Identifying heavy stellar black holes at cosmological distances with next-generation gravitational-wave observatories

We investigate the detectability of single-event coalescing black hole binaries with total mass of 100-600M(circle dot) at cosmological distances (5 less than or similar to z less than or similar to 20) with the next generation of terrestrial gravitational wave observatories, specifically Einstein Telescope and Cosmic Explorer. Our ability to observe these binaries is limited by the low-frequency performance of the detectors. Higher order multipoles of the gravitational wave signal are observable in these systems, and detection of such multipoles serves to both extend the mass range over which black hole binaries are observable and improve the recovery of their individual masses and redshift. For high-redshift systems of similar to 200M(circle dot) we will be able to confidently infer that the redshift is at least z = 12, and for systems of similar to 400M(circle dot) we can infer a minimum redshift of at least z = 8. We discuss the impact that these observations will have in narrowing uncertainties on the existence of the pair-instability mass gap, and their implications on the formation of the first stellar black holes that could be seeds for the growth of supermassive black holes powering high-z quasars.