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We analyze the radial distribution of HI gas for 23 disk galaxies withunusually high HI content from the Bluedisk sample, along with a similar-sizedsample of "normal" galaxies. We propose an empirical model to fit the radialprofile of the HI surface density, an exponential function with a depressionnear the center. The radial HI surface density profiles are very homogeneous inthe outer regions of the galaxy; the exponentially declining part of theprofile has a scale-length of $\sim 0.18$ R1, where R1 is the radius where thecolumn density of the HI is 1 M$_{\odot}$ pc$^{-2}$. This holds for allgalaxies, independent of their stellar or HI mass. The homogenous outerprofiles, combined with the limited range in HI surface density in thenon-exponential inner disk, results in the well-known tight relation between HIsize and HI mass. By comparing the radial profiles of the HI-rich galaxies withthose of the control systems, we deduce that in about half the galaxies, mostof the excess gas lies outside the stellar disk, in the exponentially decliningouter regions of the HI disk. In the other half, the excess is more centrallypeaked. We compare our results with existing smoothed-particle hydrodynamicalsimulations and semi-analytic models of disk galaxy formation in a $\Lambda$Cold Dark Matter universe. Both the hydro simulations and the semi-analyticmodels reproduce the HI surface density profiles and the HI size-mass relationwithout further tuning of the simulation and model inputs. In the semi-analyticmodels, the universal shape of the outer HI radial profiles is a consequence ofthe {\em assumption} that infalling gas is always distributed exponentially.The conversion of atomic gas to molecular form explains the limited range of HIsurface densities in the inner disk. These two factors produce the tight HImass-size relation.

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