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 of the Westerbork Synthesis Radio Telescope. 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 three-dimensional visualization technique, we find that Abell 2192 is a cluster in the process of forming, with significant substructure in it. We distinguish four 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 similar to 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.