mJIVE-20: a survey for compact mJy radio objects with the Very Long
  Baseline Array
  We present the description and early results of the mJy Imaging VLBA
Exploration at 20 cm (mJIVE-20). mJIVE-20 is a large project on the Very Long
Baseline Array which uses "filler" time to systematically inspect a large
sample of mJy radio sources, pre-selected from the FIRST survey made with the
Very Large Array, to identify any compact emission which may be present. The
newly available multifield capability of the VLBA makes it possible for us to
inspect of order 100 sources per hour of observing time with a 6.75 sigma
detection sensitivity of approximately 1 mJy/beam. The results of the mJIVE-20
survey are made publicly available as soon as the data are calibrated. After 18
months of observing, over 20,000 FIRST sources have been inspected, with 4,336
VLBI detections. These initial results suggest that within the range 1 - 200
mJy, fainter sources are somewhat more likely to be dominated by a very compact
component than brighter sources. Over half of all arcsecond-scale mJy radio
sources contain a compact component, although the fraction of sources which are
dominated by milliarcsecond scale structure is smaller at 30-35%, increasing
towards lower flux densities. Significant differences are seen depending on the
optical classification of the source. Radio sources with a stellar/point-like
counterpart in the Sloan Digital Sky Survey (SDSS) are more likely to be
detected overall, but this detection likelihood appears to be independent of
the arcsecond-scale radio flux density; the trend towards higher radio
compactness for fainter sources is confined to SDSS galaxies and
non-detections. These results are consistent with a unification model of AGN in
which less luminous sources have on average slower radio jets, with lower
Doppler suppression of compact core emission over a wider range of viewing
angles. (abridged)
