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Supermassive black hole binaries (SMBHs) are expected to result from galaxymergers, and thus are natural byproducts (and probes) of hierarchical structureformation in the Universe. They are also the primary expected source oflow-frequency gravitational wave emission. We search for binary BHs usingtime-variable velocity shifts in broad Mg II emission lines of quasars withmulti-epoch observations. First, we inspect velocity shifts of the binary SMBHcandidates identified in Ju et al. (2013), using SDSS spectra with anadditional epoch of data that lengthens the typical baseline to ~10 yr. We findvariations in the line-of-sight velocity shifts over 10 years that arecomparable to the shifts observed over 1-2 years, ruling out the binary modelfor the bulk of our candidates. We then analyze 1438 objects with 8 yr mediantime baselines, from which we would expect to see velocity shifts >1000 km/sfrom sub-pc binaries. We find only one object with an outlying velocity of 448km/s, indicating, based on our modeling, that ~< 1 per cent (the value varieswith different assumptions) of SMBHs that are active as quasars reside inbinaries with ~0.1 pc separations. Binaries either sweep through these smallseparations rapidly or stall at larger radii.

Searching for Binary Supermassive Black Holes via Variable Broad Emission Line Shifts: Low Binary Fraction