The Search for Signatures of Supermassive Black Hole Binaries (and the Remnants of those Mergers) in the Time Domain

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Part 1Binary supermassive black holes (SMBHs) should be an inevitable consequence of the growth of massive galaxies through mergers, and the strongest source of gravitational waves in the universe. However, observational evidence for this fundamental phase of changes in galaxy and black hole has remained hard to find due to the small orbital separations of gravitationally bound SMBHs. In this program, the proposers will exploit a theoretically predicted signature of a SMBH binary: periodic changes of the object's brightness over time, caused by a mass accretion rate that is linked to the binary's orbital motion.Part 2Theoretical estimates assuming reasonable values for the galaxy merger rates, quasar lifetime, and brightness variations, predict that the recently completed Pan-STARRS1 Medium Deep Survey (PS1 MDS) has the area (80 square degrees), cadence (3 days), baseline (4.2 years), and magnitude sensitivity required to detect a significant number of periodic quasars during the phase of their binary SMBHs's orbital decay cause by gravity waves. In this project the PIs will analyze all 10 PS1 MDS fields, including over 7,000 photometrically-selected quasars. Furthermore, they will exploit the wide area and excellent cadence of the upcoming Zwicky Transient Facility (ZTF) to use difference imaging to search for active galactic nuclei spatially offset from their host galaxy center, potentially indicative of a recoiling SMBH merger remnant. Both searches will provide benchmark studies for the exciting capabilities of Large Synoptic Survey Telescope (LSST), which will have orders of magnitude larger survey power.