How Universal Are the Density Profiles of Dark Halos?

We use high-resolution N-body simulations to investigate the formation of virialized halos due to the gravitational collapse of collisionless matter. A variety of formation scenarios are studied, ranging from hierarchical clustering to monolithic radial collapse. The goal of these experiments was to study departures from the universal density profiles recently found to arise in cosmological settings. However, we found that even for models that exhibit quite a different formation history, the density and velocity dispersion profiles of the virialized halos are strikingly similar.

Power-law density profiles do not result even in models with initial power-law profiles and without initial substructure or nonradial motions. Such initial conditions give rise to a radial orbit instability that leads to curved velocity dispersion and density profiles. The shapes of the density profiles in all our models are well parameterized by the profiles of halos formed in a generic cosmological setting. Our results show that the universality of dark halo density profiles does not depend crucially on hierarchical merging as has been suggested recently in the literature. Rather it arises because apparently different collapse histories produce a near universal angular momentum distribution among the halo particles. We conclude that the general form of the density and velocity dispersion profiles of virialized halos in an expanding universe are robust outcomes of gravitational collapse, nearly independent of the initial conditions and the formation history.