Richness Dependence of the Recent Evolution of Clusters of Galaxies
  We revisit the issue of the recent dynamical evolution of clusters of
galaxies using a sample of ACO clusters with z<0.14, which has been selected
such that it does not contain clusters with multiple velocity components nor
strongly merging or interacting clusters, as revealed in X-rays. We use as
proxies of the cluster dynamical state the projected cluster ellipticity,
velocity dispersion and X-ray luminosity. We find indications for a recent
dynamical evolution of this cluster population, which however strongly depends
on the cluster richness. Poor clusters appear to be undergoing their primary
phase of virialization, with their ellipticity increasing with redshift with a
rate de/dz ~ 2.5, while the richest clusters show an ellipticity evolution in
the opposite direction (with de/dz ~ -1.2), which could be due to secondary
infall. When taking into account sampling effects due to the magnitude-limited
nature of the ACO cluster catalogue we find no significant evolution of the
cluster X-ray luminosity, while the velocity dispersion increases with
decreasing redshift, independent of the cluster richness, at a rate dsigma/dz ~
-1700 km/sec.
