The HeII Lyman alpha forest: evaluation of simulated data

Using an artificial HI Lyman alpha spectrum we simulate the corresponding HeII forest with fixed values of eta and a Doppler parameter consisting of a thermal and a turbulent part. In addition metal lines with line strengths and line density as expected in the case of HS 1700+6416 are superimposed. FUSE-like noise is added. The analysis of the simulated spectra in terms of Doppler profiles recovers the input eta = N(HeII)/N(HI) with a scatter by a factor of 10. About 10 % have significantly lower eta-values for various reasons. The majority of the extremely high eta-values (up to 1000) are evidently caused by metal lines. We conclude that part of the scatter in eta of previous analyses of the HeII forest in HE 2347-4342 can be regarded as an artifact. However, we confirm that the correlation between small column densities (in voids) and high eta-values is not a methodical artifact and appears to be a true phenomenon.