Numerical Simulations in Cosmology III: Dark Matter Halos

We give a review of different properties of dark matter halos. Taken from different publications, we present results on (1) the mass and velocity functions, (2) density and velocity profiles, and (3) concentration of halos. The results are not sensitive to parameters of cosmological models, but formally most of them were derived for popular flat ΛCDM model. In the range of radii r = (0.005 − 1)rvir the density profile for a quiet isolated halo is very accurately approximated by a fit suggested by Moore et al.(1997): ρ ∝ 1/x1.5(1 + x1.5), where x = r/rs and rs is a characteristic radius. The fit suggested by Navarro et al.(1995) ρ ∝ 1/x(1 + x)2, also gives a very satisfactory approximation with relative errors of about 10% for radii not smaller than 1% of the virial radius. The mass function of z = 0 halos with mass below ≈ 10hM⊙ is approximated by a power law with slope α = −1.85. The slope increases with the redshift. The velocity function of halos with Vmax < 500 km/s is also a power law with the slope β = −3.8− 4. The power-law extends to halos at least down to 10 km/s. It is also valid for halos inside larger virialized halos. The concentration of halos depends on mass (more massive halos are less concentrated) and environment, with isolated halos being less concentrated than halos of the same mass inside clusters. Halos have intrinsic scatter of concentration: at 1σ level halos with the same mass have ∆(log cvir) = 0.18 or, equivalently, ∆Vmax/Vmax = 0.12. Velocity anisotropyfor both subhalos and the dark matter is approximated by β(r) = 0.15 + 2x/[x2 +4], where x is radius in units of the virial radius. Subject headings: cosmology:theory – galaxy structure – methods: numerical