Uncertainties in the interpretation of the Lyman alpha forest lines

Details Uncertainties in the interpretation of the Lyman alpha forest lines Uncertainties in the interpretation of the Lyman alpha forest lines

Jan 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996mnras.278..497l&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 278, Issue 2, pp. 497-505.

Astronomy and Astrophysics

Astronomy

15

Line: Formation, Line: Profiles, Methods: Observational, Quasars: Absorption Lines

Scientific paper

We study the formation of Lyalpha 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 Lyalpha and higher order Lyman lines. In particular, it is shown that the ratio N_meso/N_micro may range from 1 to ~200 for a given Lyalpha cloud, N_micro and N_meso 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 Lyalpha forest in the range 10^14<=N<=10^16 cm^-2 may simply be an artefact. A test of the dynamical evolution of the intergalactic Lyalpha 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 Lyalpha clouds.

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