Microlensing of Quasars by Stars in Their Damped Lyα 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 by an individual star should magnify the continuum but not the broad emission lines of the quasars. Consequently, the equivalent width distribution of microlensed quasars would be distorted. We model a representative spiral galaxy as a closed system composed of a bulge, a disk, and a halo, and we evolve the mass fraction of stars in the disk based on the observed metallicity of DLAs at high redshifts. The microlensing signatures are stronger 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 H I column densities N ≳ 10²¹ cm^-2 and absorption redshifts z_abs ≲ 1. About one-tenth of all quasars with DLAs (N ≳ 10²⁰ cm^-2) might show excess variability on timescales shorter than 5 yr. A search for these signals would complement microlensing searches in local galaxies and calibrate the MACHO mass fraction in galactic halos at high redshifts.