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

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 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 <1 % of BHs with mass mBH > 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.

GW190521: A Binary Black Hole Merger with a Total Mass of 150 M⊙

“…For GW190521, the mass of the heavier binary component has a high probability to be within the PISN mass gap [17,[20][21][22]. In dense stellar systems or active galactic nuclei disks, BHs with mass in the PISN gap might form via hierarchical coalescence of smaller BHs [23][24][25][26][27][28], or via direct collapse of a stellar merger between an evolved star and a main sequence companion [29,30].…”

Constraints from Gravitational-wave Detections of Binary Black Hole Mergers on the 12C(α, γ)16O Rate

“…The first-generation black holes in these environments will be limited by the PISN mass gap, but higher generation mergers would not be limited and could populate the gap (Di Carlo et al 2020b). Their effect on the inference of a g C , O…”

Measuring precession in asymmetric compact binaries

“…This makes the recent observation of GW190814, a highly asymmetric binary with a mass ratio of q ∼ 9, something of an enigma. Plausible formation channels for such asymmetric binaries include dynamical [95,96] and hierarchical merger scenarios [56,58,80,97,98]. For isolated binary formation channels, the prevalence of asymmetric compact binaries can be sensitive to the metallicity of the environment, with asymmetric binaries being preferred in low-metallicity environments [71,99,100].…”