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| Paper IPM / Astronomy / 15712 |
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We present a detailed analysis of an H2-rich, extremely strong intervening damped Lyâα absorption system (DLA) at zabs = 2.786 towards the quasar Jâ0843+0221, observed with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope. The total column density of molecular (resp. atomic) hydrogen is logâN(H2) = 21.21 ± 0.02 (resp. logâN(HâI) = 21.82 ± 0.11), making it to be the first case in quasar absorption line studies with H2 column density as high as what is seen in 13CO-selected clouds in the Milky Way. We find that this system has one of the lowest metallicity detected among H2-bearing DLAs, with [Zn/H]=â1.52+0.08â0.10â . This can be the reason for the marked differences compared to systems with similar H2 column densities in the local Universe: (i) the kinetic temperature, T â¼ 120 K, derived from the J = 0, 1 H2 rotational levels is at least twice higher than expected; (ii) there is little dust extinction with AV < 0.1; (iii) no CO molecules are detected, putting a constraint on the XCO factor XCO > 2 à 1023 cmâ2/(km sâ1 K), in the very low metallicity gas. Low CO and high H2 contents indicate that this system represents âCO-dark/faintâ gas. We investigate the physical conditions in the H2-bearing gas using the fine-structure levels of CâI, CâII, SiâII and the rotational levels of HD and H2. We find the number density to be about n â¼ 260â380 cmâ3, implying a high thermal pressure of 3â5 à 104 cmâ3 K. We further identify a trend of increasing pressure with increasing total hydrogen column density. This independently supports the suggestion that extremely strong DLAs (with logâ N(H) â¼22) probe high-z galaxies at low impact parameters.
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