Chirp mass and spin of binary black holes from first star remnants

We performed Population III (Pop III) binary evolution using population synthesis simulations for seven different models. We found that Pop III binaries tend to be binary black holes (BBHs) with chirp mass M_chirp ∼ 30 M_⊙ and they can merge in the present day, due to a long merger time. The merger rate densities of Pop III BBHs at z = 0 are in the range 3.34-21.2 $\rm yr^{-1}\,Gpc^{-3}$ which is consistent with the Advanced Laser Interferometer Gravitational Wave Observatory (aLIGO)/Advanced Virgo (aVIRGO) result of 9.7-101 $\rm yr^{-1}\,Gpc^{-3}$ . These Pop III binaries might contribute some portion of the massive BBH gravitational wave (GW) sources detected by aLIGO/aVIRGO. We also calculated the redshift dependence of Pop III BBH mergers. We found that Pop III low-spin BBHs tend to merge at low redshift, while Pop III high-spin BBHs merge at high redshift, which can be confirmed by future GW detectors such as Einstein Telescope (ET), Cosmic Explorer (CE), and DECi-hertz Interferometer Gravitational wave Observatory (DECIGO). These detectors can also check the redshift dependence of the BBH merger rate and spin distribution. Our results show that, except for one model, the mean effective spin <χ_eff> at z = 0 lies in the range 0.02-0.3, while at z = 10 it is 0.16-0.64. Therefore, massive stellar-mass BBH detection by GWs will be key for stellar evolution study in the early Universe.