Redshift Evolution of Galaxy Cluster Densities

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Abstract
The number of rich galaxy clusters per unit volume is a strong function of Ω, the cosmological density parameter, and σ8, the linear extrapolation to z = 0 of the density contrast in 8 h-1 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 Medium-Sensitivity Survey clusters with bolometric LX ≥ 1045 ergs 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 cold dark matter (CDM) style structure models, the density data are best described with σ8 0.75 ± 0.1 and Ω 0.4 ± 0.2 (90% confidence). The cluster dynamical analysis gives Ω = 0.2 ± 0.1 for which σ8 = 0.95 ± 0.1 (90% confidence). The predicted cluster density evolution in an Ω = 1 CDM model exceeds that observed by more than 1 order of magnitude.