OBSERVING GRAVITATIONAL WAVES FROM THE FIRST GENERATION OF BLACK HOLES

The properties of the first generation of black hole seeds trace and distinguish different models of formation of cosmic structure in the high-redshift universe. The observational challenge lies in identifying black holes in the mass range similar to 100-1000 M-circle dot at redshift z similar to 10. The typical frequencies of gravitational waves produced by the coalescence of the first generation of light seed black hole binaries fall in the gap between the spectral ranges of low-frequency space-borne detectors (e. g., LISA) and high-frequency ground-based detectors (e. g., LIGO, Virgo, and GEO 600). As such, these sources are targets for proposed third-generation ground-based instruments, such as the Einstein Telescope which is currently in design study. Using galaxy merger trees and four different models of black hole accretion-which are meant to illustrate the potential of this new type of source rather than to yield precise event-rate predictions-we find that such detectors could observe a few to a few tens of seed black hole merger events in three years and provide possibly unique information on the evolution of structure in the corresponding era. We show further that a network of detectors may be able to measure the luminosity distance to sources to a precision of similar to 40%, allowing us to be confident of the high-redshift nature of the sources.