Multicomponent H2 in DLA at zabs = 2.05: physical conditions through observations and numerical models
Résumé
We perform detailed spectroscopic analysis and numerical modelling of an H_2-bearing damped Lyman-α absorber (DLA) at z_abs = 2.05 towards the quasar FBQS J2340-0053. Metal absorption features arise from 14 components spread over Δv_90 = 114 km s^−1, seven of which harbour H_2. Column densities of atomic and molecular species are derived through Voigt profile analysis of their absorption lines. We measure total N(H i), N(H_2), and N(HD) to be 20.35 ± 0.05, 17.99 ± 0.05, and 14.28 ± 0.08 (log cm^−2), respectively. H_2 is detected in the lowest six rotational levels of the ground vibrational state. The DLA has metallicity, Z = 0.3 Z_|$\odot$| ([S/H] = −0.52 ± 0.06) and dust-to-gas ratio, κ = 0.34 ± 0.07. Numerical models of the H_2 components are constrained individually to understand the physical structure of the DLA. We conclude that the DLA is subjected to the metagalactic background radiation and cosmic ray ionization rate of ∼10^−15.37 s^−1. Dust grains in this DLA are smaller than grains in the Galactic interstellar medium. The inner molecular regions of the H_2 components have density, temperature, and gas pressure in the range 30–120 cm^−3, 140–360 K, and 7000–23 000 cm^−3 K, respectively. Micro-turbulent pressure is a significant constituent of the total pressure, and can play an important role in these innermost regions. Our H_2 component models enable us to constrain component-wise N(H i), and elemental abundances of sulphur, silicon, iron, and carbon. We deduce the line-of-sight thickness of the H_2-bearing parts of the DLA to be 7.2 pc.
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