Redshift Evolution of Galaxy Cluster

The number of rich galaxy clusters per unit volume is a strong function of , the cosmological density parameter, and 8 , the linear extrapolation to z = 0 of the density contrast in 8h ?1 Mpc spheres. The CNOC cluster redshift survey provides a sample of clusters whose average mass prooles are accurately known, which enables a secure association between cluster numbers and the ltered density perturbation spectrum. We select from the CNOC cluster survey those EMSS clusters with bolometric L x 10 45 erg s ?1 and a velocity dispersion exceeding 800 km s ?1 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 CDM style structure models, the density data are best described with 8 ' 0:75 0:1 and ' 0:4 0:2 (90% conndence). The cluster dynamical analysis gives = 0:2 0:1 for which 8 = 0:95 0:1 (90% conndence). The predicted cluster density evolution in an = 1 CDM model exceeds that observed by more than an order of magnitude.