2017MNRAS.470.2890B

We present a detailed analysis of an H₂-rich, extremely strong intervening damped Ly α absorption system (DLA) at z_abs = 2.786 towards the quasar J 0843+0221, observed with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope. The total column density of molecular (resp. atomic) hydrogen is log N(H₂) = 21.21 ± 0.02 (resp. log N(H I) = 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] = -1.52^+0.08_–0.10. 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 ∼ 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 A_V < 0.1; (iii) no CO molecules are detected, putting a constraint on the X_CO factor X_CO > 2 x 10²³ cm^–2/(km s^–1 K), in the very low metallicity gas. Low CO and high H₂ contents indicate that this system represents 'CO-dark/faint' gas. We investigate the physical conditions in the H₂-bearing gas using the fine-structure levels of C I, C II, Si II and the rotational levels of HD and H₂. We find the number density to be about n ∼ 260-380 cm^–3, implying a high thermal pressure of 3-5 x 10⁴ cm^–3 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 log N(H) ∼22) probe high-z galaxies at low impact parameters.