Using gravitational waves and multi-messenger Astronomy to reverse-engineer the properties of galactic nuclei

Abstract: Active galactic nuclei (AGN) are powered by accretion disks onto supermassive black holes in the the centers of galaxies. AGN are believed to play important roles in the evolution of both supermassive black holes and their host galaxies over cosmic time. AGN and the nuclear star clusters (NSCs) that interact with them remain unresolved with present and planned telescopes. As a result, the properties of AGN and NSCs are highly uncertain. Here we review how binary black hole (BBH) mergers can occur in AGN disks and how both the gravitational wave (GW) and electromagnetic wave (EM) properties of such mergers allow us to reverse-engineer the properties of AGN disks and NSCs over cosmic time. We point out that the feature in the BBH mass spectrum around $\sim 35M_{\odot}$ is an excellent probe of hierarchical merger models. Likewise constraints on the spins of upper-mass gap BH ($\gtrsim 50M_{\odot}$) test the AGN channel. The effective spin ($\chi_{\rm eff}$) distribution, including asymmetry, islands of structure and magnitudes are excellent tests of AGN model predictions. We also argue, that the rate of AGN-driven BBH mergers as a function of redshift should scale slightly shallower than the AGN number density, at least out to redshifts of $\sim 2$, and should turnover at the same redshift as the AGN number density. Finally, we emphasize a determination of an AGN fraction of observed BBH mergers ($f_{\rm BBH,AGN}$), \emph{regardless of the actual value}, allows us to infer the average properties of AGN disks and NSCs out to high redshift.