2015A&A...577A..24N

We present a detailed analysis of three extremely strong, intervening damped Lyman-α systems (ESDLAs, with logN(HI)≥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 SDSSJ214043.02-032139.2 (logN(HI)=22.4±0.1), z_abs=3.35 towards SDSSJ145646.48+160939.3 (logN(HI)=21.7±0.1), and z_abs=2.25 towards SDSSJ015445.22+193515.8 (logN(HI)=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 (logN(H₂)=20.13±0.07) and J1456+1609 (logN(H₂)=17.10±0.09) and argue for a tentative detection towards J0154+1935. Absorption from the excited fine-structure levels of OI, CI, and SiII are detected in the system towards J2140-0321, which has the largest HI column density detected so far in an intervening DLA. This is the first detection of OI 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 CI most likely stem from a dense (logn_H∼2.5-3) and cold (80K) phase and from a warm (250K) phase. They contain a fraction of the total HI, while a warmer (T>1000K) phase probably contributes significantly to the high excitation of OI fine-structure levels. The observed CI/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 0.7dex 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.