The H I column density distribution function in faint dwarf galaxies

We present the HI column density distribution function, f(N-H (I)), as measured from dwarf galaxies observed as part of the Faint Irregular Galaxy Giant Metrewave Radio Telescope (GMRT) Survey (FIGGS). We find that the shape of the dwarf galaxy f (N-H (I)) is significantly different from the f (N-H (I)) for high-redshift damped Lyman alpha absorbers (DLAs) or the f (N-H (I)) for a representative sample of z = 0 gas-rich galaxies. The dwarf f (N-H (I)) falls much more steeply at high HI column densities as compared to the other determinations. While similar to 10 per cent of the cross-section above N-H (I) = 10(20.3) atoms cm-2 at z = 0 is provided by dwarf galaxies, the fraction falls to less than or similar to 1 per cent by N-H (I) similar to 10(21.5) atoms cm(-2). In the local universe, the contribution to the high N-H (I) end of the f (N-H (I)) distribution comes predominantly from the inclined discs of large galaxies. Dwarf galaxies, both because of their smaller scalelengths and their larger intrinsic axial ratios, do not produce large HI column densities even when viewed edge-on. If high-column-density DLAs/Gamma Ray Burst (GRB) hosts correspond to galaxies like the local dwarfs, this would require that either (i) the absorption arises from merging and not isolated systems or (ii) the observed lines of sight are strongly biased towards high-column-density regions.