A VLBA search for binary black holes in active galactic nuclei with
  double-peaked optical emission line spectra
  We have examined a subset of 11 active galactic nuclei (AGN) drawn from a
sample of 87 objects that possess double-peaked optical emission line spectra,
as put forward by Wang et al. (2009a) and are detectable in the FIRST survey at
radio wavelengths. The double-peaked nature of the optical emission line
spectra has been suggested as evidence for the existence of binary black holes
in these AGN, although this interpretation is controversial. We make a simple
suggestion, that direct evidence of binary black holes in these objects could
be searched for in the form of dual sources of compact radio emission
associated with the AGN. To explore this idea, we have used the Very Long
Baseline Array to observe these 11 objects from the Wang et al. (2009a) sample.
Of the 11 objects, we detect compact radio emission from two, SDSS
J151709+335324 and SDSS J160024+264035. Both objects show single components of
compact radio emission. The morphology of SDSS J151709+335324 is consistent
with a recent comprehensive multi-wavelength study of this object by Rosario et
al. (2010). Assuming that the entire sample consists of binary black holes, we
would expect of order one double radio core to be detected, based on radio
wavelength detection rates from FIRST and VLBI surveys. We have not detected
any double cores, thus this work does not substantially support the idea that
AGN with double-peaked optical emission lines contain binary black holes.
However, the study of larger samples should be undertaken to provide a more
secure statistical result, given the estimated detection rates.
