VLT/UVES observations of extremely strong intervening damped Lyman-α systems. Molecular hydrogen and excited carbon, oxygen, and silicon at log N(H i) = 22.4

We present a detailed analysis of three extremely strong, intervening damped Lyman-α systems (ESDLAs, with log N(H i) ≥ 21.7) observed towards quasars with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope. We measure overall metallicities of [Zn/H] ~ -1.2, -1.3, and -0.7 at, respectively, z_abs = 2.34 towards SDSS J214043.02-032139.2 (log N(H i) = 22.4 ± 0.1), z_abs = 3.35 towards SDSS J145646.48+160939.3 (log N(H i) = 21.7 ± 0.1), and z_abs = 2.25 towards SDSS J015445.22+193515.8 (log N(H i) = 21.75 ± 0.15). Iron depletion of about a factor 15 compared to volatile elements is seen in the DLA towards J2140-0321, while the other two show deletion that is typical of known DLAs. We detect H₂ towards J2140-0321 (log N(H₂) = 20.13 ± 0.07) and J1456+1609 (log N(H₂) = 17.10 ± 0.09) and argue for a tentative detection towards J0154+1935. Absorption from the excited fine-structure levels of O i, C i, and Si ii are detected in the system towards J2140-0321, which has the largest H i column density detected so far in an intervening DLA. This is the first detection of O i fine-structure lines in a QSO-DLA, which also provides us with a rare possibility to study the chemical abundances of less abundant atoms like Co and Ge. Simple single-phase photo-ionisation models fail to reproduce all the observed quantities. Instead, we suggest that the cloud has a stratified structure: H₂ and C i most likely stem from a dense (log n_H ~ 2.5-3) and cold (80 K) phase and from a warm (250 K) phase. They contain a fraction of the total H i, while a warmer (T> 1000 K) phase probably contributes significantly to the high excitation of O i fine-structure levels. The observed C i/H₂ column density ratio is surprisingly low compared to model predictions, and we do not detect CO molecules: this suggests a possible underabundance of C by 0.7 dex compared to other alpha elements. The absorber could be a photo-dissociation region close to a bright star (or a star cluster) where higher temperature occurs in the illuminated region. Direct detection of on-going star formation through e.g. near-infrared emission lines in the surroundings of the gas would enable a detailed physical modelling of the system.

Based on observations collected with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under Programme ID 091.A-0370(A).