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
here.
INTERNAL DYNAMICS OF GALAXY CLUSTERS
David Merritt
Canadian Institute for Theoretical Astrophysics,
University of
Toronto, Toronto, Ontario M5S 1A1, Canada
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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