On black hole sub-systems in idealized nuclear star clusters

TL;DR

This paper discusses how black hole sub-systems can persist in nuclear star clusters longer than previously thought, due to balanced evolution regulated by the host system's longer relaxation times.

Contribution

It introduces a model showing that black hole sub-systems in nuclear star clusters can survive longer through balanced evolution, challenging earlier assumptions.

Findings

Black hole sub-systems can persist in NSC longer than previously believed.

Balanced evolution regulates black hole subsystem dynamics.

Many NSC may host substantial black hole populations.

Abstract

Recent observational evidence, numerical simulations and theoretical arguments seem to indicate that stellar mass black holes (BH) persist in stellar systems such as globular star clusters for much longer than previously thought. Previously, theory implied that the BH would segregate into a compact system with short dynamical time scales, and that the BH would escape long before the present. But stellar systems can exist in a state of balanced evolution, where the energy generated in the core is regulated by the process of two-body relaxation in the bulk of the system. If the system has a centrally concentrated BH subsystem and there is no massive central BH, then the energy is generated by three-body encounters in the core of this subsystem. Therefore, in balanced evolution, the evolution of the BH subsystem is regulated by the much longer time scales of the host system. In the present paper the implications of these results for idealized nuclear star clusters (NSC) are discussed. Though previous theory implied that BH would be almost absent from many NSC -- those with relatively short dynamical time scales -- it is argued here that, based on the results of idealized models, many such NSC could still be host to substantial BH subsystems.