On black hole subsystems in idealized nuclear star clusters

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.