Molecular hydrogen in the diffuse interstellar medium at high redshift

The physical conditions within damped Lyα systems (DLAs) can reveal the star formation history, determine the chemical composition of the associated ISM, and hence document the first steps in the formation of present day galaxies. Here we present calculations that self-consistently determine the gas ionization, level populations (atomic fine-structure levels and rotational levels of H2), grain physics, and chemistry. We show that for a low-density gas (nH6 0.1 cm ) the meta-galactic UV background due to quasars is sufficient to maintain H2 column densities below the detection limit (i.e N(H2)6 10 14 cm) irrespective of the metallicity and dust content in the gas. Such a gas will have a 21 cm spin temperature in excess of 7000 K and very low C i and C ii∗ column densities for H i column densities typically observed 50 per cent in DLAs. We show that the observed properties of the ∼ 15 per cent of the DLAs that do show detectable H2 absorption cannot be reproduced with only the quasar dominated meta-galactic UV radiation field. Gas with higher densities (nH> 10 cm ), a moderate radiation field (flux density one to ten times that of the background radiation of the Galactic ISM), the observed range of metallicity and dust-to-gas ratio reproduce all the observed properties of the DLAs that show H2 absorption lines. This favors the presence of ongoing star formation in DLAs with H2. The absence of detectable H2 and C i absorption in a large fraction of DLAs can be explained if they originate either in a low-density gas or in a high-density gas with a large ambient radiation field. The absence of 21 cm absorption and C ii∗ absorption will be consistent with the first possibility. The presence of 21 cm absorption and strong C ii∗ without H2 and C i absorption will suggest the second alternative. The N(Al ii)/N(Al iii) ratio can be used to understand the physical properties when only C ii∗ absorption is present. We find nH in components that show C ii ∗ (without H2) is less than that typically inferred from the components with H2 absorption. We also calculate the column density of various atoms in the excited fine-structure levels. The expected column densities of O i∗, O i∗∗, and Si ii∗ in a high-density cold gas is in the range of 10−10 cm for log N(H i)> 20 and the observed range of metallicities. It will be possible to confirm whether DLAs that do not show H2 originate predominantly in a high-density gas by detecting these lines in very high S/N ratio spectra.