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.
