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Astron. Astrophys. 342, 395-407 (1999)

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1. Introduction

Quasar absorption lines offer a unique possibility to study the physical properties of intervening gas clouds as well as the metagalactic UV radiation field at higher redshifts. The highly ionized absorber systems have only few resonance absorption lines like C IV 1550, Si IV 1400, N V 1240, O VI 1334 accessible from the ground in high redshift QSOs. Nearly all strong resonance lines of abundant ions (O III -O V , Ne III -Ne VIII , S III -S V , N III -N IV etc.) are in the intrinsic EUV part of the spectrum. This means, that abundance studies in the highly ionized component of the IGM of elements like O, Ne, S or N require the observation of the intrinsic EUV spectra of high-redshift QSOs.

In ultraviolet spectra obtained with the Hubble Space Telescope (HST) of the UV-bright, high-redshift ([FORMULA]) QSO HS 1700+6416 we detected at least 16 heavy-element absorption systems thereby 7 Lyman limit systems (LLS). Prominent absorption lines which have been seen for the first time in a cosmic object except the Sun are: O V 629, O IV 554, 553, 608, 787, O III 702, 833, Ne IV 543, Ne V 480, 568, Ne VI 401, Ne VII 465, S III 677, 698, S IV 657, S V 786, N III 374, 685, 989 and many others (Reimers et al. 1992, Vogel & Reimers 1995, Köhler et al. 1996).

The rich absorption line spectra consisting mainly of high-ionization lines of abundant elements are in agreement with what we expected on the basis of photoionization calculations. It appears from our analysis that typically at high redshift the metallicity is low ([C/H] [FORMULA]) with a clear tendency of increasing abundances with lower redshifts and that the abundances of O, Ne, Si, and S relative to C seem to behave as expected from nucleosynthesis in SN II explosions (Vogel & Reimers 1995; Köhler et al. 1996; Sperhake & Reimers 1997). The analysis of HS 1700+6416 data suffered from the extremely high line density in this high-redshift QSO and the lack of high-resolution optical data.

In the course of the Hamburg Quasar Survey (Hagen et al. 1995) we discovered HS 1103+6416, a quasar brighter than HS 1700+6416 and at a lower redshift of [FORMULA]. Ultraviolet observations of HS 1103+6416 with IUE revealed flux down to 1250 Å and a strong LLS at [FORMULA] (Reimers et al. 1995a). Due to the lower redshift the absorption line density should be smaller and we obtained ultraviolet spectra with the HST to study the metal line systems (MLS).

With the limited spectral resolution achievable with HST in the UV even for the brightest high-redshift QSOs, the common splitting of strong MLSs into subcomponents is not resolved and hidden saturation effects can lead to false column densities. This difficulty can be overcome only with optical observations at the highest possible resolution in order to be able to synthesize the unresolved UV lines on the basis of splitting into subcomponents observed in the optical regime. For this purpose, a high-resolution spectrum has been taken with HIRES at the Keck 10 m telescope.

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© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999