.5 Mpc radius is low (Bahcall 1981). Therefore, the dominance of elliptical and S0 galaxies in the center of Abell 194 is consistent with the suggested creation of S0 galaxies in high-density regions.

Additional arguments that all S0s are not stripped spirals are based on the presence of field S0s, the colors of early-type galaxies (Sandage & Visvanathan 1978), the larger bulges and bulge-to-disk ratios of S0s compared with spirals, and the correlation of S0 fraction with galaxy density (Dressler 1980a, Burstein 1979). Larson et al. (1980) suggested an origin for S0 galaxies in which the gas that replenishes that consumed in star formation is stored in the galaxy halo, where it can be more easily stripped than gas in the disk. Larson et al. postulated that S0 galaxies were disk systems that lost this gas-rich envelope at an early stage and subsequently exhausted the remaining disk gas through star formation. Another hypothesis, motivated by the increased fraction of elliptical galaxies with density, is that systems with large bulge-to-disk ratios are created preferentially in dense regions (Faber & Gallagher 1976, Gott & Thuan 1976, Dressler 1980a). (Gott & Thuan argued that elliptical galaxies collapse from large initial density fluctuations with their star formation nearly complete, while spiral galaxies form from smaller density fluctuations and have slower collapse and star formation rates. Thus gas-rich systems should dominate in the field and in less dense regions.

Observations of clusters like Abell 194 illustrate the problem of using spiral fraction as an evolutionary indicator and add support to the hypothesis that at least some S0 galaxies do not evolve from spiral galaxies.

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