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

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Distant clusters are of interest because they contain younger galaxies than those in present-epoch clusters and as a class, they may be dynamically less evolved than nearby clusters. Observations of distant (and nearby) clusters have been used in attempts to determine the cosmological parameters H0 and q0 (Gunn 1978, Bahcall 1975, Silk & White 1978, Schwartz 1976, White et al. 1981, Birkinshaw et al. 1981a, Boynton et al. 1982, Cavaliere et al. 1979).

In an Einstein imaging survey of 58 distant clusters (z geq 0.1), Henry et al. (1982) found X-ray emission associated with 41 clusters, 25 with z > 0.2. The X-ray luminosities ranged from ~ 1043 to 2.5 x 1045 erg s-1. Changes in the X-ray luminosity and temperature as the cluster potential evolves have been modeled by Perrenod (1978), who used White's (1976a) numerical simulation to describe the potential. Assuming a constant rate of mass injection from the galaxies with differing amounts of primordial gas, Perrenod computed that the X-ray luminosity typically increases by about a factor of ten between z = 1 and z = 0. However, Falle & Meszaros (1980) cautioned that including the additional effects of galactic friction and heating due to violent relaxation of the cluster would modify Perrenod's predictions, since these effects tend to increase the temperature and decrease the X-ray luminosity. Since present-epoch clusters show a wide range in their dynamical evolution, Henry et al. (1982) used a luminosity-limited ensemble of 25 distant clusters to search for cosmological evolution. They concluded that the slope of the luminosity function for distant clusters is consistent with that of nearby clusters and independent of redshift for z leq 0.5.

The X-ray emission from three luminous, distant clusters has been analyzed in detail. From both low- and high-resolution X-ray observations of the 3C 295 cluster (z = 0.461), Henry & Henriksen (1982) found that its morphology is similar to that of Abell 85 with the X-ray emission peaked around the central, dominant galaxy. Although 3C 295 is one of the distant clusters observed by Butcher & Oemler (1978) to have a high fraction of blue, presumably spiral galaxies, the X-ray luminosity (9.2 ± 0.8 x 1044 erg s-1 in the 0.5-4.5 keV band) and core radius (~ 0.2 Mpc) imply a central gas density of ~ 10-2 cm-3, so that ram-pressure stripping and evaporation of the interstellar material should effectively deplete spirals. Therefore Henry et al. (1979) concluded that there should be an absence of spiral galaxies in the core of the 3C 295 cluster. Spinrad (1977) found that the proportion of blue galaxies increases toward the outside of the 3C 295 cluster. Specifically, within a projected radius of 0.4 Mpc (1.0') of the center, he found 20 red galaxies and only 2 blue galaxies. White et al. (1981a) have measured X-ray properties for the distant Bautz-Morgan type I cluster SC2059-247 (z = 0.19) that are similar to those of the 3C 295 cluster. They found the luminosity to be 7.0 ± 2.5 x 1044 erg s-1 and the core radius to be ltapprox 0.17 Mpc. Like many of the cD clusters, this cluster contains an unresolved radio source.

For the distant cluster 0016 + 16 (z = 0.541), White et al. (1981) found the X-ray morphology to be similar to that of Coma. The X-ray luminosity (0.5-4.5 keV) of 3.0 x 1045 erg s-1 is one of the highest observed. Koo (1981) estimated that 0016 + 16 is 2-4 times richer than Coma and does not have a central dominant galaxy. Koo also concluded that most of the cluster galaxies are intrinsically red, implying that star formation ceased several billion years ago.

Initially it seems somewhat of a surprise that as one looks back 7 billion years to 0016 + 16, 3C 295, and other distant clusters, the X-ray properties of these systems appear similar to those of rich, nearby clusters. This result and the redshift independence of the shape of the luminosity function (Henry et al. 1981) might suggest that one must look at still more distant clusters to observe fundamental differences, such as the fraction of primordial gas which depends on the efficiency of galaxy formation. However, the apparent similarities between distant clusters and nearby, rich systems may be primarily a selection effect. Distant clusters like 3C 295, which are selected because they contain a strong, unresolved radio source, tend to be cD systems, while the optically most easily distinguishable clusters tend to be dense and rich. Such clusters have short collapse times and already may have undergone considerable dynamical evolution. A complete and unbiased sample of distant clusters would provide a better comparison. Optical identification of clusters from X-ray surveys should lead to a less biased (although perhaps smaller) sample.

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