Lensing properties of 7 damped Lyman-alpha absorbing galaxy-QSO pairs

Le Brun et al. (1997) presented the first identifications of the galaxies giving rise to 7 intermediate redshift damped Ly-alpha (DLA) absorption systems. Here, we study the gravitational lensing properties of these foreground galaxies based on their observed optical appearance and on the absence of any secondary lensed quasar image. We consider the possibility that any secondary image be hidden due to extinction by dust, but find it unlikely. We derive upper limits on the amplification factor affecting the luminosity of the background quasars; in each case, this factor is found to be less than 0.3 mag. We also obtain upper limits on the total mass of the damped Ly-alpha galaxies, within radii equal to the quasar impact parameters. Mass-to-light ratios are found to be consistent with existing estimates based on X-ray emission or on motion of dwarf satellites. Although we show that lensing is not important in this sample, we note that existing DLA surveys used to determine the cosmological density of gas at z<1 are based on samples of quasars brighter than the ones considered here and for which the amplification bias is likely to be stronger.