The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman α systems: physical conditions in the neutral gas

Abstract

We study the physical conditions in damped Lyman systems (DLAs), using a sample of 33 systems towards 26 quasi-stellar objects (QSOs) acquired for a recently completed survey of H₂ by Ledoux, Petitjean & Srianand. We use the column densities of H₂ in different rotational levels, together with those of C i, C i*, C i**, C ii* and singly ionized atomic species to discuss the kinetic temperature, the density of hydrogen and the electronic density in the gas together with the ambient ultraviolet (UV) radiation field. Detailed comparisons are made between the observed properties in DLAs, the interstellar medium (ISM) of the Galaxy and the Large and Small Magellanic Clouds (LMCs and SMCs). The mean kinetic temperature of the gas corresponding to DLA subcomponents in which an H₂ absorption line is detected, derived from the ortho-to-para ratio (OPR) (153 ± 78 Κ), is higher than that measured in the ISM (77 ± 17 Κ) and in Magellanic Clouds (82 ± 21 Κ). Typical pressure in these components (corresponding to T= 100-300 Κ and n_H= 10-200 cm^-3), measured using C i fine-structure excitation, are higher than what is measured along ISM sightlines. This is consistent with the corresponding higher values for N(H₂, J= 2)/N(H₂, J= 0) seen in DLAs. From the column densities of the high-J rotational levels, we derive that the typical radiation field in the H₂-bearing components is of the order of or slightly higher than the mean UV field in the Galactic ISM. Determination of electron density in the gas with H₂ and C i shows that the ionization rate is similar to that of a cold neutral medium (CNM) in a moderate radiation field. This, together with the fact that we see H₂ in 13-20 per cent of the DLAs, can be used to conclude that DLAs at z > 1.9 could contribute as much as 50 per cent star formation rate (SFR) density seen in Lyman break galaxies (LBGs). C ii absorption line is detected in all the components where H₂ absorption line is seen. The excitation of C ii in these systems is consistent with the physical parameters derived from the excitation of H₂ and C i. We detect C ii in about 50 per cent of the DLAs and, therefore, in a considerable fraction of DLAs that do not show H₂. In part of the later systems, physical conditions could be similar to that in the CNM gas of the Galaxy. However, the absence of C i absorption line and the presence of Al iii absorption lines with a profile similar to the profiles of singly ionized species suggest an appreciable contribution from a warm neutral medium (WNM) and/or partially ionized gas. The absence of H₂, for the level of metallicity and dust depletion seen in these systems, is consistent with low densities (i.e. n_H≤ 1 cm^-3) for a radiation field similar to the mean Galactic UV field.