Sub-Parsec Supermassive Binary Quasars: Expectations at z < 1

We investigate the theoretical expectations for detections of supermassive binary black holes that can be identified as sub-parsec luminous quasars. To date, only two candidates have been selected in a sample comprising 17,500 sources selected from the Sloan Digital Sky Survey (SDSS) Quasar Catalog at z < 0.70. In this Letter, we use models of assembly and growth of supermassive black holes (SMBHs) in hierarchical cosmologies to study the statistics and observability of binary quasars at sub-parsec separations. Our goal is twofold: (1) to test if such a scarce number of binaries is consistent with theoretical prediction of SMBH merger rates and (2) to provide additional predictions at higher redshifts and at lower flux levels. We determine the cumulative number of expected binaries in a complete volume-limited sample. Motivated by Boroson & Lauer, we apply the SDSS quasar luminosity cut (Mi < -22) to our theoretical sample, deriving an upper limit to the observable binary fraction. We find that sub-parsec quasar binaries are intrinsically rare. Our best models predict ∼0.01 deg-2 sub-parsec binary quasars with separations below 104 Schwarzschild radii (v orb > 2000 kms) at z < 0.7, which represent a fraction ∼6 × 10-4 of unabsorbed quasars in our theoretical sample. In a complete sample of ∼10,000 sources, we therefore predict an upper limit of ∼10 sub-parsec binary quasars. The number of binaries increases rapidly with increasing redshift. The decreasing lifetime with SMBH binary mass suggests that lowering the luminosity threshold does not lead to a significant increase in the number of detectable sub-parsec binary quasars. © 2009. The American Astronomical Society. All rights reserved.