Molecular Hydrogen in Damped Ly-alpha Systems: Spatial Distribution

Abstract

To interpret H_2 quasar absorption line observations in Damped Ly-alpha clouds (DLAs), we model the H_2 spatial distribution within a DLA. Based on numerical simulations of disk 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 $\la 10$% of the disk 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.

5 pages, 3 figures, 1 table, accepted by MNRAS Letter

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• A high molecular fraction in a sub-damped absorber at z=0.56
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• Fate of clumps in damped Ly$\alpha$ systems
• Search for high column density systems with gamma ray bursts
• Constraints on variations of m${}_{\mathrm{p}}$/m${}_{\mathrm{e}}$ based on UVES observations of H${}_2$