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Paper   IPM / Astronomy / 15712
School of Astronomy
  Title:   CO-dark molecular gas at high redshift: very large H2 content and high pressure in a low metallicity damped Lyman-alpha system
1.  S. A. . Balashev
2.  P. . Noterdaeme
3.  H. . Rahmani
4.  V.V. . Klimenko
5.  C. . Ledoux
6.  P. . Petitjean
7.  R. . Srianand
8.  A.V. I. vanchik
9.  D.A. . Varshalovich
  Status:   Published
  Journal: MNRAS
  Vol.:  470
  Year:  2017
  Supported by:            ipm IPM
We present a detailed analysis of an H2-rich, extremely strong intervening damped Ly α absorption system (DLA) at zabs = 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 AV < 0.1; (iii) no CO molecules are detected, putting a constraint on the XCO factor XCO > 2 × 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 × 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.

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