Gravitational Lensing of Quasi-stellar Objects by Their Damped LY alpha Absorbers

Abstract

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 cut-off to the observed neutral hydrogen (HI) column density. The combination of these effects yields a pronounced peak in the observed abundance of absorbers with high column densities (>2*10^{21} cm^{-2}) and low redshifts (z 10^{21} cm^{-2} and redshifts 0.5<z<1 are reliable flags for lensed QSOs with a close pair of images separated by 0.3 arcsec. 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.