Mosaiced wide-field VLBI observations of the Lockman Hole/XMM
  Active Galactic Nuclei (AGN) play a decisive role in galaxy evolution,
particularly so when operating in a radiatively inefficient mode, where they
launch powerful jets that reshape their surroundings. However, identifying them
is difficult, since radio observations commonly have resolutions of between 1
arcsec and 10 arcsec, which is equally sensitive to radio emission from
star-forming activity and from AGN. Very Long Baseline Interferometry (VLBI)
observations allow one to filter out all but the most compact non-thermal
emission from radio survey data. The observational and computational demands to
do this in large surveys have been, until recently, too high to make such
undertakings feasible. Only the recent advent of wide-field observing
techniques have facilitated such observations, and we here present the results
from a survey of 217 radio sources in the Lockman Hole/XMM field. We describe
in detail some new aspects of the calibration, including primary beam
correction, multi-source self-calibration, and mosaicing. As a result, we
detected 65 out of the 217 radio sources and were able to construct, for the
first time, the source counts of VLBI-detected AGN. They indicate that at least
15%-25% of the sub-mJy radio sources are AGN-driven, consistent with recent
findings using other AGN selection techniques. We have used ancillary data to
investigate the AGN hosts. We find that among the sources nearby enough to be
resolved in the optical images, 88% (23/26) could be classified as early-type
or bulge-dominated galaxies. While 50% of these sources are correctly
represented by the SED of an early-type galaxy, for the rest the best fit was
obtained with a heavily extinct starburst template, an effect we ascribe to a
degeneracy in the fit. Overall, the typical hosts of VLBI-detected sources are
in good agreement with being early-type or bulge-dominated galaxies.
