Binary black hole mergers from young massive clusters in the
  pair-instability supernova mass gap
  The recent discovery of the binary black hole (BBH) merger event GW190521,
between two black holes (BHs) of $\approx100M_\odot$, and as well as other
massive BBH merger events involving BHs within the pair-instability supernova
(PSN) mass gap have sparked widespread debate on the origin of such extreme
gravitational-wave (GW) events. In this study, I investigate whether dynamical
interactions in young massive clusters (YMCs) serves as a viable scenario for
assembling PSN-gap BBH mergers. To that end, I explore a grid of 40 new
evolutionary models of a representative YMC of initial mass $M_{\rm
cl}=7.5\times10^4M_\odot$ ($N\approx1.28\times10^5$) and size $r_h=2$ pc, with
all BH progenitor stars being initially in primordial binaries. All cluster
models are evolved with the direct, relativistic N-body code NBODY7
incorporating up to date remnant formation, BH natal spin, and
general-relativistic (GR) merger recoil schemes. The BBH mergers from these
model cluster computations agree well with the masses and effective spin
parameters of the GW events in the latest GW transient catalogue (GWTC). In
particular, GW190521-like, i.e., $\approx200M_\odot$, low aligned spin events
are produced via dynamical merger among BHs derived from star-star merger
products. GW190403-like, i.e., PSN-gap, highly asymmetric and aligned events
result from mergers involving BHs that are spun up via matter accretion or
binary interaction. The present YMC models yield a present day, intrinsic
merger rate density of $0-3.8\times10^{-2}{{\rm~yr}^{-1}{\rm Gpc}^{-3}}$ for
GW190521-type events. They produce GW190403_051519-like events at a rate within
$0-1.6\times10^{-1}{{\rm~yr}^{-1}{\rm Gpc}^{-3}}$ and their total BBH-merger
yield within the PSN gap is $0-8.4\times10^{-1}{{\rm~yr}^{-1}{\rm Gpc}^{-3}}$.
