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We have analyzed the properties of low column density Lyalpha forest clouds (12. 8 -1. 55 for clouds with NHI <= 10 (14. 3) \ cm (-2) and changes extremely slowly with redshift over this NHI range. 2) The deviation of f (NHI) from this power-law distribution at higher column densities depends on redshift: for higher redshifts, the departure from a power-law distribution occurs at higher NHI, which means that higher column density clouds rapidly disappear with decreasing redshift. This may be consistent with an invariant shape for the different column density distributions, but with a shift to a lower column density normalization for systems at different redshifts driven by the overall expansion of the universe. 3) The line widths of the forest clouds increase as redshift decreases. 4) The correlation strength of the forest clouds seems to increase as redshift decreases. We discuss these results in terms of the evolution of the IGM comparing the results to models and analytic descriptions of the evolution of structure in the gas.
Kim et al. (Tue,) studied this question.