Effects of Disks on Gravitational Lensing by Spiral Galaxies

Gravitational lensing of a quasar by a spiral galaxy should often beaccompanied by damped Lyman-alpha absorption and dust extinction due to theintervening gaseous disk. In nearly edge-on configurations, the surface massdensity of the gas and stars in the disk could by itself split the quasar imageand contribute significantly to the overall lensing cross section. We calculatethe lensing probability of a disk+halo mass model for spiral galaxies,including cosmic evolution of the lens parameters. A considerable fraction ofthe lens systems contains two images with sub-arcsecond separation, straddlinga nearly edge-on disk. Because of that, extinction by dust together withobservational selection effects (involving a minimum separation and a maximumflux ratio for the lensed images), suppress the detection efficiency of spirallenses in optical wavebands by at least an order of magnitude. The missinglenses could be recovered in radio surveys. In modifying the statistics ofdamped Lyman-alpha absorbers, the effect of extinction dominates over themagnification bias due to lensing.Comment: 19 pages, 12 figures; submitted to Ap