Multiwavelength investigation of a near‐solar metallicity sub‐DLA at z abs = 1.3647 towards PKS 0237−233 ★

We searched for 21‐cm absorption associated with the z abs = 1.3647 absorption system towards the gigahertz peaked‐spectrum source PKS 0237−233 using the Giant Metrewave Radio Telescope (GMRT). A high quality Ultraviolet and Visible Echelle Spectrograph (UVES) spectrum shows that C  i and C  i * are detected at this redshift together with C  ii *, Mg  i , Mg  ii , Si  ii , Al  ii , Fe  ii and Mn  ii . The complex profiles, spread over ∼300 km s −1 , are fitted with 21 Voigt profile components. None of these components is detected in 21‐cm absorption down to a detection limit of τ(3σ) ≤ 3 × 10 −3 (or N (H  i )/ T S ≤ 10 17  cm −2  K −1 ). We derive log [ N (H  i ) cm −2 ]≤ 19.30 ± 0.30 using the Lyα absorption line detected in the IUE ( International Ultraviolet Explorer ) spectrum of the quasar. Mg  ii , Si  ii and Al  ii column densities are consistent with near‐solar metallicity and we measure [O/H]≥−0.33. Using photoionization models constrained by the fine‐structure excitations of C  i and C  ii , and the 21‐cm optical depth, we show that the C  i absorption arises predominantly either in a warm ionized medium (WIM) or warm neutral medium (WNM) in ionization and thermal equilibrium with the metagalactic ultraviolet (UV) background dominated by quasi‐stellar objects (QSOs) and star‐forming galaxies. The estimated thermal pressure of the gas is of the same order of magnitude over different velocity ranges through the absorption profile (2.6 ≤ log [ P / k  (cm −3  K)]≤ 4.0) . The gas‐phase metallicity corrected for ionization is Z ≥ 0.5 Z ⊙ with a signature of iron coproduction elements being under abundant compared to α process elements by ∼0.5 dex. At z abs ≥ 1.9, C  i absorption is usually associated with H 2 absorption arising from cold gas in damped Lyα systems (DLAs). This system and the z abs = 2.139 towards Tol 1037−270 are the only two systems known which show that C  i absorption can also be detected in warm gas provided the metallicity is high enough. Interestingly, both the systems are part of unusual concentrations of absorption lines.