Radial velocity moments of dark matter haloes

Using cosmological N-body simulations, we study the radial velocity distribution in dark matter haloes, focusing on the lowest-order even moments, dispersion and kurtosis. We determine the properties of 10 massive haloes in the simulation box, approximating their density distribution by the Navarro, Frenk & White (NFW) formula characterized by the virial mass and concentration. We also calculate the velocity anisotropy parameter of the haloes, and find it mildly radial and increasing with distance from the halo centre. The radial velocity dispersion of the haloes shows a characteristic profile with a maximum, while the radial kurtosis profile decreases with distance starting from a value close to Gaussian near the centre. We therefore confirm that dark matter haloes possess intrinsically non-Gaussian, flat-topped velocity distributions. We find that the radial velocity moments of the simulated haloes are quite well reproduced by the solutions of the Jeans equations obtained for the halo parameters with the anisotropy measured in the simulations. We also study the radial velocity moments for a composite cluster made of 10 haloes out to 10 virial radii. In this region the velocity dispersion decreases systematically to reach the value of the background, while kurtosis increases from below to above the Gaussian value of 3, signifying a transition from a flat-topped to a strongly peaked velocity distribution with respect to the Gaussian, which can be interpreted as the dominance of ordered flow with a small dispersion. We illustrate the transition by showing explicitly the velocity distribution of the composite cluster in a few radial bins