Gravitational wave backgrounds from coalescing black hole binaries at
  cosmic dawn: an upper bound
  The successive discoveries of binary merger events by Advanced LIGO-Virgo
have been revealing the statistical properties of binary black hole (BBH)
populations. A stochastic gravitational wave background (GWB) is a useful tool
to probe the cosmological evolution of those compact mergers. In this paper, we
study the upper bound on a GWB produced by BBH mergers, whose stellar
progenitors dominate the reionization process at the cosmic dawn. Since early
reionization by those progenitors yields a high optical depth of the universe
inconsistent with the {\it Planck} measurements, the cumulative mass density is
limited to $\rho_\star \lesssim 10^7~M_\odot~{\rm Mpc}^{-3}$. Even with this
upper bound, the amplitude of a GWB owing to the high-$z$ BBH mergers is
expected to be as high as $\Omega_{\rm gw}\simeq 1.48_{-1.27}^{+1.80}\times
10^{-9}$ at $f\simeq 25$ Hz, while their merger rate at the present-day is
consistent or lower than the observed GW event rate. This level of GWB is
detectable at the design sensitivity of Advanced LIGO-Virgo and would indicate
a major contribution of the high-$z$ BBH population to the local GW events. The
spectral index is expected to be substantially flatter than the canonical value
of $\simeq 2/3$ generically produced by lower-redshift and less massive BBHs.
Moreover, if their mass function is more top-heavy than in the local universe,
the GWB spectrum is even more skewed toward lower frequencies, which would
allow us to extract information on the mass function of merging BBHs at high
redshifts.
