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. We present 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. We predict the local number density of SMBHBs implied by the common-process signal in the NANOGrav 12.5-yr dataset and compare to SMBHB population models which do not take the GWB as input. Finally, we show how our model can be used to place constraints on the population of SMBHBs with associated quasar activity.