Multi-phase gas properties of extremely strong intervening DLAs towards quasars

International audienceWe present the results of a spectroscopic analysis of extremely strong damped Lyα absorbers (ESDLAs; log N(H I) ≳ 21.7) observed with the medium resolution spectrograph, X-shooter at the Very Large Telescope (VLT). Recent studies in the literature indicate that ESDLAs probe gas from within the star-forming disk of the associated galaxies and thus ESDLAs provide a unique opportunity to study the interstellar medium of galaxies at high redshift. We report column densities (N), equivalent widths (w, for Mg II and Ca II transitions), and the kinematic spread (Δv90) of species from neutral (namely O I, Ar I, Cl I, N I, and Na I), singly ionised (Mg II, Ca II, S II, Ni II, Mn II, Ti II, and P II), and higher ionisation (C IV, Si IV, N V, and O VI) species. We estimate the dust-corrected metallicity measured using different singly ionised gas species such as P II, S II, Si II, Mn II and Cr II, and Zn II. We find that, using the dust correction prescription, the measured metallicities are consistent for all mentioned species in all ESDLAs within 3σ uncertainty. We further perform a quantitative comparison of column densities, equivalent widths, and kinematic spreads of ESDLAs with other samples that are associated with galaxies and detected in absorption along the line of sight towards high-redshift quasars (QSOs). We find that the distributions of the Ar I to H I column density ratio (N(Ar I)/N(H I)) in DLAs and ESDLAs are similar. We further report that ESDLAs do not show a strong deficiency of Ar I relative to other α-capture elements as is seen in DLAs. This supports the idea that the mentioned under-abundance of Ar I in DLAs is possibly caused by the presence of background UV photons that penetrate the low N(H I) clouds to ionise Ar I, but they cannot penetrate deep enough in the high N(H I) ESDLA environment. The w(Mg II λ2796) distribution in ESDLAs is found to be similar to that of metal-rich C I-selected absorbers, but the velocity spread of their Mg II profile is different. The dust content (measured by modelling the quasar extinction) and w(Ca II λ3934) distributions are similar in ESDLAs and Ca II-selected absorbers, yet we do not see any correlation between w(Ca II λ3934) and dust content. The Δv90 velocity spread of singly ionised species in ESDLAs is statistically smaller than that of DLAs. For higher ionisation species (such as C IV and Si IV) that trace the warm ionised medium, Δv90 is similar in the two populations. This suggests that the ESDLAs sample a different H I region of their associated galaxy compared to the general DLA population. We further study the N(Cl I) distribution in high-redshift DLA and ESDLA sightlines, as Cl I is a good tracer of H2 gas. The N(Cl I)−N(H2) correlation is followed by all the clouds (ESDLAs and otherwise) having log N(H2) < 22