A universal density slope - velocity anisotropy relation for relaxed structures

Abstract

We identify a universal relation between the radial density slope \alpha (r) and the velocity anisotropy \beta (r) for equilibrated structures. This relation holds for a variety of systems, including disk galaxy mergers, spherical collapses, cold dark matter (CDM) halos both with and without cooling. We argue that the shape of the relation is reasonable from fundamental principles when the dark matter or stars are assumed to obey Tsallis statistics, and in that case we fit the \alpha - \beta relation with just one free parameter. One can use this result to close the Jeans equations, for example to construct mass models of elliptical galaxies from observational data or to tune dark matter direct detection experiments. We also predict the asymptotic central slope and anisotropy of CDM halos to be approximately -1 and 0.