Astronomy and Astrophysics – Astrophysics
Scientific paper
2004-10-26
Astronomy and Astrophysics
Astrophysics
3 pages, 1 figure, to appear in Astrophysics in the Far Ultraviolet: Five Years of Discovery with FUSE, ASP Conf. series, eds.
Scientific paper
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.
Fechner Cora
Reimers Dieter
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