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The nanohertz gravitational wave background (GWB) is believed to be dominatedby GW emission from supermassive black hole binaries (SMBHBs). Observations ofseveral dual active galactic nuclei (AGN) strongly suggest a link between AGNand SMBHBs, given that these dual AGN systems will eventually form bound binarypairs. Here we develop an exploratory SMBHB population model based onempirically constrained quasar populations, allowing us to decompose the GWBamplitude into an underlying distribution of SMBH masses, SMBHB number density,and volume enclosing the GWB. Our approach also allows us to self-consistentlypredict the number of local SMBHB systems from the GWB amplitude.Interestingly, we find the local number density of SMBHBs implied by thecommon-process signal in the NANOGrav 12.5-yr dataset to be roughly five timeslarger than previously predicted by other models. We also find that at most$\sim 25 \%$ of SMBHBs can be associated with quasars. Furthermore, ourquasar-based approach predicts $\gtrsim 95\%$ of the GWB signal comes from $z\lesssim 2.5$, and that SMBHBs contributing to the GWB have masses $\gtrsim10^8 M_\odot$. We also explore how different empirical galaxy-black holescaling relations affect the local number density of GW sources, and find thatrelations predicting more massive black holes decrease the local number densityof SMBHBs. Overall, our results point to the important role that a measurementof the GWB will play in directly constraining the cosmic population of SMBHBs,as well as their connections to quasars and galaxy mergers.

A quasar-based supermassive black hole binary population model: implications for the gravitational-wave background