  Supermassive black hole binaries (SMBHBs) should form frequently in galactic
nuclei as a result of galaxy mergers. At sub-parsec separations, binaries
become strong sources of low-frequency gravitational waves (GWs), targeted by
Pulsar Timing Arrays (PTAs). We used recent upper limits on continuous GWs from
the North American Nanohertz Observatory for Gravitational Waves (NANOGrav)
11yr dataset to place constraints on putative SMBHBs in nearby massive
galaxies. We compiled a comprehensive catalog of ~44,000 galaxies in the local
universe (up to redshift ~0.05) and populated them with hypothetical binaries,
assuming that the total mass of the binary is equal to the SMBH mass derived
from global scaling relations. Assuming circular equal-mass binaries emitting
at NANOGrav's most sensitive frequency of 8nHz, we found that 216 galaxies are
within NANOGrav's sensitivity volume. We ranked the potential SMBHBs based on
GW detectability by calculating the total signal-to-noise ratio (S/N) such
binaries would induce within the NANOGrav array. We placed constraints on the
chirp mass and mass ratio of the 216 hypothetical binaries. For 19 galaxies,
only very unequal-mass binaries are allowed, with the mass of the secondary
less than 10 percent that of the primary, roughly comparable to constraints on
a SMBHB in the Milky Way. Additionally, we were able to exclude binaries
delivered by major mergers (mass ratio of at least 1/4) for several of these
galaxies. We also derived the first limit on the density of binaries delivered
by major mergers purely based on GW data.
