The density profile of equilibrium and nonequilibrium dark matter halos

We study the diversity of the density profiles of dark matter halos based on a large set of high-resolution cosmological simulations of 256(3) particles. The cosmological models include four scale-free models and three representative cold dark matter models. The simulations have good force resolution. In each cosmological model, there are about 400 massive halos that have more than 10(4) particles within the virial radius. Our unbiased selection of all massive halos enables us to quantify how well the bulk of dark matter halos can be described by the Navarro, Frenk, & White (NFW) profile, which was established for equilibrium halos. We find that about 70% of the halos can be fitted by the NFW profile with a fitting residual dvi(max) < 0.3 in Omega(0) = 1 universes. This percentage is higher in the low-density cosmological models of Omega(0) = 0.3. The rest of the halos exhibit larger deviations from the NFW profile for more significant internal substructures. There is a considerable amount of variation in the density profile even among the halos that can be fitted by the NFW profile (i.e., dvi(max) < 0.30). The distribution of the profile parameter, the concentration c, can be described well by a lognormal function with the mean value (c) over bar slightly smaller (15%) than the NFW result and the dispersion, a,, in In c, of about 0.25. More virialized halos with dvi(max) < 0.15 have a mean value (c) over bar in better agreement with the NFW result, and their dispersion, a,, is also slightly smaller (about 0.2). Our results can alleviate some of the conflicts found recently between the theoretical NFW profile and observational results. Implications for theoretical and observational studies of galaxy formation are discussed.