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

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1. Introduction

Very high redshift ([FORMULA]) radio galaxies (hereafter HZRG) show emission lines of varying degree of excitation. In virtually all objects, the Ly[FORMULA] line is the strongest and is usually accompanied by high excitation lines of C IV [FORMULA]1549 @ C III ][FORMULA]1909 @ [FORMULA] and, at times, N V [FORMULA]1240 (Röttgering et al. 1997 and references therein). An important characteristic of the emision gas is its spatial scale. The sizes of the Ly[FORMULA] emission region range from 15 to 120 kpc (van Ojik et al. 1997).

Most ground work on HZRG is performed at rather low resolution ([FORMULA]Å) to maximize the probability of line detection and the S/N. However a very potent discovery was made by van Ojik et al. (1997, hereafter vO97) at much higher resolution, that of extended H I absorption gas. In effect, out of 18 HZRG spectra taken at the unusually high resolution of [FORMULA]-3Å, v097 found -in 60% of the objects- deep absorption troughs superimposed on the Ly[FORMULA] emission profiles. Furthermore, out of the 10 radio galaxies smaller than 50 kpc, strong H I absorption is found in 9 of them. The absorption gas appears to have a covering factor near unity over very large scales, namely as large as the underlying emission gas.

The current paper addresses the problem of the ionization state of both the absorption and the emission gas as well as the interconnection between the two. The main justifications behind this work are the following: HZRG are probably the progenitors of the massive central cluster galaxies (Pentericci et al. 1999) and as such are an important means by which we can study large ellipticals and their environment at such high redshift, a time not so long after, or even during their formation. Furthermore, the extended gas as detected in C IV (see below) is chemically enriched and therefore represents debris of past intense stellar formation periods and is interesting to study in their own right. What is the fate of such gas? How quickly has the enrichment of this large scale gas proceeded? Will this gas be heated up into a hot wind and enrich the intergalactic X-ray gas in cluster of galaxies? Will it on the contrary condense into sheets or condensations? A better understanding of the various gaseous phases which co-exist in high redshift objects would help anwering these questions.

To determine the physical conditions of the absorption gas, new observations were carried out at the wavelength of C IV and [FORMULA] in 0943-242, the first radio galaxy reported to show large scale absorption troughs (Röttgering et al. 1995, hereafter RO95). The new spectrum shows the C IV absorption doublet at the same redshift 1 [FORMULA] as the Ly[FORMULA] absorption trough (RO95). Clearly and surprisingly the gas in absorption is highly ionized and probably of comparable excitation to the gas seen in emission.

This paper is structured as follows. We first present observations which show C IV in absorption in 0943-242 (Sect. 2). In Sect. 3 we derive a ratio ([FORMULA]) relating the observed emission and absorption quantities which depends somewhat on the ionization fraction of H but not explicitely on the C/H metallicity ratio. At first, we postulate that the emission and absorption gas components are co-spatial and share the same excitation mechanism and physical conditions and proceed to model [FORMULA] with a one-zone equilibrium photoionization model. We improve on the model using a stratified photoionized slab. As the observed ratio cannot be reproduced even in the case of collisional ionization, we discuss in Sect. 4.1 two alternative interpretations of this significant discrepancy. We demonstrate the many advantages of the winning scenario in which the absorption gas is further out and of much lower density, pressure and metallicity than the emission gas.

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000