Gravitational waves from Population III binary black holes formed by dynamical capture

ABSTRACT We use cosmological hydrodynamic simulations to study the gravitational wave (GW) signals from high-redshift binary black holes (BBHs) formed by dynamical capture (ex situ formation channel). We in particular focus on black holes (BHs) originating from the first generation of massive, metal-poor, so-called Population III (Pop III) stars. An alternative (in situ) formation pathway arises in Pop III binary stars whose GWligi signature has been intensively studied. In our optimistic model, we predict a local GW event rate density for ex situ BBHs (formed at z > 4) of ∼0.04 yr−1 Gpc−3. This is comparable to or even higher than the conservative predictions of the rate density for in situ BBHs ∼0.01–0.1 yr−1 Gpc−3, indicating that the ex situ formation channel may be as important as the in situ one for producing GW events. We also evaluate the detectability of our simulated GW events for selected planned GW instruments, such as the Einstein Telescope (ET). For instance, we find the all-sky detection rate with signal-to-noise ratios above 10 to be ${\lesssim} 100\, \rm {yr^{-1}}$ for the xylophone configuration of ET. However, our results are highly sensitive to the subgrid models for BBH identification and evolution, such that the GW event efficiency (rate) is reduced by a factor of 4 (20) in the pessimistic case. The ex situ channel of Pop III BBHs deserves further investigation with better modelling of the environments around Pop III-seeded BHs.