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Supermassive black hole (SMBH) binaries are expected in a Lambda CDMcosmology given that most (if not all) massive galaxies contain a massive blackhole at their center. So far, however, direct evidence for such binaries hasbeen elusive. We use cross-correlation to search for temporal velocity shiftsin the MgII broad emission lines of 0.36 < z < 2 quasars with multipleobservations in the Sloan Digital Sky Survey. For ~ 10^9 Msun BHs in SMBHbinaries, we are sensitive to velocity drifts for binary separations of ~ 0.1pc with orbital periods of ~100 years. We find seven candidate sub-pc--scalebinaries with velocity shifts > 3.4 sigma ~ 280 km/s, where sigma is oursystematic error. Comparing the detectability of SMBH binaries with the numberof candidates (N < 7), we can rule out that most 10^9 Msun BHs exist in ~0.03-0.2 pc scale binaries, in a scenario where binaries stall at sub-pc scalesfor a Hubble time. We further constrain that < one-third of quasars host SMBHbinaries after considering gas-assisted sub-pc evolution of SMBH binaries,although this result is very sensitive to the assumed size of the broad lineregion. We estimate the detectability of SMBH binaries with ongoing ornext-generation surveys (e.g., BOSS, Subaru Prime Focus Spectrograph), takinginto account the evolution of the sub-parsec binary in circumbinary gas disks.These future observations will provide longer time baselines for searchessimilar to ours and may in turn constrain the evolutionary scenarios of SMBHbinaries.

Search for Supermassive Black Hole Binaries in the Sloan Digital Sky Survey Spectroscopic Sample