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

We analyse the radial distribution of HI gas for 23 disc galaxies with unusually high HI 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 HI surface density, an exponential function with a depression near the centre. The radial HI surface density profiles are very homogeneous in the outer regions of the galaxy; the exponentially declining part of the profile has a scalelength of ~0.18 R1, where R1 is the radius where the column density of the HI is 1 M⊙ pc-2. This holds for all galaxies, independent of their stellar or HI mass. The homogenous outer profiles, combined with the limited range in HI surface density in the nonexponential inner disc, results in the well-known tight relation between HI size and HI mass. By comparing the radial profiles of the HI-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 HI 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 Λ cold dark matter universe. Both the hydro simulations and the semi-analytic models reproduce the HI surface density profiles and the HI size-mass relation without further tuning of the simulation and model inputs. In the semi-analytic models, the universal shape of the outer HI radial profiles is a consequence of the assumption that infalling gas is always distributed exponentially. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.