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Session 86 - Cosmology: Theory.
Display session, Friday, January 09
High-resolution N-body simulations of hierarchical clustering show that the radially averaged density profiles of dark matter halos follow the same functionality in a wide variety of cosmogonies, with low mass halos being denser than more massive ones. This mass-density correlation is interpreted as reflecting the typical earlier formation time of less massive objects. We investigate this hypothesis in the light of formation times defined as the epoch at which dark matter halos experience their last important structural rearrangement owing to the merger of progenitors with comparable masses. Such typical halo formation times are calculated by means of a modification of the Lacey-Cole (1993) clustering model that includes a phenomenological frontier, \Delta_m, between tiny and notable relative mass captures, leading to the distinction between continuous accretion and discrete mergers. For \Delta_m\sim 0.6, we confirm the previous claim that the characteristic density of halos is essentially proportional to the mean density of the universe at their formation time, although a proportionality with respect to the critical density yields slightly better results for open universes. In addition, we find that the scale radius of the halos is proportional to their virial radius at formation.
We show that the characteristics of these two relations are consistent with the following simple picture. Violent relaxation caused by major mergers rearranges the structure of dark halos according to their mass and the critical density of the universe at that epoch, leading to the same density profile with universal values, for any particular cosmogony, of the dimensionless characteristic density and scale radius. Then, until the next major mergers take place, the halos keep their central part steady and gently grow through the accretion of surrounding layers, gradually expanding their virial radius as the critical density of the universe diminishes.
Program listing for Friday