Cusps in CDM halos: The density profile of a billion particle halo

We resolve the inner region of a massive cluster forming in a cosmological ΛCDM simulation with a mass resolution of up to 3 × 105M⊙. This is a billion times less than the clusters virial mass and is about two orders of magnitude better than the current highest resolution ΛCDM simulations. We achieve this resolution using a new multi-mass refinement procedure and are now able to probe a dark matter halo density profile down to 0.1 percent of the virial radius. The inner density profile of this cluster halo is well fitted by a power-law ρ ∝ r down to the smallest resolved scale. An inner region with constant logarithmic slope is now resolved, which indicates that cuspy profiles describe the inner profile better than recently proposed profiles with a core. For this cluster the inner slope is about γ = 1.2, close to the medium value in a sample of six high resolution cluster simulations of Diemand et al. (2004b).