Wide-field VLBA Observations of the Chandra Deep Field South
  Wide-field surveys are a commonly-used method for studying thousands of
objects simultaneously, to investigate, e.g., the joint evolution of
star-forming galaxies and active galactic nuclei. VLBI observations can yield
valuable input to such studies because they are able to identify AGN. However,
VLBI observations of large swaths of the sky are impractical using standard
methods, because the fields of view of VLBI observations are of the order of
10" or less. We have embarked on a project to carry out Very Long Baseline
Array (VLBA) observations of all 96 known radio sources in one of the
best-studied areas in the sky, the Chandra Deep Field South (CDFS). The
challenge was to develop methods which could significantly reduce the amount of
observing (and post-processing) time. We have developed an extension to the
DiFX software correlator which allows one to correlate hundreds of positions
within the primary beams. This extension enabled us to target many sources, at
full resolution and high sensitivity, using only a small amount of observing
time. The combination of wide fields-of-view and high sensitivity across the
field in this survey is unprecedented. We have observed with the VLBA a single
pointing containing the Chandra Deep Field South, in which 96 radio sources
were known from previous observations with the ATCA. From our input sample, 20
were detected with the VLBA. The majority of objects have flux densities in
agreement with arcsec-scale observations, implying that their radio emission
comes from very small regions. One VLBI-detected object had earlier been
classified as a star-forming galaxy. Comparing the VLBI detections to sources
found in sensitive, co-located X-ray observations we find that X-ray detections
are not a good indicator for VLBI detections. Wide-field VLBI survey science is
now coming of age.
