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One of the striking findings of QSO absorption-line studies is that there appears to be substantially more neutral hydrogen at redshift z ≈ 2.5 than there is at present in nearby galaxies. This result comes from the statistics of the “damped Lyman α” (DLα) class of absorption system (Wolfe et al. 1986, Lanzetta et al. 1995). Pushing the observations and analysis to redshifts greater than 4, Storrie-Lombardi et al. (1995) find evidence for a maximum in the integral HI content, ΩHI, at z ≈ 2 to 3 (see review in this volume by Norman & Braun 1996). In addition to uncertainty in the conclusions due to small number statistics, uncertainties may enter the interpretation due to possible biases and selection effects. Fall and Pei (1993) have argued that dust in galaxies will skew the statistical arguments against inclusion of dusty lines of sight through spiral galaxies, causing the DLα statistics to under-estimate the true HI content. Gravitational lensing may also influence the QSO absorption statistics (Thomas & Webster 1990, Bartelmann & Loeb 1995, Smette et al. 1995), since the amplification provided by the intervening galaxy may preferentially bring dim background QSOs into the sample selected for spectroscopic observation, thus overemphasizing the DLα systems. |
