An observational and theoretical view of the radial distribution of HI gas in galaxies

We analyse the radial distribution of H i gas for 23 disc galaxies with unusually high H i content from the Bluedisk sample, along with a similar-sized sample of 'normal' galaxies. We propose an empirical model to fit the radial profile of the H i surface density, an exponential function with a depression near the centre. The radial H i surface density profiles are very homogeneous in the outer regions of the galaxy; the exponentially declining part of the profile has a scalelength of similar to 0.18 R1, where R1 is the radius where the column density of the H i is 1 M-aS (TM) pc(-2). This holds for all galaxies, independent of their stellar or H i mass. The homogenous outer profiles, combined with the limited range in H i surface density in the non-exponential inner disc, results in the well-known tight relation between H i size and H i mass. By comparing the radial profiles of the H i-rich galaxies with those of the control systems, we deduce that in about half the galaxies, most of the excess gas lies outside the stellar disc, in the exponentially declining outer regions of the H i disc. In the other half, the excess is more centrally peaked. We compare our results with existing smoothed particle hydrodynamical simulations and semi-analytic models of disc galaxy formation in a I > cold dark matter universe. Both the hydro simulations and the semi-analytic models reproduce the H i surface density profiles and the H i size-mass relation without further tuning of the simulation and model inputs. In the semi-analytic models, the universal shape of the outer H i radial profiles is a consequence of the assumption that infalling gas is always distributed exponentially.