A vestige low metallicity gas shell surrounding the radio galaxy 0943 – 242 at z = 2 . 92

Observations are presented showing the doublet C ivλλ1548, 1551 absorption lines superimposed on the C iv emission in the radio galaxy 0943–242. Within the errors, the redshift of the absorption system that has a column density of NCIV = 1014.5±0.1 cm−2 coincides with that of the deep Lyα absorption trough observed by Röttgering et al. (1995). The gas seen in absorption has a resolved spatial extent of at least 13 kpc (the size of the extended emission line region). We first model the absorption and emission gas as co-spatial components with the same metallicity and degree of excitation. Using the information provided by the emission and absorption line ratios of C iv and Lyα, we find that the observed quantities are incompatible with photoionization or collisional ionization of cloudlets with uniform properties. We therefore reject the possibility that the absorption and emission phases are co-spatial and favour the explanation that the absorption gas has low metallicity and is located further away from the host galaxy (than the emission line gas). The larger size considered for the outer halo makes plausible the proposed metallicity drop relative to the inner emission gas. In absence of confining pressure comparable to that of the emission gas, the outer halo of 0943–242 is considered to have a very low density allowing the metagalactic ionizing radiation to keep it higly ionized. In other radio galaxies where the jet has pressurized the outer halo, the same gas would be seen in emission (since the emissivity scales as nH ) and not in absorption as a result of the lower filling factor of the denser condensations. This would explain the anticorrelation found by van Ojik et al. (1997) between Lyα emission sizes (or radio jet sizes) and the observation (or not) of H i in absorption. The estimated low metallicity for the absorption gas in 0943–242 (Z ∼ 0.01Z ) and its proposed location –outer halo outside the radio cocoon– suggest that its existence preceeds the observed AGN phase and is a vestige of the initial starburst at the onset of formation of the parent galaxy.