Damped Lyman alpha absorbers are believed to be associated with galactic disks. We show that gravitational lensing can therefore affect the statistics of these systems. First, the magnification bias due to lensing raises faint QSOs above a given magnitude threshold and thereby enhances the probability for observing damped absorption systems. Second, the bending of light rays from the source effectively limits the minimum impact parameter of the line of sight relative to the center of the absorber, thus providing an upper cutoff to the observed neutral hydrogen (H I) column density. The ray bending also reduces a possible obscuration of the QSO by dust. The combination of the lensing effects yields a pronounced peak in the observed abundance of absorbers with high column densities (≳2 × 10²¹ cm⁻²) and low redshifts (zabs ≲ 1) in the spectra of bright QSOs (B ≲ 18 mag) with redshifts zQSO ≳ 2. The inferred value of the cosmological density parameter of neutral hydrogen, ΩH I, in creases with increasing redshift and luminosity of the sources even if the true H I density remains constant. This trend is consistent with the observed evolution of ΩH I(z). Damped Lyman alpha absorbers with column densities ≳ 10²¹ cm⁻² and redshifts 0.5 ≲ zabs ≲ 1 are reliable flags for lensed QSOs with a close pair of images separated by ∼0".3 × (υc/220 km s⁻¹)², where υc is the rotational velocity of the lens. Detection of these gravitational lensing signatures with the Hubble Space Telescope can be used to constrain the depth of the absorber potential wells and the cosmological constant.
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Bartelmann et al. (1996) studied this question.