2021ApJ...910...30T

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 ∼10¹³M_☉ 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.