Resolving the Structure of Cold Dark Matter Halos

We examine the effects of mass resolution and force softening on the density profiles of cold dark matter halos that form within cosmological N-body simulations. As we increase the mass and force resolution, the halo profiles become steeper and denser in the central regions and do not converge to a unique result. At our highest resolution we have nearly 3 million particles within the virial radius, several orders of magnitude more than previously used and we can resolve close to a thousand surviving halos within this one object. As the mass resolution is increased we resolve progenitor halos that collapse at higher redshifts and have very high densities. This material ends up deeper within the central regions of the final halo, leading to higher densities at a fixed radius. The steep inner density profiles, ρ(r) ∼ r −1.4, will prove problematic for the CDM model when compared with the observed properties of halos on galactic and cluster scales.