Redshift Evolution of Galaxy Cluster Densities

The number of rich galaxy clusters per unit volume is a strong function of V, the cosmological density parameter, and s8, the linear extrapolation to z 5 0 of the density contrast in 8 h Mpc spheres. The Canadian Network for Observational Cosmology (CNOC) cluster redshift survey provides a sample of clusters, the average mass profiles of which are accurately known, which enables a secure association between cluster numbers and the filtered density perturbation spectrum. We select from the CNOC cluster survey those Extended MediumSensitivity Survey clusters with bolometric LX $ 10 ergs s and a velocity dispersion exceeding 800 km s in the redshift ranges 0.18–0.35 and 0.35–0.55. We compare the number density of these subsamples with similar samples at both high and low redshift. Using the Press-Schechter formalism and cold dark matter (CDM) style structure models, the density data are best described with s8 3 0.75 H 0.1 and V 3 0.4 H 0.2 (90% confidence). The cluster dynamical analysis gives V 5 0.2 H 0.1 for which s8 5 0.95 H 0.1 (90% confidence). The predicted cluster density evolution in an V 5 1 CDM model exceeds that observed by more than 1 order of magnitude. Subject headings: cosmology: large-scale structure of universe— galaxies: clusters