EFFECTS OF DUST ON GRAVITATIONAL LENSING BY SPIRAL GALAXIES

Gravitational lensing of an optical QSO by a spiral galaxy is often co unteracted by dust obscuration, since the line of sight to the QSO pas ses close to the center of the galactic disk. The dust in the lens is likely to be correlated with neutral hydrogen, which in turn should le ave an Ly alpha absorption signature on the QSO spectrum. We use the e stimated dust-to-gas ratio of the Milky Way galaxy as a mean and allow a spread in its values to calculate the effects of dust on lensing by low-redshift spiral galaxies. Using a no-evolution model for spirals at z less than or similar to 0.8, we find, in Lambda = 0 cosmologies, that the magnification bias due to lensing is stronger than dust obscu ration for QSO samples with a magnitude limit of B less than or simila r to 16. The density parameter of neutral hydrogen, Omega(H I), is ove restimated in such samples and is underestimated for fainter QSOs.