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 .
( 29 ) 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 \sim 2.6 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 .