2022A&A...661A.134R

We present the results of a spectroscopic analysis of extremely strong damped Lyα absorbers (ESDLAs; log N(HI) ≥ 21.7) observed with the medium resolution spectrograph, X-shooter at theVery 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 MgII and CaII transitions), and the kinematic spread (Δv₉₀) of species from neutral (namely OI, ArI, ClI, NI, and NaI), singly ionised (MgII, CaII, SII, NiII, MnII, TiII, and PII), and higher ionisation (CIV, SiIV, NV, and OVI) species. We estimate the dust-corrected metallicity measured using different singly ionised gas species such as PII, SII, SiII, MnII and CrII, and ZnII. 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 ArI to HI column density ratio (N(ArI)/N(HI)) in DLAs and ESDLAs are similar. We further report that ESDLAs do not show a strong deficiency of ArI relative to other α-capture elements as is seen in DLAs. This supports the idea that the mentioned under-abundance of ArI in DLAs is possibly caused by the presence of background UV photons that penetrate the low N(HI) clouds to ionise ArI, but they cannot penetrate deep enough in the high N(HI) ESDLA environment. The w(MgII λ2796) distribution in ESDLAs is found to be similar to that of metal-rich CI-selected absorbers, but the velocity spread of their MgII profile is different. The dust content (measured by modelling the quasar extinction) and w(CaII λ3934) distributions are similar in ESDLAs and CaII-selected absorbers, yet we do not see any correlation between w(CaII 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 CIV and SiIV) that trace the warm ionised medium, Δv₉₀ is similar in the two populations. This suggests that the ESDLAs sample a different HI region of their associated galaxy compared to the general DLA population. We further study the N(ClI) distribution in high-redshift DLA and ESDLA sightlines, as ClI is a good tracer of H₂ gas. The N(ClI)-N(H₂) correlation is followed by all the clouds (ESDLAs and otherwise) having log N(H₂) < 22.