Gravitational lensing at milliarcsecond angular resolution with VLBI
  observations
  Gravitational lensing is a powerful tool for quantifying the mass content and
distribution in distant galaxies. By using milliarcsecond angular resolution
observations of radio-loud gravitationally lensed sources it is also possible
to detect and quantify small deviations from a smooth mass density
distribution, which can be due to low mass substructures in the lensing galaxy.
We present high-resolution global VLBI observations of the gravitationally
lensed radio source MG J0751+2716 (at z = 3.2), that shows evidence of both
compact and extended structure (core-jet morphology) across several
gravitational arcs. These data provide a wealth of observational constraints
that are used to determine the inner (baryonic and dark matter) mass profile of
a group of galaxies and also investigate the smoothness of the dark matter
distribution on mas-scales, which is sensitive to possible structures of
$10^{6-7}$ M$_{\odot}$ within the lensing halo or along the line-of-sight. Our
lens modelling finds evidence for astrometric anomalies in this system, which
suggest presence of extra mass structure in the lens model. To date this kind
of detailed studies of gravitational lensing systems like MG J0751+2716 has
been limited by the currently small sample of radio-loud gravitational lenses.
In this context, we also present a new pilot gravitational lens search in the
VLBI survey mJIVE-20, in perspective of future surveys with the next generation
of radio interferometers.
