We searched for 21-cm absorption associated with the z _ { abs } = 1.3647 absorption system toward the gigahertz peaked-spectrum source PKS 0237 - 233 using the Giant Metrewave Radio Telescope .
A high quality 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 \sim 300 km s ^ { -1 } , are fitted with 21 Voigt profile components .
None of these components are detected in 21-cm absorption down to a detection limit of \tau ( 3 \sigma ) \leq 3 \times 10 ^ { -3 } ( or N ( H i ) / T _ { S } \leq 10 ^ { 17 } cm ^ { -2 } K ^ { -1 } ) .
We derive log [ N ( H i ) cm ^ { -2 } ] \leq 19.30 \pm 0.30 using the Ly \alpha absorption line detected in the IUE spectrum of the quasar .
Mg ii , Si ii and Al ii column densities are consistent with near solar metallicity and we measure [ O/H ] \geq - 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 meta-galactic UV background dominated by 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 \leq { log [ } P / k { ( cm ^ { -3 } K ) ] } \leq 4.0 ) .
The gas-phase metallicity corrected for ionization is Z \geq 0.5 Z _ { \odot } with a signature of iron co-production elements being under abundant compared to \alpha - process elements by \sim 0.5 dex .
At z _ { abs } \geq 1.9 , C i absorption is usually associated with H _ { 2 } absorption arising from cold gas in damped Lyman- \alpha systems .
This system and the z _ { abs } = 2.139 toward 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 .