Evolution of the Internal Dynamics of Galaxy Clusters

We consider a sample of 51 distant galaxy clusters at 0.15 ∼< z ∼< 0.9 (< z >∼ 0.3), each cluster having at least 10 galaxies with available redshift in the literature. We select member galaxies, analyze the velocity dispersion profiles, and evaluate in a homogeneous way cluster velocity dispersions and virial masses. We apply the same procedures already recently applied on a sample of nearby clusters (z < 0.15, Girardi et al. 1998b) in order to properly analyze the possible dynamical evolution of galaxy clusters. We remark problems induced by the poor sampling and the small spatial extension of the sampled cluster region in the computation of velocity dispersion. We do not find any significant difference between nearby and distant clusters. In particular, we consider the galaxy spatial distribution, the shape of the velocity dispersion profile, and the relations between velocity dispersion and X–ray luminosity and temperature. Our results imply little dynamical evolution in the range of redshift spanned by our cluster sample, and suggest that the typical redshift of cluster formation is higher than that of the sample we analyze. Subject headings: galaxies: distances and redshifts X-rays: galaxies cosmology: observations.