Microlensing of Quasars By Stars In Their Damped Ly α Absorbers

The damped Lyα absorbers (DLAs) in quasar spectra are believed to be the progenitors of present–day disk galaxies. We examine the probability for microlensing of background quasars by stars in their DLAs. Microlensing should magnify the continuum but not the broad emission lines of the quasars. We model a representative spiral galaxy as a closed system composed of a bulge, a disk, and a halo, and evolve the mass fraction of stars in the disk based on the observed metallicity of DLAs at high redshifts. The microlensing signatures are strongly enhanced if the halo of the galaxy is made of Massive Compact Halo Objects (MACHOs). In this case, the distortion imprinted by microlensing on the equivalent width distribution of quasar emission lines can be detected with high significance in a sample of ∼ < 10 DLAs with HI column densities N ∼ > 10 21 cm −2 and absorption redshifts z abs ∼ < 1. About a tenth of all quasars with DLAs (N ∼ > 10 20 cm −2) might show excess variability on timescales shorter than five years. A search for these signals would complement microlensing searches in local galaxies and calibrate the MACHO mass fraction in galactic halos at high redshifts.