Formation of Binary Black Holes Similar to GW190521 with a Total Mass of $\sim 150\,M_{\odot}$ from Population III Binary Star Evolution

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 $\sim 150 \,M_{\odot}$, 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$~(\rho_{\rm SFR}/(6\times10^5~M_{\odot}/{\rm Mpc}^3))~Err_{\rm sys}~{\rm yr^{-1}~Gpc^{-3}}$, where $\rho_{\rm SFR}$ and $Err_{\rm sys}$ 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 $\rho_{\rm SFR}=6\times10^5~M_{\odot}/{\rm Mpc}^3$ and $ Err_{\rm sys}=1$ is 0.13--0.66$~{\rm yr^{-1}~Gpc^{-3}}$, which is consistent with the observed value of 0.02--0.43$~{\rm yr^{-1}~Gpc^{-3}}$.
Comment: accepted by MNRAS Letter. comments welcome