Uncertainties in the interpretation of the Lyman alpha forest lines

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Abstract
We study the formation of Lyα forest lines in media with a stochastic velocity field when the line broadening is caused mainly by macroscopic large-scale, rather than thermal, motions. Our results show clearly that a finite velocity correlation length affects strongly (i) the line profiles, (ii) the equivalent widths and (iii) the intensity ratios of Lyα and higher order Lyman lines. In particular, it is shown that the ratio Nmeso/Nmicro may range from 1 to ∼ 200 for a given Lycc cloud, Nmicro and Nmeso denoting the hydrogen column density calculated in the microturbulent limit, i.e. for vanishing correlation length (l → 0), and for finite correlation length (mesoturbulence), respectively. We argue that the column density distribution function f(N) has to be re-examined. The deficit of absorption systems revealed in the Lyα forest in the range 1014 ≤ N ≤ 1016 cm−2 may simply be an artefact. A test of the dynamical evolution of the intergalactic Lyα clouds is suggested. It is based on the fact that the statistical properties of the velocity field affect optically thin and thick lines in different ways, allowing us to measure eventually turbulent characteristics of the gas in the Lya clouds.

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