Multiwavelength investigation of a near-solar metallicity sub-DLA at zabs= 1.3647 towards PKS 0237-233

Abstract

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 x 10 -3 (or N(H I)/Ts ≤ 10 17 cm -2 K -1 ). We derive log [N(H I) cm -2 ]≤ 19.30 ± 0.30 using the Lya 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 a process elements by ∼0.5dex. At z abs ≥ 1.9, C I absorption is usually associated with H 2 absorption arising from cold gas in damped Lya 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.