Gravitational lensing of quasi-stellar objects by their damped Ly alpha absorbers

Damped Ly alpha absorbers are believed to be associated with galactic disks. We show that gravitational lensing call 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 (greater than or similar to 2 x 10(21) cm(-2)) and low redshifts (z(abs) less than or similar to 1) in the spectra of bright QSOs (B less than or similar to 18 mag) with redshifts z(QSO) greater than or similar to 2. The inferred value of the cosmological density parameter of neutral hydrogen, Omega(H I), increases 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 Omega(H I)(z). Damped Ly alpha absorbers with column densities greater than or similar to 10(21) cm(-2) and redshifts 0.5 less than or similar to z(abs) less than or similar to 1 are reliable flags for lensed QSOs with a close pair of images separated by similar to 0 ''.3 x (upsilon(c)/220 km s(-1))(2), where upsilon(c) 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.