SIMBAD references

We introduce the `Bluedisk' project, a large programme at the Westerbork Synthesis Radio Telescope that has mapped the Hi in a sample of 23 nearby galaxies with unusually high Hi mass fractions, along with a similar-sized sample of control galaxies. This paper presents the sample selection, observational set-up, data reduction strategy and a first analysis of the sizes and structural properties of the Hi discs. We find that the Hi-rich galaxies lie on the same Hi mass versus Hi size relation as normal spiral galaxies, extending it to total Hi masses of 2x1010M☉ and radii R1 of ∼ 100kpc. The Hi-rich galaxies have significantly larger values of Hi-to-optical size ratio and more clumpy Hi discs than those of normal spirals. There is no evidence that the discs of Hi-rich galaxies are more disturbed. In fact, the centre of the Hi distribution corresponds more closely with the centre of the optical light in the Hi-rich galaxies than in the controls. All these results argue against a scenario in which new gas has been brought in by mergers. It is possible that they may be more consistent with cooling from a surrounding quasi-static halo of warm/hot gas.