Effects of disks on gravitational lensing by spiral galaxies

Gravitational lensing of a quasar by a spiral galaxy should often be accomp anied by damped Ly alpha absorption and dust extinction due to the interven ing gaseous disk. In nearly edge-on configurations, the surface mass densit y of the gas and stars in the disk could by itself split the quasar image a nd contribute significantly to the overall lensing cross section. We calcul ate the lensing probability of a disk-plus-halo mass model for spiral galax ies, including the cosmic evolution of the lens parameters. A considerable fraction of the lens systems produce two images with subarcsecond separatio n, straddling a nearly edge-on disk. Because of that, extinction by dust to gether with observational selection effects (involving a minimum separation and a maximum flux ratio for the lensed images) suppress the detection eff iciency of spiral lenses in optical wave bands by at least an order of magn itude. The missing lenses could be recovered in radio surveys. In modifying the statistics of damped Lya absorbers, the effect of extinction dominates over the magnification bias due to lensing.