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 st atistics of these systems. First, the magnification bias due to lensin g raises faint QSOs above a given magnitude threshold and thereby enha nces the probability for observing damped absorption systems. Second, the bending of light rays from the source effectively limits the minim um impact parameter of the line of sight relative to the center of the absorber, thus providing an upper cutoff to the observed neutral hydr ogen (H I) column density. The ray bending also reduces a possible obs curation of the QSO by dust. The combination of the lensing effects yi elds a pronounced peak in the observed abundance of absorbers with hig h 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 bri ght QSOs (B less than or similar to 18 mag) with redshifts z(QSO) grea ter than or similar to 2. The inferred value of the cosmological densi ty parameter of neutral hydrogen, Omega(H I), increases with increasin g 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 gre ater than or similar to 10(21) cm(-2) and redshifts 0.5 less than or s imilar to z(abs) less than or similar to 1 are reliable flags for lens ed 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 veloci ty of the lens. Detection of these gravitational lensing signatures wi th the Hubble Space Telescope can be used to constrain the depth of th e absorber potential wells and the cosmological constant.