Metallicities, Dust, and Molecular Content of a QSO-damped Lyα System Reaching log N(H I) = 22: An Analog to GRB-DLAs

International audienceWe present the elemental abundance and H2 content measurements of a damped Lyα (DLA) system with an extremely large H I column density, log N(H I) (cm-2) = 22.0 ± 0.10, at z abs = 3.287 toward the QSO SDSS J081634+144612. We measure column densities of H2, C I, C I sstarf, Zn II, Fe II, Cr II, Ni II, and Si II from a high signal-to-noise and high spectral resolution VLT-UVES spectrum. The overall metallicity of the system is [Zn/H] = -1.10 ± 0.10 relative to solar. Two molecular hydrogen absorption components are seen at z = 3.28667 and 3.28742 (a velocity separation of ≈52 km s-1) in rotational levels up to J = 3. We derive a total H2 column density of log N(H2) (cm-2) = 18.66 and a mean molecular fraction of f = 2N(H2)/[2N(H2) + N(H I)] = 10-3.04 ± 0.37, typical of known H2-bearing DLA systems. From the observed abundance ratios we conclude that dust is present in the interstellar medium of this galaxy, with an enhanced abundance in the H2-bearing clouds. However, the total amount of dust along the line of sight is not large and does not produce any significant reddening of the background QSO. The physical conditions in the H2-bearing clouds are constrained directly from the column densities of H2 in different rotational levels, C I and C I sstarf. The kinetic temperature is found to be T ≈ 75 K and the particle density lies in the range n H = 50-80 cm-3. The neutral hydrogen column density of this DLA is similar to the mean H I column density of DLAs observed at the redshift of γ-ray bursts (GRBs). We explore the relationship between GRB-DLAs and the high column density end of QSO-DLAs finding that the properties (metallicity and depletion) of DLAs with log N(H I) > 21.5 in the two populations do not appear to be significantly different