The First Stellar Binary Black Holes: The Strongest Gravitational Wave Burst Sources

The evolution of the first populations of massive metal-free and metal-poor binary stars is followed. Such stars may form with large initial masses and evolve without significant mass loss. Stellar evolution at low metallicity may lead to the formation of intermediate-mass black holes (~100-500 M_solar) in the early universe, in contrast to the much lower mass black holes (~10 M_solar) formed at present. Following the assumption that some of these Population III stars have formed in binaries, we present the physical properties of the first stellar binary black holes. We find that a significant fraction of such binary black holes coalesce within the Hubble time. We point out that a burst of gravitational waves from the final coalescences and the following ringdown of these binary black hole mergers can be observed in the interferometric detectors. We estimate that the advanced Laser Interferometer Gravitational-Wave Observatory detection rate of such mergers ranges from several to a thousand events per year with a high signal-to-noise ratio (>~10) for a number of evolutionary models. We also identify assumptions that preclude the formation of massive binary black holes and point out what could be learned from the lack of detection of their mergers.