Molecular hydrogen from z = 0.0963 DLA towards the QSO J1619+3342

Abstract

We report the detection of H₂ in a z_abs = 0.0963 Damped Lyman α (DLA) system towards z_em = 0.4716 QSO J1619+3342. This DLA has log N(H I) = 20.55 ± 0.10, 18.13 ≤ log N(H₂) ≤ 18.40, [S/H] = -0.62 ± 0.13, [Fe/S] = -1.00 ± 0.17 and the molecular fraction - 2.11 ≤ log[f(H₂)] ≤ -1.85. The inferred gas kinetic temperature using the rotational level population is in the range 95-132 K. We do not detect C I or C II* absorption from this system. Using R- and V-band deep images, we identify a sub-L_* galaxy at an impact parameter of 14 kpc from the line of sight, having consistent photometric redshift, as a possible host for the absorber. We use the photoionization code CLOUDY to get the physical conditions in the H₂ component using the observational constrains from H₂, C I, C II* and Mg I. All the observations can be consistently explained if one or more of the following is true: (i) carbon is underabundant by more than 0.6 dex as seen in halo stars with Z ∼ 0.1 Z_⊙, (ii) H I associated with H₂ component is less than 50 per cent of the H I measured along the line of sight and (iii) the H₂ formation rate on the dust grains is at least a factor of 2 higher than what is typically used in analytic calculations for Milky Way interstellar medium. Even when these are satisfied, the gas kinetic temperature in the models is much lower than what is inferred from the ortho-to-para ratio of the molecular hydrogen. Alternatively, the high kinetic temperature could be a consequence of contribution to the gas heating from non-radiative heating processes seen in hydrodynamical simulations.