A Large Systematic Search for Close Supermassive Binary and Rapidly Recoiling Black Holes. IV. Ultraviolet Spectroscopy

We present Hubble Space Telescope ultraviolet (UV) spectroscopy of 13 quasars at z < 0.7, along with contemporaneous optical spectra from ground-based telescopes. The targets were selected to have broad Hβ emission lines with substantial velocity offsets relative to the rest frame of their host galaxy. By analogy to single-line spectroscopic binary stars, these objects have been regarded as supermassive black hole binary (SBHB) candidates where the offset emission lines may be caused by bulk orbital motion. The best alternative explanation is that the Hβ line profile is the result of nonaxisymmetric emission from a disk-like broad-line region associated with a single supermassive black hole. We use the broad UV line profiles to discriminate between these two scenarios. We describe our methodology for isolating the broad optical and UV line profiles and the criteria we apply for comparing them. Of the 13 SBHB candidates, three have strong evidence in support of the SBHB hypothesis, five have tentative support, one is disfavored, and four have such severely absorbed UV line profiles that the results are inconclusive.