The Edges of Things

¹ Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK
² Observatorio Astronómico Nacional, C/Alfonso XII 3, 28014 Madrid, Spain
³ Max–Plank–Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Univ. of Cape Town, Dept. of Astronomy, Private Bag X3, Rondebosch 7701, South Africa
⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Abstract

We investigate the radial column density distribution in a sample of 9 galaxies taken from the THINGS survey. The HI data have been used to probe the extent of the neutral (atomic) gas disks down to column densities of less than 0.1 ℳ_⊙ pc⁻². The size of the disks might be set simply by the amount of available gas that has contributed to the formation of the galaxy or truncated by ionisation by the extragalactic radiation field once the HI column density has dropped below a certain threshold.