Merger rate density of Population III binary black holes below, above, and in the pair-instability mass gap

Ataru Tanikawa; Hajime Susa; Takashi Yoshida; Alessandro A. Trani; Tomoya Kinugawa

doi:10.3847/1538-4357/abe40d

paper

Merger rate density of Population III binary black holes below, above, and in the pair-instability mass gap

arXiv:2008.01890 · doi:10.3847/1538-4357/abe40d

Abstract

We present the merger rate density of Population (Pop.) 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: $5 - 50 M_{⊙}$ ) but also high-mass BHs (hBHs: $130 - 200 M_{⊙}$ ), where lBHs and hBHs are below and above the pair-instability mass gap ( $50 - 130 M_{⊙}$ ), respectively. Pop. III BH-BHs can be categorized into three subpopulations: BH-BHs without hBHs (hBH0s: $m_{tot} ≲ 100 M_{⊙}$ ), with one hBH (hBH1s: $m_{tot} \sim 130 - 260 M_{⊙}$ ), and with two hBHs (hBH2s: $m_{tot} \sim 270 - 400 M_{⊙}$ ), where $m_{tot}$ is the total mass of a BH-BH. Their merger rate densities at the current universe are $\sim 0.1$ yr $^{- 1}$ Gpc $^{- 3}$ for hBH0s, and $\sim 0.01$ yr $^{- 1}$ Gpc $^{- 3}$ for the sum of hBH1s and hBH2s, provided that the mass density of Pop. III stars is $\sim 1 0^{13} M_{⊙}$ 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 near future. They have low effective spins $≲ 0.2$ in the current universe. The number ratio of the hBH2s to the hBH1s is high, $≳ 0.1$ . We also find BHs in the mass gap (up to $\sim 85 M_{⊙}$ ) merge. These merger rates can be reduced to nearly zero if Pop. III binaries are always wide ( $≳ 100 R_{⊙}$ ), and if Pop. III stars always enter into chemically homogeneous evolution. The presence of close Pop. III binaries ( $\sim 10 R_{⊙}$ ) are crucial for avoiding the worst scenario.

arXiv comment38 pages, 33 figures, accepted for publication in ApJ

Merger rate density of Population III binary black holes below, above, and in the pair-instability mass gap

Abstract

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