From Megaparsecs To Milliparsecs: Galaxy Evolution & Supermassive Black Holes with NANOGrav and the ngVLA

The dynamical evolution of supermassive black-hole binary systems is tethered on large scales to the merger rate of massive galaxies, and on small scales to the stellar and gaseous environments of galactic cores. The population of these systems will create an ensemble gravitational-wave signal at nanohertz frequencies that is detectable by Pulsar Timing Arrays (PTA), such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). By measuring nanohertz gravitational waves, we will be able to definitively demonstrate that supermassive black-hole binaries reach milliparsec separations, characterize their sub-parsec dynamical evolution, constrain scaling relationships between the bulge and central black-hole mass, and understand how gas inflow at large scales is fed to central galactic regions. The ngVLA will be an essential complement to this search; its goal resolution will allow for dual AGN to be distinguished, giving merger-rate estimates that can constrain PTA searches. If intercontinental VLBI capabilities are added, then parsec-scale resolution of binary evolution may be possible, indicating whether binaries stall or are driven efficiently to sub-parsec separations. NANOGrav gravitational-wave analysis, in concert with observations by the ngVLA and complementary facilities, will paint a multi-messenger portrait of galaxy evolution and the dynamics of the most massive black-holes in the Universe.