Gas reservoirs and star formation in a forming galaxy cluster at z=0.2
  We present first results from the Blind Ultra Deep HI Environmental Survey
(BUDHIES) of the Westerbork Synthesis Radio Telescope (WSRT). Our survey is the
first direct imaging study of neutral atomic hydrogen gas in galaxies at a
redshift where evolutionary processes begin to show. In this letter we
investigate star formation, HI-content, and galaxy morphology, as a function of
environment in Abell 2192 (at z=0.1876). Using a 3-dimensional visualization
technique, we find that Abell 2192 is a cluster in the process of forming, with
significant substructure in it. We distinguish 4 structures that are separated
in redshift and/or space. The richest structure is the baby cluster itself,
with a core of elliptical galaxies that coincides with (weak) X-ray emission,
almost no HI-detections, and suppressed star formation. Surrounding the
cluster, we find a compact group where galaxies pre-process before falling into
the cluster, and a scattered population of "field-like" galaxies showing more
star formation and HI-detections. This cluster proves to be an excellent
laboratory to understand the fate of the HI gas in the framework of galaxy
evolution. We clearly see that the HI gas and the star formation correlate with
morphology and environment at z=0.2. In particular, the fraction of
HI-detections is significantly affected by the environment. The effect starts
to kick in in low mass groups that pre-process the galaxies before they enter
the cluster. Our results suggest that by the time the group galaxies fall into
the cluster, they are already devoid of HI.
