Binary black holes in young star clusters: the impact of metallicity

Young star clusters are the most common birthplace 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(h) >= 3.4 x 10(4) and >= 1.5 x 10(2) M-circle dot pc(-3) in dense and loose star clusters, respectively). Metal-poor clusters tend to form more massive BHs than metal-rich ones. We find similar to 6, similar to 2, and <1 per cent of BHs with mass m(BH) > 60 M-circle dot at Z = 0.0002, 0.002, and 0.02, respectively. In metal-poor clusters, we form intermediate-mass BHs with mass up to similar to 320 M-circle dot. 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 similar to 140 M-circle dot (similar to 80 M-circle dot). The most massive BBH merger in our simulations has primary mass similar to 88 M-circle dot, inside the pair-instability mass gap, and a mass ratio of similar to 0.5. Only BBHs born in young star clusters from metal-poor progenitors can match the masses of GW 170729, the most massive event in first and second observing run (O1 and O2), and those of GW 190412, the first unequal-mass merger. We estimate a local BBH merger rate density similar to 110 and similar to 55 Gpc(-3) yr(-1), if we assume that all stars form in loose and dense star clusters, respectively.