The Thermodynamics of Rotating Black-Hole Star Clusters

TL;DR

This paper explores the thermodynamics of rotating black-hole star clusters, revealing symmetry breaking, equilibrium states, and the behavior of black holes and lighter bodies in various thermodynamic conditions.

Contribution

It introduces a numerical method for equilibrium states, proves symmetry breaking at low temperatures, and analyzes the effects of rotation on cluster configurations and black hole orbits.

Findings

Lopsided equilibria precess with a temperature ratio.

Black holes form flattened, eccentric, or stationary orbits in rotating clusters.

Light bodies tend to form spherically symmetric sub-clusters with maximum-entropy eccentricity distribution.

Abstract

Rotating star clusters near supermassive black holes are studied using Touma-Tremaine thermodynamics of gravitationally interacting orbital ellipses. A simple numerical procedure for calculating thermodynamic equilibrium states for an arbitrary distribution of stars over masses and semimajor axes is described. Spontaneous symmetry breaking and breakdown of thermodynamics at low positive temperatures are rigorously proven for non-rotating clusters. Rotation is introduced through a second temperature-like parameter. Both axially symmetric and lopsided rotational equilibria are found; the lopsided equilibria precess with the angular velocity that is given by the ratio of the two temperatures. Eccentric stellar disc in the nucleus of Andromeda galaxy may be an example of a lopsided thermodynamic equilibrium of a rotating black hole star cluster. Stellar-mass black holes occupy highly eccentric orbits in broken-symmetry star clusters, and form flattened disc-like configurations in rotating star clusters. They are attracted to orbits that are stationary in the frame of reference rotating with the angular velocity of the cluster. In spherical clusters, stellar-mass black holes' orbits are significantly more eccentric than those of the lighter stars if the temperature is negative, and more circular if the temperature is positive. Finally we note that planets, comets, dark matter particles and other light bodies tend to form a spherically symmetric non-rotating sub-cluster with maximum-entropy eccentricity distribution $\mathscr{P}(e)=2e$, even if their host cluster is rotating and lopsided.