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Astron. Astrophys. 345, 73-80 (1999)

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1. Introduction

The QSO J 2233-606 (zem = 2.24) has been given tremendous interest as it is located in the middle of the STIS Hubble deep field south making this field an ideal target for studying the connection between the diffuse gaseous component of the universe and galaxies (Ferguson 1998). The spectrum of this QSO shows several associated systems, (i.e. systems with zabs [FORMULA] zem), at zabs [FORMULA] 2.2 with broad C IV and N V absorption lines (Sealey et al. 1998, Savaglio 1998, Outram et al. 1998). HST STIS spectra, together with the available ground-based data, provide different pieces of information about these systems over the rest-wavelength range 375-2800 Å.

Associated systems have been intensively studied in the past few years because they are believed to be intimately related to the central engines of AGNs as: (i) they frequently show absorption due to high-ionization lines; (ii) they have been convincingly shown to have metallicities of the order of or above solar (Petitjean et al. 1994, Savaglio et al. 1994, Hamann 1997); (iii) the absorbing gas does not cover the background emitting region completely (Petitjean et al. 1994, Hamann et al. 1997b, Barlow & Sargent 1997). In a few cases it has been shown that the optical depth of the high-ionization lines varies with time on scales of a year (Hamann et al. 1997b) as is the case for some broad absorption line systems (e.g. Barlow et al. 1992). This requires a recombination time-scale of less than a year and hence high particle density in the absorbing gas if the observed variability is caused by the change in the ionizing conditions.

Soft X-ray spectra of an appreciable fraction of Seyfert-I galaxies show K-shell absorption edges of ionized oxygen (O VII and O VIII ; Reynolds 1997, George et al. 1998). Rapid variability of these absorption edges suggests that the absorbing gas is very close to the central engine. Moreover, there is a one-to-one correspondence between the presence of associated absorption systems and X-ray "warm absorbers" in Seyfert galaxies (Crenshaw et al. 1998). The case for a unified model for X-ray and UV absorbers, although attractive, is not completely convincing yet however. Even though Mathur et al. (1994) showed that the X-ray and UV absorptions seen in 3C351 can be reproduced by a single-cloud model, there are cases where two different ionized zones are needed even to produce the optical depth ratios of O VII and O VIII (see Reynolds 1997).

Some Seyfert-I galaxies show signatures of the existence of optically thin emitting clouds in the BLR. Shields, Ferland & Peterson (1995) have shown that the properties of the optically thin clouds in the inner BLR are consistent with that of warm absorbers (see also Porquet et al. 1998). Analysis of the broad Ne VIII [FORMULA]774 emission line in QSOs shows that the Ne VIII -emitting regions have ionization parameters in the range 5-30, total hydrogen column densities of the order of [FORMULA] cm-2 and average covering factors [FORMULA]30% for solar abundances and a nominal QSO spectrum (Hamann et al. 1998). The ionization conditions in these emitting clouds are similar to those of warm absorbers.

In order to investigate in more detail the nature of associated systems and their possible connection to warm absorbers, absorptions from species with a wide range of excitation should be studied. Morevover column densities should be determined taking into account the effect of partial coverage. In this prospect, absorption from Ne VIII , if present, is crucial as its ionization potential, 207 eV, is much higher than the ionization potential of other easily observable species. To our knowledge the Ne VIII [FORMULA]770,780 absorption doublet has been detected in only two associated systems, in the line of sight to HS1700+6416 at zabs = 2.7126 (Petitjean et al. 1996; see the HST spectrum in Vogel & Reimers 1995) and in the line of sight to UM675 (Hamann et al. 1995, see the spectrum in Hamann et al. 1997a) at zabs = 2.1340. Analysis of the latter system leads the authors to conclude that, although the detection of Ne VIII provides strong evidence for a link between the associated system and the warm absorber, the total hydrogen column density in the Ne VIII phase is too small to produce the warm absorber phenomenon. Recently Telfer et al. (1998) have done a similar study (including detection of Ne VIII ) of the broad absorption line system present in QSO SBS 1542+531. Note that in all previous studies Ne VIII absorption is investigated with low dispersion FOS spectra.

In the following we discuss in detail the associated systems in the line of sight to QSO J 2233-606 and especially the unambiguous detection of strong Ne VIII absorption. We briefly present the method to derive column densities in case of partial coverage in Sect. 2; describe the data in Sect. 3 and the individual absorption systems in Sect. 4, discuss the physical consequences of the observations in Sect. 5 and draw our conclusions in Sect. 6.

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999