MERGER VERSUS ACCRETION AND THE STRUCTURE OF DARK-MATTER HALOS

High-resolution N-body simulations of hierarchical clustering in a wid e variety of cosmogonies show that the density profiles of dark matter halos are universal, with low-mass halos being denser than their more massive counterparts. This mass-density correlation is interpreted as reflecting the earlier typical formation time of less massive objects . We investigate this hypothesis in the light of formation times defin ed as the epoch at which halos experience their last major merger. Suc h halo formation times are calculated by means of a modification of th e extended Press & Schechter formalism that includes a phenomenologica l frontier, Delta(m), between tiny and notable relative mass captures leading to the distinction between merger and accretion. For Delta(m) similar to 0.6, we confirm that the characteristic density of halos is essentially proportional to the mean density of the universe at their time of formation. Yet, proportionality with respect to the critical density yields slightly better results for open universes. In addition , we find that the scale radius of halos is also essentially proportio nal to their virial radius at the time of formation. We show that thes e two relations are consistent with the following simple scenario. Vio lent relaxation caused by mergers rearranges the structure of halos le ading to the same density profile with universal values of the dimensi onless characteristic density and scale radius. Between mergers, halos grow gradually through the accretion of surrounding layers by keeping their central parts steady and expanding their virial radius as the c ritical density of the universe diminishes.