The UV spectrum of HS1700+6416 I. Predicting the metal line content of the far UV spectrum

Aims. In preparation of a study of the He II/H I ratio towards the bright QSO HS 1700+ 6416, we predict the metal line content of the far-UV spectral range by modelling 18 metal absorption line systems with redshifts 0.2 < z < 2.6 identified in the spectrum of this quasar. Methods. For that purpose, we investigate the spectral energy distribution of the metagalactic ionizing radiation field. Simple photoionization models based on 8 different shapes of the ionizing background are tested for each system. The adopted energy distributions comprise the Haardt & Madau ( 2001, HM) model of metagalactic UV background as well as typical spectra of AGN and starburst galaxies. The models are evaluated and the favoured one is estimated. Results. We find that the majority of the considered systems is best reproduced with a HM-like ionizing radiation, where the He II break, formally located at 4 Ryd, is shifted to lower energies (similar to 3Ryd), probably due to the opacity of the higher He II Lyman series lines. The remaining systems can be reasonably described with models based on the unmodified HM background or the spectra of AGN or starburst galaxies. This finding supports the idea that the UV background is spatially variable due to both IGM opacity variations and to local sources. In comparison to an unmodified HM background, the resulting ionizing spectrum leads to carbon abundances lower by similar to 0.5 dex. Furthermore, if the ionizing radiation field as determined from metal line systems was typical for the IGM, the expected He II/ H I ratio would be 150 to 190.