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Abstract

We investigate the detectability of single-event coalescing black hole
binaries with total mass of $100-600 M_{\odot}$ at cosmological distances ($5
\lesssim z \lesssim 20$) 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 b
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
$\sim 200 M_{\odot}$ we will be able to confidently infer that the redshift is
at least $z=12$, and for systems of $\sim 400 M_{\odot}$ 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.