  The existence of massive stellar black hole binaries (MBHBs), with primary
black hole (BH) masses $\ge 31 \, M_\odot$, was proven by the detection of the
gravitational wave (GW) event GW150914 during the first LIGO/Virgo observing
run (O1), and successively confirmed by seven additional GW signals discovered
in the O1 and O2 data. By adopting the galaxy formation model \texttt{GAMESH}
coupled with binary population synthesis (BPS) calculations, here we
investigate the origin of these MBHBs by selecting simulated binaries
compatible in mass and coalescence redshifts. We find that their cosmic birth
rates peak in the redshift range $6.5 \leq z \leq 10$, regardless of the
adopted BPS. These MBHBs are then old systems forming in low-metallicity ($Z
\sim [0.01-0.1] \, Z_{\odot}$), low-stellar-mass galaxies, before the end of
cosmic reionization, i.e. significantly beyond the peak of cosmic star
formation. GW signals generated by coalescing MBHBs open up new possibilities
to probe the nature of stellar populations in remote galaxies, at present too
faint to be detected by available electromagnetic facilities.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics;General Relativity and Quantum Cosmology