Counter-rotating stellar discs around a massive black hole: self-consistent, time-dependent dynamics

Abstract

We formulate the collisionless Boltzmann equation (CBE) for dense star clusters that lie within the radius of influence of a massive black hole in galactic nuclei. Our approach to these nearly Keplerian systems follows that of Sridhar and Touma (1999): Delaunay canonical variables are used to describe stellar orbits and we average over the fast Keplerian orbital phases. The stellar distribution function (DF) evolves on the longer time scale of precessional motions, whose dynamics is governed by a Hamiltonian, given by the orbit-averaged self-gravitational potential of the cluster. We specialize to razor-thin, planar discs and consider two counter-rotating ("$\pm$") populations of stars. To describe discs of small eccentricities, we expand the $\pm$ Hamiltonian to fourth order in the eccentricities, with coefficients that depend self-consistently on the $\pm$ DFs. We construct approximate $\pm$ dynamical invariants and use Jeans' theorem to construct time-dependent $\pm$ DFs, which are completely described by their centroid coordinates and shape matrices. When the centroid eccentricities are larger than the dispersion in eccentricities, the $\pm$ centroids obey a set of 4 autonomous ordinary differential equations. We show that these can be cast as a two-degree of freedom Hamiltonian system which is nonlinear, yet integrable. We study the linear instability of initially circular discs and derive a criterion for the counter-rotating instability. We then explore the rich nonlinear dynamics of counter-rotating discs, with focus on the variety of steadily precessing eccentric configurations that are allowed. The stability and properties of these configurations are studied as functions of parameters such as the disc mass ratios and angular momentum.