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

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) less than or similar to 100M), with one hBH (hBH1s: m(tot) similar to 130-260M), and with two hBHs (hBH2s: m(tot) similar to 270-400M), where m(tot) is the total mass of a BH-BH. Their merger rate densities at the current universe are similar to 0.1 yr(-1) Gpc(-3) for hBH0s, and similar to 0.01 yr(-1) Gpc(-3) for the sum of hBH1s and hBH2s, provided that the mass density of Population III stars is similar to 10(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 the near future. They have low effective spins less than or similar to 0.2 in the current universe. The number ratio of hBH2s to hBH1s is high, greater than or similar to 0.1. We also find that BHs in the mass gap (up to similar to 85M) merge. These merger rates can be reduced to nearly zero if Population III binaries are always wide (greater than or similar to 100R), and if Population III stars always enter into chemically homogeneous evolution. The presence of close Population III binaries (similar to 10R) is crucial for avoiding the worst scenario.

Automated summary

The study presents the merger rate density of Population III binary black holes, revealing that mergers with high-mass black holes may dominate in the future. The effective spins of these mergers are low and the number ratio of different types of mergers varies.