The properties of merging black holes and neutron stars across cosmic time

The next generation ground-based gravitational wave interferometers will possibly observe mergers of binary black holes (BBHs) and binary neutron stars (BNSs) to redshift $z\gtrsim{}10$ and $z\gtrsim{}2$, respectively. Here, we characterize the properties of merging BBHs, BNSs and neutron star-black hole binaries across cosmic time, by means of population-synthesis simulations combined with the Illustris cosmological simulation. We find that the mass of merging compact objects does not depend (or depends very mildly) on the merger redshift. Even the mass distribution of black holes depends only mildly on redshift, because BBHs originating from metal-poor progenitors ($Z\leq{}4\times{}10^{-3}$) dominate the entire population of merging BBHs across cosmic time. For a common-envelope efficiency $\alpha{}\ge{}3$, the main difference between the mass distribution of BBHs merging in the last Gyr and that of BBHs merging more than 11 Gyr ago is that there is an excess of heavy merging black holes ($20-35$ M$_\odot$) in the last Gyr. This excess is explained by the longer delay time of massive BBHs.