Binary black holes in young star clusters: the impact of metallicity
  Young star clusters are the most common birth-place of massive stars and are
dynamically active environments. Here, we study the formation of black holes
(BHs) and binary black holes (BBHs) in young star clusters, by means of 6000
N-body simulations coupled with binary population synthesis. We probe three
different stellar metallicities (Z=0.02, 0.002 and 0.0002) and two initial
density regimes (density at the half-mass radius $\rho_{\rm
h}\ge{}3.4\times10^4$ and $\ge{1.5\times10^2}$ M$_\odot$ pc$^{-3}$ in dense and
loose star clusters, respectively). Metal-poor clusters tend to form more
massive BHs than metal-rich ones. We find $\sim{}6$, $\sim{}2$, and $<1$% of
BHs with mass $m_{\rm BH}>60$ M$_\odot$ at Z=0.0002, 0.002 and 0.02,
respectively. In metal-poor clusters, we form intermediate-mass BHs with mass
up to $\sim{}320$ M$_\odot$. BBH mergers born via dynamical exchanges
(exchanged BBHs) can be more massive than BBH mergers formed from binary
evolution: the former (latter) reach total mass up to $\sim{}140$ M$_\odot$
($\sim{}80$ M$_\odot$). The most massive BBH merger in our simulations has
primary mass $\sim{}88$ M$_\odot$, inside the pair-instability mass gap, and a
mass ratio of $\sim{}0.5$. Only BBHs born in young star clusters from
metal-poor progenitors can match the masses of GW170729, the most massive event
in O1 and O2, and those of GW190412, the first unequal-mass merger. We estimate
a local BBH merger rate density $\sim{}110$ and $\sim{}55$ Gpc$^{-3}$
yr$^{-1}$, if we assume that all stars form in loose and dense star clusters,
respectively.
