Can quasars, triggered by mergers, account for NANOGrav's stochastic gravitational wave background?

Abstract

The stochastic gravitational wave background (GWB) recently discovered by several pulsar timing array (PTA) experiments is consistent with arising from a population of coalescing super-massive black hole binaries (SMBHBs). The amplitude of the background is somewhat higher than expected in most previous population models or from the local mass density of SMBHs. SMBHBs are expected to be produced in galaxy mergers, which are also thought to trigger bright quasar activity. Under the assumptions that (i) a fraction $f_{bin} \sim 1$ of all quasars are associated with SMBHB mergers, (ii) the typical quasar lifetime is $t_{Q} \sim 10^{8} yr$, and (iii) adopting Eddington ratios $f_{Edd} \sim 0.3$ for the luminosity of bright quasars, we compute the GWB associated directly with the empirically measured quasar luminosity function (QLF). This approach bypasses the need to model the cosmological evolution of SMBH or galaxy mergers from simulations or semi-analytical models. We find a GWB amplitude approximately matching the value measured by NANOGrav. Our results are consistent with most quasars being associated with SMBH binaries and being the sources of the GWB, and imply a joint constraint on $t_{Q}$, $f_{Edd}$ and the typical mass ratio $q \equiv M_{2}/M_{1}$. The GWB in this case would be dominated by relatively distant $\sim 10^{9} M_{\odot}$ SMBHs at $z \approx 2 - 3$, at the peak of quasar activity. Similarly to other population models, our results remain in tension with the local SMBH mass density.