On the likelihood of detecting gravitational waves from Population III compact object binaries

We study the contribution of binary black hole (BH-BH) mergers from the first, metal-free stars in the Universe (Pop III) to gravitational wave detection rates. Our study combines initial conditions for the formation of Pop III stars based on N-body simulations of binary formation (including rates, binary fraction, initial mass function, orbital separation and eccentricity distributions) with an updated model of stellar evolution specific for Pop III stars. We find that the merger rate of these Pop III BH-BH systems is relatively small (≲ 0.1 Gpc<SUP>-3</SUP> yr<SUP>-1</SUP>) at low redshifts (z < 2), where it can be compared with the LIGO empirical estimate of 9-240 Gpc<SUP>-3</SUP> yr<SUP>-1</SUP>. The predicted rates are even smaller for Pop III double neutron star (NS) and BH-NS mergers. Our rates are compatible with those of Hartwig et al., but significantly smaller than those found in previous work . We explain the reasons for this discrepancy by means of detailed model comparisons and point out that (I) identification of Pop III BH-BH mergers may not be possible by Advanced LIGO, and (II) the stochastic gravitational wave background from Pop III mergers may be lower than recently estimated. We further estimate gravitational wave detection rates for third-generation interferometric detectors. Our calculations are relevant for low to moderately rotating Pop III stars. We can now exclude a significant (>1 per cent) contribution of these stars to low-redshift BH-BH mergers. However, it remains to be tested whether (and at what level) rapidly spinning Pop III stars in the homogeneous evolution scenario can contribute to BH-BH mergers in the local Universe.