SIMBAD references

In the case of zero-metal (Population III or Pop III) stars, we show that the total mass of binary black holes from binary Pop III star evolution can be ∼150 M_☉, which agrees with the mass of the binary black hole GW190521 recently discovered by LIGO/Virgo. The event rate of such binary black hole mergers is estimated as 0.13-0.66 (ρ_SFR/(6 x 10⁵ M_☉ Mpc^–3)) Err_sys yr^–1 Gpc^–3, where ρ_SFR and Err_sys are the cumulative comoving mass density of Pop III stars depending on star formation rate and the systematic errors depending on uncertainties in the Pop III binary parameters, respectively. The event rate in our fiducial model with ρ_SFR = 6 x 10⁵ M_☉ Mpc^–3 and Err_sys = 1 is 0.13-0.66 yr^–1 Gpc^–3, which is consistent with the observed value of 0.02-0.43 yr^–1 Gpc^–3.