Published in The Minnesota lectures on clusters of
galaxies and large-scale structure (A90-36758 15-90). San Francisco, CA,
Astronomical Society of the Pacific, 1988, p. 175-196.
For a PDF version of the article, click
For a PDF version of the article, click here.
Abstract. Recent results on the dynamics and post-collapse evolution of rich clusters are reviewed, with an emphasis on the Coma cluster. Current constraints on the distribution of dark matter in clusters are still rather weak; contrary to general belief, the limits derivable from kinematical data are about as good as those derivable from the existing X-ray data. In the case of the Coma cluster, the allowed values of M/L span nearly an order of magnitude; high-mass models require galaxy orbits that are very radial in the outer parts. If the dark matter is distributed like the galaxies, then tidal forces are sufficiently strong to limit the masses of dark halos to ~ 5 × 1011 M in the centers of clusters like Coma. The orbits of such massive galaxies could decay appreciably over a cluster lifetime due to dynamcal friction; this decay has probably been observed as an enhancement in the density of galaxies very near the centers of many rich clusters. Observations of the morphology and kinematics of multiple-nucleus cD galaxies suggest that "cannibalism" is a real effect, although it does not appear to be occurring at a fast enough rate to produce a cD galaxy in a cluster that did not contain one initially. Thus, if cD galaxies are the endproducts of repeated mergers, it is likely that most of these mergers took place before or during cluster formation. N-body simulations of cluster formation suggest that this is a viable hypothesis.
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