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We investigate the formation of virialized halos due to the gravitationalcollapse of collisionless matter using high-resolution N-body simulations. Avariety of formation scenarios are studied, ranging from hierarchicalclustering to monolithic radial collapse. The goal of these experiments was tostudy departures from the universal density profiles recently found to arise incosmological settings. However, we found that even for models which exhibitquite a different formation history, the density and velocity dispersionprofiles of the virialized halos are strikingly similar.  Power law density profiles do not result even in models with initial powerlaw profiles and no initial substructure or non-radial motions. Such initialconditions give rise to a radial orbit instability which leads to curvedvelocity dispersion and density profiles. The shapes of the density profiles inall our models are well parameterized by the profiles of halos formed in ageneric cosmological setting. Our results show that the universality of darkhalo density profiles does not depend crucially on hierarchical merging as hasbeen suggested recently in the literature. Rather it arises because apparentlydifferent collapse histories produces a near universal angular momentumdistribution among the halo particles. We conclude that the density andvelocity dispersion profiles of virialized halos in an expanding universe arerobust outcomes of gravitational collapse, nearly independent of the initialconditions and the formation history.

How Universal Are the Density Profiles of Dark Halos?