An Observational Search for Close, Supermassive Binary Black Holes

In our standard scenarios galaxy evolution via mergers, supermassive binary black holes appear to be inevitable. Yet, close supermassive binaries, at separations of a parsec or less have remained elusive. Finding such binaries constitutes a test of galaxy evolution models as well as models that invoke them in order to explain a variety of other phenomena. Separations of about a parsec are particularly interesting because they are the progenitors of gravitational wave sources. Moreover, early models suggested that binaries may spend a great deal of time at these separations. Motivate by the above considerations, our group embarked a few years ago on a search for close supermassive binary black holes. I will describe the methodology that we are using, the results we have so far, and our strategy for future observations.