The Search for Binary Supermassive Black Holes amongst Quasars with Offset Broad Lines using the Very Long Baseline Array

In several previous studies, quasars exhibiting broad emission lines with &1000 km s􀀀1 velocity 16 osets with respect to the host galaxy rest frame have been discovered. One leading hypothesis for the 17 origin of these velocity-oset broad lines is the dynamics of a binary supermassive black hole (SMBH). 18 We present high-resolution radio imaging of 34 quasars showing these velocity-oset broad lines with 19 the Very Long Baseline Array (VLBA), aimed at nding evidence for the putative binary SMBHs (such 20 as dual radio cores), and testing the competing physical models. We detect exactly half of the target 21 sample from our VLBA imaging, after implementing a 5 detection limit. While we do not resolve 22 double radio sources in any of the targets, we obtain limits on the instantaneous projected separations 23 of a radio-emitting binary for all of the detected sources under the assumption that a binary still 24 exists within our VLBA angular resolution limits. We also assess the likelihood that a radio-emitting 25 companion SMBH exists outside of our angular resolution limits, but its radio luminosity is too weak 26 to produce a detectable signal in the VLBA data. Additionally, we compare the precise sky positions 27 aorded by these data to optical positions from both the SDSS and Gaia DR2 source catalogs. We nd 28 projected radio/optical separations on the order of 10 pc for three quasars. Finally, we explore how 29 future multi-wavelength campaigns with optical, radio, and X-ray observatories can help discriminate 30 further between the competing physical models.