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The BUDHIES Project

BUDHIES is a deep HI survey of galaxies in two clusters at 0.16 < z < 0.22 and the large-scale structure around them. The survey aims at understanding where, how, and why star-forming spiral galaxies get transformed into passive early-type galaxies. The unique aspect of our study is that, for the first time, we have accurate measurements of the HI content of galaxies in different environments at intermediate redshift. Our study is the first where optical properties and gas content are combined at a redshift where evolutionary effects begin to show, and in a volume large enough to sample all environments, ranging from voids to cluster cores.

The deep HI observations were carried out at the Westerbork Synthesis Radio Telescope, thanks to a new back end and the low system temperature of the telescope. Multi wavelength follow-ups have also been carried out in the two volumes, including Isaac Newton Telescope optical imaging and multi-object spectroscopy with the William Herschel Telescope in La Palma. The optical data has allowed us to study in detail the environments of the HI-detected galaxies and the stellar populations of the galaxies in and around the two clusters. Additional studies in the UV (GALEX) and IR (Spitzer and Herschel SPIRE and PACS imaging) are also under way.

First results

A preliminary comparative study of Abell 2192 (Jaffe et al. 2012), has already shown remarkable results. The main galaxy overdensity in Abell 2192 is an intermediate-mass cluster with a high degree of substructure, as shown in the animated 3D (ra-dec-redshift) figure below. The main cluster (red symbols) is surrounded by a spread field-like population of galaxies (gray and turquoise) and a compact group (yellow) that is infalling into the main cluster. Moreover, it is interesting to see where the HI-detected galaxies (blue transparent cubes) and the OII-emitters (orange transparent cubes) are within these structures. A final crucial peace of information is galaxy morphology, which is indicated by the different symbols (cubes with spikes correspond to late-type galaxies and solid circles are early-types).

From this cluster alone, we learned that the HI gas, as well as the star formation in late-type galaxies, correlates with environment at z~0.2. Our results suggest that the HI gas gets removed and star formation suppressed progressively, from the lowest mass galaxy groups to cluster-sized structures, as smaller structures get assembled into larger structures.

We are in the process of analysing the entire volume to carry out similar analysis. The results are being published in a series of papers, starting by BUDHIES I, which presents the optical spectroscopy.

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