Illuminating black hole subsystems in young star clusters
  There is increasing evidence that globular clusters retain sizeable black
hole populations at present day. This is supported by dynamical simulations of
cluster evolution, which have unveiled the spatial distribution and mass
spectrum of black holes in clusters across cosmic age. However, black hole
populations of young, high metallicity clusters remain unconstrained. Black
holes hosted by these clusters mass segregate early in their evolutionary
history, forming central subsystems of hundreds to thousands of black holes. We
argue that after supernova feedback has subsided ($\gtrsim 50\,{\rm Myr}$), the
host cluster can accumulate gas from its dense surroundings, from which the
black hole subsystem accretes at highly enhanced rates. The collective
accretion luminosity can be substantial and provides a novel observational
constraint for young massive clusters. We test this hypothesis by performing 3D
hydrodynamic simulations where we embed discretized potentials, representing
our black holes, within the potential of a massive cluster. This system moves
supersonically with respect to a gaseous medium from which it accretes. We
study the accretion of this black hole subsystem for different subsystem
populations and determine the integrated accretion luminosity of the black hole
subsystem. We apply our results to the young massive clusters of the Antennae
Galaxies and find that a typical subsystem accretion luminosity should be in
excess of $\approx 10^{40}\,{\rm ergs\,\,s^{-1}}$. We argue that no strong
candidates of this luminous signal have been observed and constrain the
subsystem population of a typical cluster in the Antennae Galaxies to
$\lesssim10-2\times10^2$ $10\,M_\odot$ black holes, given that feedback doesn't
significantly impede accretion and that the gas remains optically thin.
