Might we eventually understand the origin of the dark matter velocity anisotropy?

Abstract

The density profile of simulated dark matter structures is fairly well-established, and several explanations for its characteristics have been put forward. In contrast, the radial variation of the velocity anisotropy has still not been explained. We suggest a very simple origin, based on the shapes of the velocity distributions functions, which are shown to differ between the radial and tangential directions. This allows us to derive a radial variation of the anisotropy profile which is in good agreement with both simulations and observations. One of the consequences of this suggestion is that the velocity anisotropy is entirely determined once the density profile is known. We demonstrate how this explains the origin of the γ-βrelation, which is the connection between the slope of the density profile and the velocity anisotropy. These findings provide us with a powerful tool, which allows us to close the Jeans equations.