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 star clusters with $1.1\times 10^5$ stars, a
velocity dispersion of $\sim$ 10 $\mathrm{km\,s^{-1}}$, a half mass radius
$R_h=0.6$ pc, and a binary fraction of $10\%$ including updated evolution
models for stellar winds and pair-instability supernovae (PISNe). Within the
first tens of megayears of evolution, 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. We present
several possible formation paths of massive BHs above the assumed lower PISNe
mass-gap limit ($45 M_\odot$) into the intermediate-mass BH (IMBH) regime ($>
100 M_\odot$) which include collisions of stars and BHs as well as the direct
collapse of stellar merger remnants with low mass cores. The stellar evolution
updates result in the early formation of higher mass stellar BHs than for 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 in the PISN mass-gap,
which is dominated by star-BH mergers. This IMBH formation path 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$M_\odot$,173$M_\odot$) which is expected to merge within a Hubble time.
