Black hole mergers in compact star clusters and massive black hole formation beyond the mass gap

We present direct N-body simulations, carried out with nbody6+ + gpu, of young and compact low-metallicity (Z = 0.0002) star clusters with 1.1 x 10(5) stars, a velocity dispersion of similar to 15 km s(-1), a half-mass radius R-h = 0.6 pc, and a binary fraction of 10 per cent including updated evolution models for stellar winds and (pulsation) pair-instability supernovae (PSNe). Within the first tens of megayears, each cluster hosts several black hole (BH) merger events which nearly cover the complete mass range of primary and secondary BH masses for current LIGO-Virgo-KAGRA gravitational wave detections. The importance of gravitational recoil is estimated statistically during post-processing analysis. We present possible formation paths of massive BHs above the assumed lower PSN mass-gap limit (45 M-circle dot) into the intermediate-mass black hole (IMBH) regime (>100 M-circle dot) which include collisions of stars, BHs, and the direct collapse of stellar merger remnants with low core masses. The stellar evolution updates result in the early formation of heavier stellar BHs compared to the previous model. The resulting higher collision rates with massive stars support the rapid formation of massive BHs. For models assuming a high accretion efficiency for star-BH mergers, we present a first-generation formation scenario for GW190521-like events: a merger of two BHs which reached the PSN mass-gap merging with massive stars. This event is independent of gravitational recoil and therefore conceivable in dense stellar systems with low escape velocities. One simulated cluster even forms an IMBH binary (153, 173 M-circle dot) which is expected to merge within a Hubble time.

Automated summary

We present direct N-body simulations of young and compact low-metallicity star clusters and investigate the formation of massive black holes (BHs) through BH mergers and stellar mergers. The simulations show that each cluster hosts multiple BH merger events within the first tens of megayears, covering a wide range of BH masses. We also propose formation scenarios for massive BHs above the assumed lower mass-gap limit, including collisions of stars, BHs, and the direct collapse of stellar merger remnants. The results suggest that the updated stellar evolution models lead to the early formation of heavier stellar BHs and support the rapid formation of massive BHs through increased collision rates with massive stars. The simulations also reveal a possible first-generation formation scenario for GW190521-like events.