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

Ugo N. Di Carlo; Michela Mapelli; Nicola Giacobbo; Mario Spera; Yann Bouffanais; Sara Rastello; Filippo Santoliquido; Mario Pasquato; Alessandro Ballone; Alessandro A. Trani; Stefano Torniamenti; Francesco Haardt

doi:10.1093/mnras/staa2286

paper

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

arXiv:2004.09525 · doi:10.1093/mnras/staa2286

Abstract

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 $ρ_{h} \geq 3.4 \times 1 0^{4}$ and $\geq 1.5 \times 1 0^{2}$ 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 $\sim 6$ , $\sim 2$ , and $< 1$ % of BHs with mass $m_{BH} > 60$ M $_{⊙}$ at Z=0.0002, 0.002 and 0.02, respectively. In metal-poor clusters, we form intermediate-mass BHs with mass up to $\sim 320$ 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 $\sim 140$ M $_{⊙}$ ( $\sim 80$ M $_{⊙}$ ). The most massive BBH merger in our simulations has primary mass $\sim 88$ M $_{⊙}$ , 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.

arXiv comment14 pages, 10 figures, 5 tables, accepted for publication in MNRAS

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

Abstract

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