The formation and observational signatures of a new class of compact stellar-dynamical populations: the dark clusters

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In this proposed project the goal is to numerically study the end-phases of star clusters in which a significant number of stellar remnants, i.e., neutron stars (NS) and black holes (BH), are retained following their progenitor supernovae. Preliminary studies indicate that for star clusters residing within the inner few kiloparsec of the Galaxy, the low-mass main-sequence stars are stripped off from the cluster by the external Galactic tidal force on a timescale comparable to or shorter than the BH depletion time due to mutual encounters. Such rapid removal of stars results in the formation of gravitationally bound cluster remnants comprising a cluster of BHs orbited by a few luminous stars. To an observer, such an object would appear as a highly super-virial but compact star cluster with a large mass-to-light-ratio. We nickname such cluster remnants “dark clusters” which represent a predicted, totally new type of compact stellar population. The very existence and the probability of finding dark clusters will impose significant constraints on the natal kicks of BHs and establish star clusters as sites of significant dynamical activities among BHs which give rise to a wide variety of physical phenomena. In this proposed project, the formation and the properties of dark clusters will be studied in detail and systematically using direct N-body computations of realistically modelled star clusters. The aim is to deduce properties of dark clusters that can be readily utilized by observers as signatures, in a search of these newly predicted class of objects of considerable astrophysical significance.