Abstract :
Dense stellar clusters are a natural environment for black holes to be born, form binaries and merge, because of the high density that enhances close encounters. Being the remnants of the death of very massive stars, stellar black holes are generated early in the life of the cluster, when primordial gas is still present. Due to mass segregation the black holes accumulate close to the center of the cluster, where gas density is high. In this dense gaseous environment, they may accrete gas and grow in mass before the gas is depleted because of stellar formation feedback processes. In addition, the accretion hardens the binary and accelerates mass segregation in the center. I suggest that this process results in such an increase of BH masses that the upper black hole mass gap of 50-130 solar masses, suggested by supernova theory, is populated. This will enhance binary black hole mergers and induce a characteristic spin signature detectable by LIGO. I further suggest that due to inhomogeneous diffusion a black hole subcluster may be hosted inside dense stellar clusters. I will finally consider systems with regular orbits, such as black holes orbiting a supermassive black hole in galactic nuclei. Like the spins in a ferromagnet the angular momenta of the orbits present order-disorder phase transitions. In the ordered phase, angular momenta are aligned resulting in the formation of a black hole disk, where binary black hole mergers are more frequent.
