Secondary infall and the pseudo-phase-space density profiles of cold dark matter haloes.

We use N-body simulations to investigate the radial dependence of the density, ρ, and velocity dispersion, σ, in cold dark matter (CDM) haloes. In particular, we explore how closely , a surrogate measure of the phase-space density, follows a power law in radius. Our study extends earlier work by considering, in addition to spherically averaged profiles, local Q estimates for individual particles, ; profiles based on the ellipsoidal radius dictated by the triaxial structure of the halo, ; and by carefully removing substructures in order to focus on the profile of the smooth halo, . The resulting profiles follow closely a power law near the centre, but show a clear upturn from this trend near the virial radius, r₂₀₀. The location and magnitude of the deviations are in excellent agreement with the predictions from Bertschinger's spherical secondary-infall similarity solution. In this model, in the inner, virialized regions, but departures from a power-law occur near r₂₀₀ because of the proximity of this radius to the location of the first shell crossing – the shock radius in the case of a collisional fluid. Particles there have not yet fully virialized, and so Q departs from the inner power-law profile. Our results imply that the power-law nature of Q profiles only applies to the inner regions and cannot be used to predict accurately the structure of CDM haloes beyond their characteristic scale radius. [ABSTRACT FROM AUTHOR]