A new type of compact stellar population: dark star clusters
  Among the most explored directions in the study of dense stellar systems is
the investigation of the effects of the retention of supernova remnants,
especially that of the massive stellar remnant black holes (BHs), in star
clusters. By virtue of their eventual high central concentration, these stellar
mass BHs potentially invoke a wide variety of physical phenomena, the most
important ones being emission of gravitational waves (GWs), formation of X-ray
binaries, and modification of the dynamical evolution of the cluster. Here we
propose, for the first time, that rapid removal of stars from the outer parts
of a cluster by the strong tidal field in the inner region of our Galaxy can
unveil its BH sub-cluster, which appears as a star cluster that is
gravitationally bound by an invisible mass. We study the formation and
properties of such systems through direct N-body computations and estimate that
they can be present in significant numbers in the inner region of the Milky
Way. We call such objects "dark star clusters" (DSCs) as they appear dimmer
than normal star clusters of similar mass and they comprise a predicted, new
class of entities. The finding of DSCs will robustly cross-check BH retention;
they will not only constrain the uncertain natal kicks of BHs, thereby the
widely debated theoretical models of BH formation, but will also pinpoint star
clusters as potential sites for GW emission for forthcoming ground-based
detectors such as the Advanced LIGO. Finally, we also discuss the relevance of
DSCs for the nature of IRS 13E.
