  NANOGrav and other Pulsar Timing Arrays (PTAs) have discovered a
common-spectrum process in the nHz range that may be due to gravitational waves
(GWs): if so, they are likely to have been generated by black hole (BH)
binaries with total masses $> 10^9 M_{\odot}$. Using the Extended
Press-Schechter formalism to model the galactic halo mass function and a simple
relation between the halo and BH masses suggests that these binaries have
redshifts $z = {O}(1)$ and mass ratios $\gtrsim 10$, and that the GW signal at
frequencies above ${O}(10)$~nHz may be dominated by relatively few binaries
that could be distinguished experimentally and would yield observable circular
polarization. Extrapolating the model to higher frequencies indicates that
future GW detectors such as LISA and AEDGE could extend the PTA observations to
lower BH masses $\in (10^6, 10^9 ) M_{\odot}$ and $\in (10^3, 10^9) M_{\odot}$.
