The UV spectrum of HS1700+6416 II. FUSE observations of the HeII Lyman alpha forest

Aims. We present the far-UV spectrum of the quasar HS 1700+6416 taken with FUSE. This QSO provides the second line of sight with the He II absorption resolved into a Ly alpha forest structure. Since HS 1700+6416 is slightly less redshifted ( z(em) = 2.72) than HE 2347-4342, we only probe the post-reionization phase of He II, seen in the evolution of the He II opacity, which is consistent with a simple power law. Methods. The He II/ H I ratio. is estimated using a line profile-fitting procedure and an apparent optical depth approach, respectively. The expected metal line absorption in the far-UV is taken into account as well as molecular absorption of galactic H-2. About 27% of the. values are affected by metal line absorption. In order to investigate the applicability of the analysis methods, we create simple artificial spectra based on the statistical properties of the H I Ly alpha forest. Results. The analysis of the artificial data demonstrates that the apparent optical depth method as well as the line profile-fitting procedure lead to confident results for restricted data samples only ( 0.01 <= tau(HI) <= 0.1 and 12.0 = log N-HI <= 13.0, respectively). The reasons are saturation in the case of the apparent optical depth and thermal line widths in the case of the profile fits. Furthermore, applying the methods to the unrestricted data set may mimic a correlation between the He II/ Hi ratio and the strength of the Hi absorption. For the restricted data samples a scatter of 10-15% in eta would be expected even if the underlying value is constant. The observed scatter is significantly larger than expected, indicating that the intergalactic radiation background is indeed fluctuating. In the redshift range 2.58 < z < 2.72, where the data quality is best, we find eta similar to 100, suggesting a contribution of soft sources like galaxies to the UV background.