Probing Binary SMBHs with the VLBA

Current models for galaxy evolution invoke mergers between galaxies in order to explain correlations between the central supermassive black hole (SMBH) and various host galaxy properties (e.g., Ferrarese et al., 2000 and Gebhardt et al., 2000). It is believed that as a result of these mergers, dual (< ~1 kpc) and binary (< ~ 10pc) SMBHs are formed. At small separations (≪1 kpc), these binary black holes would emit strong gravitational wave signatures which could be detected by pulsar timing arrays (PTAs). In a sample of ~ 15,900 sloan digital sky survey (SDSS) quasars, Eracleous et al. (2012) found 88 objects with velocity-offset broad emission lines (originating from the quasar broad line region) with respect to the host galaxy rest frame. A leading explanation for such offset lines is that they originate from a binary SMBH system. In this talk,I will present some results from the high-resolution very long baseline array (VLBA) X-band observations of 37 candidate binary SMBH systems from the Eracleous et al. (2012) sample, aimed at directly imaging these binaries. Direct observations of binary SMBHs will help to constrain the strength and distribution of objects emitting gravitational waves detectable by PTAs. Other VLBA investigations of binary SMBH candidates will also be discussed, if time permits.