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Astron. Astrophys. 356, 23-32 (2000)

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5. Conclusions

The detection of C IV absorption in radio galaxy 0943-242 at the same redshift as the deep Ly[FORMULA] trough observed by RO95 demonstrates that the detected absorption gas is highly ionized. Having assumed that the H I and C IV columns measured from the Voigt profile fitting were representative of the dominant gas phase (by mass) in the outer halo, we have effectively ruled out that the absorption and emission gas occupy the same position in 0943-242. We subsequently reassessed the picture proposed by vO97 in which both the large scale emission gas and the absorption gas were of comparable density ([FORMULA]cm-3). In the former picture, the absorption gas was believed to lie outside the AGN ionization bicone (see their Fig. 11 in vO97). To ionize the gas to such a degree without using the AGN flux is problematic. We have proposed an alternative picture in which the absorption gas is of very low metallicity and lies far away (in the outer halo) from the inner pressurized radio jet cocoon. Since in this new scheme the density of the absorption gas is expected to be very low, the metagalactic background radiation now suffices to photoionize it. Furthermore, the structure of the absorption gas is now drastically simplified since we do not need over [FORMULA] condensations of size [FORMULA]pc and density [FORMULA]cm-3 to reach a covering factor close to unity. We can now reach similarly high covering factor using a single or few shells of very low density which have a volume filling factor close to unity (assuming a density of [FORMULA] cm-3).

It appears to us that the low metallicity inferred ([FORMULA]) and the proposed location of the absorption gas in 0943-242 -outside the radio cocoon, in an outer halo which is seen in emission in other radio galaxies (as in 1243+036)- strongly suggest that the absorbers' existence precedes the observed AGN phase. Unless this non-primordial gas has been enriched by still undetected pop III stars, we consider that it more likely corresponds to a vestige gas phase expelled from the parent galaxy during the initial starburst at the onset of its formation.

If the C IV doublet was detected in absorption in other radio galaxies with deep Ly[FORMULA] absorption troughs, there are many aspects which would be worth studying. For instance, how uniform is the excitation of the absorption gas across the region over which it is detected? Is a single phase sufficient? This could be tested by an attempt to detect absorption troughs of Mg II [FORMULA]2798 or imaging the troughs in C IV with an integral field spectrograph on an 8-m class telescope. How different is the metallicity of the absorption gas in the other radio galaxies? The information gathered could then be used to infer the enrichment history of the outer halo gas which surrounds HZRG.

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000