Molecular hydrogen in damped Lyα systems:: spatial distribution

To interpret H-2 quasar absorption-line observations in damped Lyalpha clouds (DLAs), we model the H-2 spatial distribution within a DLA. Based on numerical simulations of disc structures with parameters similar to those derived for such absorbers, we calculate the H-2 distribution as a function of ultraviolet background (UVB) intensity and dust-to-gas ratio. For typical values of these two quantities we find that the area in which the H-2 fraction exceeds 10(-6) (typical observational detection limit) only covers less than or similar to10 per cent of the disc surface, i.e. H-2 has a very inhomogeneous, clumpy distribution even at these low abundance levels. This explains the relative paucity of H-2 detections in DLAs. We also show the dependence of the covering fraction of H-2 on dust-to-gas ratio and UVB intensity, and we comment on the physics governing the H-2 chemical network at high redshift.