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 anal ytic form rho(r) = Ar-1(r + a(rho))(-2), as advocated on the basis of n-body simulations by Navarro, Frenk, & White. The predicted core radi i are a(rho) = 0.20 (in units of the radius where the mean interior de nsity is 200 times the critical density) for an Omega = 0.2 open cold dark matter model and a(rho) = 0.26 for a flat Omega = 0.2 model, with little dependence on other cosmological parameters for simulations no rmalized to the observed cluster abundance. The dynamically derived lo cal mass-to-light ratio, which has little radial variation, converts t he observed light profile to a mass profile. We find that the scale ra dius of the mass distribution, 0.20 less than or equal to a(rho) less than or equal to 0.30 (depending on modeling details, with a 95% confi dence range of 0.12-0.50), is completely consistent with the predicted values. Moreover, the profiles and total masses of the clusters as in dividuals can be acceptably predicted from the cluster rms line-of-sig ht velocity dispersion alone. This is strong support for the hierarchi cal clustering theory for the formation of galaxy clusters in a cool, collisionless, dark-matter-dominated universe.