A quasar-based supermassive black hole binary population model:
  implications for the gravitational-wave background
  The nanohertz gravitational wave background (GWB) is believed to be dominated
by GW emission from supermassive black hole binaries (SMBHBs). Observations of
several dual active galactic nuclei (AGN) strongly suggest a link between AGN
and SMBHBs, given that these dual AGN systems will eventually form bound binary
pairs. Here we develop an exploratory SMBHB population model based on
empirically constrained quasar populations, allowing us to decompose the GWB
amplitude into an underlying distribution of SMBH masses, SMBHB number density,
and volume enclosing the GWB. Our approach also allows us to self-consistently
predict the number of local SMBHB systems from the GWB amplitude.
Interestingly, we find the local number density of SMBHBs implied by the
common-process signal in the NANOGrav 12.5-yr dataset to be roughly five times
larger than previously predicted by other models. We also find that at most
$\sim 25 \%$ of SMBHBs can be associated with quasars. Furthermore, our
quasar-based approach predicts $\gtrsim 95\%$ of the GWB signal comes from $z
\lesssim 2.5$, and that SMBHBs contributing to the GWB have masses $\gtrsim
10^8 M_\odot$. We also explore how different empirical galaxy-black hole
scaling relations affect the local number density of GW sources, and find that
relations predicting more massive black holes decrease the local number density
of SMBHBs. Overall, our results point to the important role that a measurement
of the GWB will play in directly constraining the cosmic population of SMBHBs,
as well as their connections to quasars and galaxy mergers.
