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Supermassive black hole binaries (SMBHBs) should form frequently in galacticnuclei as a result of galaxy mergers. At sub-parsec separations, binariesbecome strong sources of low-frequency gravitational waves (GWs), targeted byPulsar Timing Arrays (PTAs). We used recent upper limits on continuous GWs fromthe North American Nanohertz Observatory for Gravitational Waves (NANOGrav)11yr dataset to place constraints on putative SMBHBs in nearby massivegalaxies. We compiled a comprehensive catalog of ~44,000 galaxies in the localuniverse (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 derivedfrom global scaling relations. Assuming circular equal-mass binaries emittingat NANOGrav's most sensitive frequency of 8nHz, we found that 216 galaxies arewithin NANOGrav's sensitivity volume. We ranked the potential SMBHBs based onGW detectability by calculating the total signal-to-noise ratio (S/N) suchbinaries would induce within the NANOGrav array. We placed constraints on thechirp mass and mass ratio of the 216 hypothetical binaries. For 19 galaxies,only very unequal-mass binaries are allowed, with the mass of the secondaryless than 10 percent that of the primary, roughly comparable to constraints ona SMBHB in the Milky Way. Additionally, we were able to exclude binariesdelivered by major mergers (mass ratio of at least 1/4) for several of thesegalaxies. We also derived the first limit on the density of binaries deliveredby major mergers purely based on GW data.

The NANOGrav 11yr Data Set: Limits on Supermassive Black Hole Binaries in Galaxies within 500Mpc