Mass and Redshift Dependence of Dark Halo Structure

Using a combination of N-body simulations with different resolutions, we study in detail how the concentrations of cold dark matter halos depend on halo mass at different redshifts. We confirm that halo concentrations at the present time depend strongly on halo mass, but our results also show marked differences from the predictions of some early empirical models. Our main results are that the mass dependence of the concentrations becomes weaker at higher redshifts and that at z>~3, halos of mass greater than 10¹¹ h^-1 M_solar all have a similar median concentration, c~3.5. While the median concentrations of low-mass halos grow significantly with time, those of massive halos change very little with redshift. These results are quantitatively in good agreement with the empirical model proposed by Zhao et al., which shows that halos in the early fast accretion phase all have similar concentrations.