The Average Mass Profile of Galaxy Clusters

The average mass density profile measured in the Canadian Network for Observational Cosmology cluster survey is well described with the analytic form ρ(r) = Ar-1(r + aρ)-2, as advocated on the basis of n-body simulations by Navarro, Frenk, & White. The predicted core radii are aρ = 0.20 (in units of the radius where the mean interior density is 200 times the critical density) for an Ω = 0.2 open cold dark matter model and aρ = 0.26 for a flat Ω = 0.2 model, with little dependence on other cosmological parameters for simulations normalized to the observed cluster abundance. The dynamically derived local mass-to-light ratio, which has little radial variation, converts the observed light profile to a mass profile. We find that the scale radius of the mass distribution, 0.20 ≤ aρ ≤ 0.30 (depending on modeling details, with a 95% confidence range of 0.12-0.50), is completely consistent with the predicted values. Moreover, the profiles and total masses of the clusters as individuals can be acceptably predicted from the cluster rms line-of-sight velocity dispersion alone. This is strong support for the hierarchical clustering theory for the formation of galaxy clusters in a cool, collisionless, dark-matter-dominated universe.