Studying the formation and evolution of black hole binaries (BHBs) is essential for the interpretation of current and forthcoming gravitational wave (GW) detections. We investigate the statistics of BHBs that form from isolated binaries, by means of a new version of the SEVN population-synthesis code. SEVN integrates stellar evolution by interpolation over a grid of stellar evolution tracks. We upgraded SEVN to include binary stellar evolution processes and we used it to evolve a sample of 1.5 × 10 8 binary systems, with metallicity in the range 10 −4 ; 4 × 10 −2 . From our simulations, we find that the mass distribution of black holes (BHs) in double compact-object binaries is remarkably similar to the one obtained considering only single stellar evolution. The maximum BH mass we obtain is ∼ 30, 45 and 55 M at metallicity Z = 2×10 −2 , 6×10 −3 , and 10 −4 , respectively. A few massive single BHs may also form ( < ∼ 0.1% of the total number of BHs), with mass up to ∼ 65, 90 and 145 M at Z = 2 × 10 −2 , 6 × 10 −3 , and 10 −4 , respectively. These BHs fall in the mass gap predicted from pair-instability supernovae. We also show that the most massive BHBs are unlikely to merge within a Hubble time. In our simulations, merging BHs like GW151226 and GW170608, form at all metallicities, the high-mass systems (like GW150914, GW170814 and GW170104) originate from metal poor (Z < ∼ 6 × 10 −3 ) progenitors, whereas GW170729-like systems are hard to form, even at Z = 10 −4 . The BHB merger rate in the local Universe obtained from our simulations is ∼ 90Gpc −3 yr −1 , consistent with the rate inferred from LIGO-Virgo data.

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 ρh ≥ 3.4 × 104 and ≥1.5 × 102 M⊙ pc−3 in dense and loose star clusters, respectively). Metal-poor clusters tend to form more massive BHs than metal-rich ones. We find ∼6, ∼2, and &lt;1 % of BHs with mass mBH &gt; 60 M⊙ at Z = 0.0002, 0.002 and 0.02, respectively. In metal-poor clusters, we form intermediate-mass BHs with mass up to ∼320 M⊙. 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 ∼140 M⊙ (∼80 M⊙). The most massive BBH merger in our simulations has primary mass ∼88 M⊙, inside the pair-instability mass gap, and a mass ratio of ∼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 ∼110 and ∼55 Gpc−3 yr−1, if we assume that all stars form in loose and dense star clusters, respectively.

We present the merger rate density of Population III binary black holes (BHs) by means of a widely used binary population synthesis code BSE with extensions to very massive and extreme metal-poor stars. We consider not only low-mass BHs (lBHs: 5–50M ⊙) but also high-mass BHs (hBHs: 130–200M ⊙), where lBHs and hBHs are below and above the pair-instability mass gap (50–130M ⊙), respectively. Population III BH–BHs can be categorized into three subpopulations: BH–BHs without hBHs (hBH0s: m tot ≲ 100M ⊙), with one hBH (hBH1s: m tot ∼ 130–260M ⊙), and with two hBHs (hBH2s: m tot ∼ 270–400M ⊙), where m tot is the total mass of a BH–BH. Their merger rate densities at the current universe are ∼0.1 yr−1 Gpc−3 for hBH0s, and ∼0.01 yr−1 Gpc−3 for the sum of hBH1s and hBH2s, provided that the mass density of Population III stars is ∼1013 M ⊙ Gpc−3. These rates are modestly insensitive to initial conditions and single star models. The hBH1 and hBH2 mergers can dominate BH–BHs with hBHs discovered in the near future. They have low effective spins ≲0.2 in the current universe. The number ratio of hBH2s to hBH1s is high, ≳0.1. We also find that BHs in the mass gap (up to ∼85M ⊙) merge. These merger rates can be reduced to nearly zero if Population III binaries are always wide (≳100R ⊙), and if Population III stars always enter into chemically homogeneous evolution. The presence of close Population III binaries (∼10R ⊙) is crucial for avoiding the worst scenario.