Abstract

We use cosmological hydrodynamic simulations to study the gravitational wave (GW) signals from high-redshift ex-situ binary black holes (BBHs) formed by dynamical capture. We particularly focus on BHs originated from the remnants of the first generation of massive, metal-poor stars (so-called Pop~III stars). These BHs can also be born into binary systems from Pop~III binary stars (i.e. in-situ BBHs), whose GW signals have been intensely studied. Our fiducial simulation produces a local intrinsic rate density of GW events for ex-situ BBHs (formed at z>4) as 0.01-0.1 yr^-1 Gpc^-3, comparable to the rate density predicted for in-situ BBHs, showing that the ex-situ channel of binary BH formation is can be as important as the in-situ channel for producing GWs. We also use the simulated BBHs to evaluate the detectability of BH-BH merger events for select future planned GW instruments such as the Einstein Telescope (ET) and Decihertz observatories (DOs). For instance, we found that the all-sky detection rates with signal-to-noise ratios above 10 to be 10-200 yr^-1 for the xylophone configuration of ET and the optimal DO, with uncertainties from our semi-analytical model for the evolution of ex-situ BBHs.