VLBI studies of DAGN and SMBHB hosting galaxies
  Dual active galactic nuclei (DAGN) and supermassive black hole binaries
(SMBHBs) at kpc and pc-scale separations, respectively, are expected during
stages of galaxy merger and evolution. Their observational identification can
address a range of areas of current astrophysics frontiers including the final
parsec problem and their contribution towards the emission of low-frequency
gravitational waves. This has however been difficult to achieve with current
spectroscopy and time domain strategies. Very long baseline interferometry
(VLBI) as a method of directly imaging radio structures with milli-arcsecond
(mas) and sub-mas resolutions is introduced as a possible means of detecting
DAGN and SMBHBs. We motivate its usage with expected observational signatures
and cite some studies from literature to illustrate its current status, and
present an updated list of candidates imaged with high-resolution radio
observations. We then recall some shortcomings of the method with possible
solutions and discuss future directions, relevant to large surveys with the
upcoming Square Kilometer Array and future space VLBI missions.
