The Average Mass Profile of Galaxy Clusters

The average mass density profile measured in the CNOC cluster survey is welldescribed with the analytic form rho(r)=A/[r(r+a_rho)^2], as advocated on thebasis on n-body simulations by Navarro, Frenk & White. The predicted core radiiare a_rho=0.20 (in units of the radius where the mean interior density is 200times the critical density) for an Omega=0.2 open CDM model, or a_rho=0.26 fora flat Omega=0.2 model, with little dependence on other cosmological parametersfor simulations normalized to the observed cluster abundance. The dynamicallyderived local mass-to-light ratio, which has little radial variation, convertsthe observed light profile to a mass profile. We find that the scale radius ofthe mass distribution, 0.20<= a_rho <= 0.30 (depending on modeling details,with a 95% confidence range of 0.12-0.50), is completely consistent with thepredicted values. Moreover, the profiles and total masses of the clusters asindividuals can be acceptably predicted from the cluster RMS line-of-sightvelocity dispersion alone. This is strong support of the hierarchicalclustering theory for the formation of galaxy clusters in a cool,collisionless, dark matter dominated universe.