Merger Rate Density of Population III Binary Black Holes Below, Above, and in the Pair-instability Mass Gap

Abstract

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.

Population Synthesis of Black Hole Binaries with Normal-star Companions. I. Detached Systems

“…Until now, it is still impossible to distinguish between various channels based on GW observations alone. The popular channels involve the formation through isolated binary evolution (e.g., Tutukov & Yungelson 1993;Lipunov et al 1997;Voss & Tauris 2003;Kinugawa et al 2014;Belczynski et al 2016;Eldridge & Stanway 2016;Stevenson et al 2017;van den Heuvel et al 2017;Klencki et al 2018;Spera et al 2019;Breivik et al 2020;Zevin et al 2020;Tanikawa et al 2021;Bavera et al 2021;Olejak et al 2021), and dynamical interactions in globular clusters (Downing et al 2010;Rodriguez et al 2016;Askar et al 2017;Perna et al 2019;Kremer et al 2020) or young stellar clusters (Ziosi et al 2014;Di Carlo et al 2019;Santoliquido et al 2020;Rastello et al 2020). Other channels include the formation from isolated multiple systems (Silsbee & Tremaine 2017;Liu & Lai 2018;Hoang et al 2018;Fragione & Loeb 2019), the chemically homogeneous evolution for rapidly rotating stars Marchant et al 2016), as well as the disk of active galactic nuclei (Antonini & Rasio 2016;Stone et al 2017;McKernan et al 2018).…”

On the population III binary black hole mergers beyond the pair-instability mass gap

“…In order to perform a population synthesis calculation, we use the binary population synthesis code (Tanikawa et al 2021), which is an upgraded version of BSE (Hurley et al 2000(Hurley et al , 2002 1 . In our code, the fitting formulae for extremely metal poor stars were implemented (Tanikawa et al 2020).…”

“…We use the 𝛼𝜆 formalism for common envelope evolution (Webbink 1984) and set 𝛼 CE = 1 and 𝜆 CE = 1. We adopt the 'rapid' model in Fryer et al (2012) with the modification for the pulsational pair-instability and pair-instability supernova (see equations 5-7 in Tanikawa et al 2021). We assume that the mass of a BH formed through pulsational pair-instability is 45 𝑀 .…”

“…Previously, Belczynski et al (2004), Kinugawa et al (2014Kinugawa et al ( , 2020) and Tanikawa et al (2021) performed binary population synthesis calculations for Pop. III stars and predicted the property of BBH mergers.…”

High mass but low spin: an exclusion region to rule out hierarchical black-hole mergers as a mechanism to populate the pair-instability mass gap

“…The presence of stellar companions and/or gaseous environments might also aid the formation of heavier BHs via either stellar mergers (Di Carlo et al 2019, 2020Spera et al 2019;Renzo et al 2020a;Kremer et al 2020;González et al 2021) or accretion episodes (Marchant et al 2019;Roupas & Kazanas 2019;van Son et al 2020;Safarzadeh & Haiman 2020;Rice & Zhang 2021;Natarajan 2021). Forming BHs from Population III also provides an appealing explanation to the larger masses involved (Umeda et al 2020;Liu & Bromm 2020;Song et al 2020;Tanikawa et al 2020Tanikawa et al , 2021Farrell et al 2021;Kinugawa et al 2021). More speculative explanations include primordial BHs (De Luca et al 2021), exotic compact objects (Bustillo et al 2021) and physics beyond the standard model (Sakstein et al 2020).…”