Two in one? A possible dual radio-emitting nucleus in the quasar SDSS
  J1425+3231
  The radio-emitting quasar SDSS J1425+3231 (z=0.478) was recently found to
have double-peaked narrow [O III] optical emission lines. Based on the analysis
of the optical spectrum, Peng et al. (2011) suggested that this object harbours
a dual active galactic nucleus (AGN) system, with two supermassive black holes
(SMBHs) separated on the kpc scale. SMBH pairs should be ubiquitous according
to hierarchical galaxy formation scenarios in which the host galaxies and their
central black holes grow together via interactions and eventual mergers. Yet
the number of presently-confirmed dual SMBHs on kpc or smaller scales remains
small. A possible way to obtain direct observational evidence for duality is to
conduct high-resolution radio interferometric measurements, provided that both
AGN are in an evolutionary phase when some activity is going on in the radio.
We used the technique of Very Long Baseline Interferometry (VLBI) to image SDSS
J1425+3231. Observations made with the European VLBI Network (EVN) at 1.7 GHz
and 5 GHz frequencies in 2011 revealed compact radio emission at sub-mJy flux
density levels from two components with a projected linear separation of
\sim2.6 kpc. These two components support the possibility of a dual AGN system.
The weaker component remained undetected at 5 GHz, due to its steep radio
spectrum. Further study will be necessary to securely rule out a jet--shock
interpretation of the less dominant compact radio source. Assuming the dual AGN
interpretation, we discuss black hole masses, luminosities, and accretion rates
of the two components, using available X-ray, optical, and radio data. While
high-resolution radio interferometric imaging is not an efficient technique to
search blindly for dual AGN, it is an invaluable tool to confirm the existence
of selected candidates.
