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, and there are about 400 massive halos with more than 10^4 particles within the virial radius in each cosmological model. Our unbiased selection of all massive halos enables 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 seventy percent of the halos can be fitted by the NFW profile with a fitting residual dviₘₐₓ less than 30% in Omega_0=1 universes. This percentage is higher in lower density cosmological models. The rest of the halos exhibits larger deviations from the NFW profile for more significant internal substructures. There is a considerable amount of variation in the density profile even for the halos which can be fitted by the NFW profile (i.e. dviₘₐₓ<0.30). The distribution of the profile parameter, the concentration c, can be well described by a lognormal function with the mean value \ c slightly smaller (15%) than the NFW result and the dispersion \σ_c in \ln c about 0.25. The more virialized halos with dviₘₐₓ<0.15 have the mean value recently between the theoretical NFW profile and observational results. Implications for theoretical and observational studies of galaxy formation are discussed.