Interpreting the gravitational-wave background in terms of supermassive black hole binary populations

To date, most models of the stochastic gravitational wave background have built up the supermassive black hole binary population through modeling the merger histories of massive galaxies to z ~ 1. However, the recent discovery of the dual AGN J1010+1413 by Goulding et al. (2019) has opened up the possibility of using AGN population models as a proxy for massive black hole binary populations. We present the results of recent efforts to build on this model of the gravitational wave background by combining it with estimates of the local number of binary sources from Mingarelli et al. (2017), as well as an analysis of the volume and mass containing the background that is implied by such a model. We additionally compare this AGN proxy model of the massive black hole binary population to population models predicted by galactic merger models. Finally, we examine possible improvements that can be made to this model, including allowing the mass function to vary with redshift, as well as by allowing variations in the ratio of the binary and AGN population over redshift.