Estimation of physical conditions in the cold phase of the interstellar medium in the sub-DLA system at z = 2.06 in the spectrum of the quasar J 2123–0050

An independent analysis of the molecular hydrogen absorption system at redshift z_abs = 2.059 in the spectrum of the quasar J 2123−0050 is presented. The H₂ system consists of two components (A and B) with column densities \(\log N_{{H_2}}^A = 17.94 \pm 0.01\) and \(N_{{H_2}}^B = 15.16 \pm 0.02\). The spectrum exhibits the lines of HDmolecules (logN_HD^A = 13.87±0.06) and the neutral speciesCI and Cl I associated with the H₂ absorption system. For the molecular hydrogen lines near the quasar’s Lyβ and OVI emission lines, we detect a nonzero residual flux, ~3% of the total flux, caused by the effect of partial coverage of the quasar’s broad-line region by an H₂ cloud. Due to the smallness of the residual flux, the effect does not affect the H₂ column density being determined but increases the statistics of observations of the partial coverage effect to four cases. The uniqueness of the system being investigated is manifested in a high abundance of the neutral species H₂ and CI at the lowest HI column density, logN_HI = 19.18 ± 0.15, among the highredshift systems. The H₂ and CI column densities in the system being investigated turn out to be higher than those in similar systems in our Galaxy and theMagellanic Clouds by two or three orders ofmagnitude. The \(N_{HD} /2N_{H_2 }\) ratio for component A has turned out to be also unusually high, (4.26 ± 0.60) × 10⁻⁵, which exceeds the deuterium abundance (D/H) for high-redshift systems by a factor of 1.5. Using the HI, H₂, HD, and CI column densities as well as the populations of excited H₂ and CI levels, we have investigated the physical conditions in components A and B. Component A represents the optically thick case; the gas has a low number density (~30 cm⁻³) and a temperature T ~ 140 K. In component B, the mediumis optically thin with n_H ≤ 100 cm⁻³ and T ≥ 100 K. The ultraviolet (UV) background intensity in the clouds exceeds the mean intensity in our Galaxy by almost an order ofmagnitude. A high gas ionization fraction, \(n_{H^ + } /n_H \sim 10^{ - 2}\), which can be the result of partial shielding of the systemfrom hard UV radiation, is needed to describe the high HD and CI column densities. Using our simulations with the PDRMeudon code, we can reconstruct the observed column densities of the species within the model with a constant density (n_H ~ 40 cm⁻³). A high H₂ formation rate (higher than the mean Galactic value by a factor of 10−40) and high gas ionization fraction and UV background intensity are needed in this case.