Limits on Individual Supermassive Black Hole Binaries from the NANOGrav 11-year data set*

Supermassive black hole binaries form in galaxy mergers and emit low-frequency gravitational waves (GWs) that can be detected by pulsar timing arrays.~We have searched the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11-year data set for GWs~from such systems. We placed limits on the GW strain, and used these limits to constrain the properties of supermassive black hole binaries~in the local Universe. We also used our results to constrain the merger history of nearby massive galaxies.