Abstract
We present a detailed analysis of three extremely strong intervening DLAs (log N(HI)>=21.7) observed towards quasars with VLT/UVES. We measure overall metallicities of [Zn/H]~-1.2, -1.3 and -0.7 at respectively zabs=2.34 towards SDSS J2140-0321 (log N(HI) = 22.4+/-0.1), zabs=3.35 towards SDSS J1456+1609 (log N(HI) = 21.7+/-0.1) and zabs=2.25 towards SDSS J0154+1935 (log N(HI) = 21.75+/-0.15). 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 OI, CI and SiII are detected in the system towards J2140-0321, that 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, that also provides us 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 likely originate from both a dense (log nH~2.5-3) cold (80K) and warm (250K) phase containing a fraction of the total HI while a warmer (T>1000 K) 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 0.7 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. NIR emission lines in the surrounding of the gas would enable a detailed physical modelling of the system.
Highlights
Diffuse gaseous clouds in the Universe are primarily described by their neutral hydrogen column density, which is a quantity that can be directly and accurately measured in absorption against background sources
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
Assuming 0.4 dex depletion of C onto dust grains as seen in the cold interstellar medium (ISM), the true abundance of carbon in the DLA should be of the order of [C/O] ∼ −0.7 or even less for the gas phase column density to be consistent with the models
Summary
Diffuse gaseous clouds in the Universe are primarily described by their neutral hydrogen column density, which is a quantity that can be directly and accurately measured in absorption against background sources. The corresponding absorbers are called damped Lyman-α systems (DLAs) when log N(H i) ≥ 20.3 (Wolfe et al 1986) These systems are thought to arise when the line of sight passes a small impact parameter to a foreground galaxy. Evidence of this came mostly from the similarity with H i column densities in nearby galactic discs and the presence of metals at different levels of chemical enrichment: the smoking gun of star-formation activity. Ellison et al 2005; Jorgenson et al 2006), at least in terms of H i-content Another explanation, proposed by Schaye (2001), is that the high-column-density gas collapses into molecular hydrogen, eventually leading to star formation.
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