ABSTRACT:
Sub-parsec binary supermassive black holes (BBHs) are predicted products of hierarchical galaxy mergers, and are of great interest to both galaxy formation studies and gravitational physics. Finding these systems in the EM domain, however, is challenging given the stringent spatial resolution requirement. Peculiar spectroscopic features of the broad emission lines of quasars have been used to identify spatially-unresolved sub-pc BBH candidates. But followup spectroscopic monitoring of these candidates is required to confirm their binary nature. I will review the current status of searching for sub-pc BBHs in quasars with this spectroscopic technique, and describe our long-term effort in searching for such objects in the era of massive spectroscopic surveys.