Observations are presented showing the doublet C iv \lambda \lambda 1548 , 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 \hbox { $N _ { CIV } $ } = 10 ^ { 14.5 \pm 0.1 } \hbox { $ { cm ^ { -2 } } $ } coincides with that of the deep Ly \alpha 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 Ly \alpha , 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 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 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 metallicity for the absorption gas in 0943–242 ( Z \sim 0.01 \hbox { $Z _ { \sun } $ } ) 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 .