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 α systems (DLAs), using a sample of 33 systems towards 26 quasi-stellar objects (QSOs) acquired for a recently completed survey of H 2 by Ledoux, Petitjean & Srianand. We use the column densities of H 2 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 H 2 absorption line is detected, derived from the ortho-to-para ratio (OPR) (153 ± 78 K), is higher than that measured in the ISM (77 ± 17 K) and in Magellanic Clouds (82 ± 21 K). Typical pressure in these components (corresponding to T = 100-300 K 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 (H 2 , J = 2)/N (H 2 , J = 0) seen in DLAs. From the column densities of the high-J rotational levels, we derive that the typical radiation field in the H 2 -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 H 2 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 H 2 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 H 2 absorption line is seen. The excitation of C II in these systems is consistent with the physical parameters derived from the excitation of H 2 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 H 2 . 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 H 2 , 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.

The ESO UVES advanced data products quasar sample – III. Evidence of bimodality in the [N/α] distribution

“…The column density, redshift, and b-parameter of each velocity component is given in Table 7. Note that Srianand et al (2005) derived log N(Si ii)= 14.86 ± 0.02 and log N(S ii)= 14.50 ± 0.02, where former is in good agreement with our result. B 2222-396, z abs =2.1539: the line profiles of this DLA are characterised by a simple velocity structure composed of two components.…”

“…The properties of the velocity components are described in Table 1. Note that Srianand et al (2005) derived log N(S ii)= 14.30 ± 0.04, consistent to our results within the errors. The Si ii lines are either saturated or blended.…”

Quasar 2175 Å dust absorbers – II. Correlation analysis and relationship with other absorption line systems

“…More importantly, the cold molecular gas can be missed due to relatively small cross-sections compared to the more pervasive warm neutral ISM (Zwaan & Prochaska 2006). Neutral carbon has been considered an excellent tracer of molecular hydrogen (H 2 ) because the ionization potential of C i (11.26 eV) is very close to the dissociation energy of H 2 (e.g., Srianand et al 2005). When H 2 is detected, C i is always present in the same system (e.g., Ge & Bechtold 1999;Ge, Bechtold, & Kulkarni 2001;Srianand et al 2008;Guimarães et al 2012;Noterdaeme et al 2015;Balashev et al 2017.)…”

Partial covering of the emission regions of Q 0528−250 by intervening H2 clouds

“…Since the first identification of the H2 system in the quasar (Levshakov & Varshalovich 1985) other studies have been conducted, providing discordant results (see Table 2). Srianand et al (2005) used a two-component model, while King et al (2011) pointed out that a three-component model is very strongly preferred over two-component model. King et al (2011) used a fitting procedure where they increased the number of components in the absorption system in order to minimize the corrected Akaike information criterion, AICC 2 (Sugiura 1978;King et al 2011).…”

“…Molecular hydrogen absorption systems, a subset of damped Lyα systems (DLAs) and sub-damped Lyα systems (sub-DLAs), reveal diffuse and translucent interstellar clouds in high redshift intervening galaxies (Noterdaeme et al 2008). An analysis of H2 absorption systems allows for examining the physical conditions of diffuse clouds in distant galaxies (Srianand et al 2005;Noterdaeme et al 2007). It has been shown that the gas is a part of the cold neutral medium with comparatively low kinetic temperature (T ∼ 50 − 500 K) and high densities (nH > 10 cm −3 ), thus compact sizes (l < ∼ 1 pc).…”