A vestige low metallicity gas shell surrounding the radio galaxy 0943-242 at z=2.92

Observations are presented showing the doublet C IV lambda lambda 1548, 155 1 absorption lines superimposed on the C rv emission in the radio galaxy 09 43-242. Within the errors, the redshift of the absorption system that has a column density of N-CIV = 10(14.5+/-0.1) cm(-2) coincides with that of the deep Ly alpha absorption trough observed by Rottgering et al. (1995). The gas seen in absorption has a resolved spatial extent of at least 13 kpc (th e size of the extended emission line region). We first model the absorption and emission gas as co-spatial components with the same metallicity and de gree of excitation. Using the information provided by the emission and abso rption line ratios of C IV and Ly alpha, we find that the observed quantiti es are incompatible with photoionization or collisional ionization of cloud lets with uniform properties. We therefore reject the possibility that the absorption and emission phases are co-spatial and favour the explanation th at the absorption gas has low metallicity and is located further away from the host galaxy (than the emission line gas). The larger size considered fo r 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 ve ry low density allowing the metagalactic ionizing radiation to keep it higl y 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 a s n(H)(2)) 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 Ly alpha emission sizes (or radio jet sizes) and the observation (or not) of H I in absorption. The estimated low metal licity for the absorption gas in 0943-242 (Z similar to 0.01Z.) and its pro posed location -outer halo outside the radio cocoon-suggest that its existe nce preceeds the observed AGN phase and is a vestige of the initial starbur st at the onset of formation of the parent galaxy.