The paper by Navarro, Frenk, and White explores the equilibrium density profiles of dark matter halos in hierarchically clustering universes using high-resolution N-body simulations. They find that all such profiles have a universal shape, independent of halo mass, initial density fluctuation spectrum, and cosmological parameters. The spherically averaged equilibrium profiles are well-fit by a simple formula originally proposed for galaxy clusters in a cold dark matter universe. The two scale parameters of the fit, the halo mass and its characteristic density, are strongly correlated. Low-mass halos are significantly denser than more massive systems, reflecting their higher collapse redshifts. The characteristic density of an equilibrium halo is proportional to the density of the universe at the time it was assembled. The authors compare their results with previous work and provide an analytic procedure based on the Press-Schechter formalism to calculate equilibrium profiles for any hierarchical model. They conclude that the shape of halo density profiles is independent of cosmological context, and that mergers and collisions act as a "relaxation" mechanism to produce a universal profile.The paper by Navarro, Frenk, and White explores the equilibrium density profiles of dark matter halos in hierarchically clustering universes using high-resolution N-body simulations. They find that all such profiles have a universal shape, independent of halo mass, initial density fluctuation spectrum, and cosmological parameters. The spherically averaged equilibrium profiles are well-fit by a simple formula originally proposed for galaxy clusters in a cold dark matter universe. The two scale parameters of the fit, the halo mass and its characteristic density, are strongly correlated. Low-mass halos are significantly denser than more massive systems, reflecting their higher collapse redshifts. The characteristic density of an equilibrium halo is proportional to the density of the universe at the time it was assembled. The authors compare their results with previous work and provide an analytic procedure based on the Press-Schechter formalism to calculate equilibrium profiles for any hierarchical model. They conclude that the shape of halo density profiles is independent of cosmological context, and that mergers and collisions act as a "relaxation" mechanism to produce a universal profile.