Intermediate mass black hole formation in compact young massive star clusters

Young dense massive star clusters are promising environments for the formation of intermediate mass black holes (IMBHs) through collisions. We present a set of 80 simulations carried out with nbody6++gpu of 10 models of compact star clusters with half-mass radii R-h less than or similar to 1 pc, central densities rho(core) greater than or similar to 10(5)M(circle dot) pc(-3), and resolved stellar populations with 10 percent primordial binaries. Very massive stars (VMSs) up to similar to 400M(circle dot) grow rapidly by binary exchange and three-body scattering with stars in hard binaries. Assuming that in VMS-stellar black hole (BH) collisions all stellar material is accreted on to the BH, IMBHs with masses up to M-BH similar to 350M(circle dot) can form on time-scales of less than or similar to 15 Myr, as qualitatively predicted from Monte Carlo MOCCA simulations. One model forms an IMBH of 140 M-circle dot by three BH mergers with masses of 17:28, 25:45, and 68:70 M-circle dot within similar to 90 Myr. Despite the stochastic nature of the process, formation efficiencies are higher in more compact clusters. Lower accretion fractions of 0.5 also result in IMBH formation. The process might fail for values as low as 0.1. The IMBHs can merge with stellar mass BHs in intermediate mass ratio inspiral events on a 100 Myr time-scale. With 10(5) stars, 10 per cent binaries, stellar evolution, all relevant dynamical processes, and 300 Myr simulation time, our large suite of 80 simulations indicate another rapid IMBH formation channel in young and compact massive star clusters.

Automated summary

By simulating young dense massive star clusters, a potential mechanism for the formation of intermediate mass black holes (IMBHs) through star collisions has been identified. Particularly, very massive stars grow rapidly through binary exchange and three-body scattering in hard binaries, potentially leading to the formation of IMBHs in a short period of time.