The Local Nanohertz Gravitational-Wave Landscape From Supermassive Black
  Hole Binaries
  Supermassive black hole binaries (SMBHBs) in the 10 million to 10 billion
$M_\odot$ range form in galaxy mergers, and live in galactic nuclei with large
and poorly constrained concentrations of gas and stars. There are currently no
observations of merging SMBHBs--- it is in fact possible that they stall at
their final parsec of separation and never merge. While LIGO has detected high
frequency GWs, SMBHBs emit GWs in the nanohertz to millihertz band. This is
inaccessible to ground-based interferometers, but possible with Pulsar Timing
Arrays (PTAs). Using data from local galaxies in the 2 Micron All-Sky Survey,
together with galaxy merger rates from Illustris, we find that there are on
average $91\pm7$ sources emitting GWs in the PTA band, and $7\pm2$ binaries
which will never merge. Local unresolved SMBHBs can contribute to GW background
anisotropy at a level of $\sim20\%$, and if the GW background can be
successfully isolated, GWs from at least one local SMBHB can be detected in 10
years.
