HI in the outskirts of Nearby Spirals

We analyse nine galaxies taken from the THINGS survey to investigate the H I extent of spiral galaxy disks. We exploit the high spatial and velocity resolution, and the sensitivity of THINGS to investigate where the atomic gas disks end and what might shape their outskirts. We find that the atomic gas surface density across most of the disk is constant at 5 to 10 M· pc^-2 and declines at large radius. The shape of the H I distribution can be described by a Sérsic-type function with a slope index n = 0.18-0.36. The H I column density at which radial profiles turn over is found to be at too high a level for it to be caused by ionisation by a meta-galactic UV field. Instead we suggest the H I extent is rather set by how galaxy disks form.