Quantifying Departures from Equilibrium with the Spherical Jeans Equation

Abstract

Proper motions of collisionless, pointlike objects in a spherically symmetric system—for example, stars in a galaxy—can be used to test whether that system is in equilibrium, with no assumptions regarding isotropy. In particular, the fourth-order spherical Jeans equation yields expressions for two observable quantities characterizing the departure from equilibrium, both of which can be expressed in terms of time derivatives of first and third moments of the velocities. As illustrations, we compute these quantities for tracer distributions drawn from an exact equilibrium configuration, and also from near-equilibrium configurations generated using the N -body code GALIC.