1997/01/15 by Abraham Loeb, Loeb, Abraham
Physics and Astronomy · #Adaptive optics and wavefront sensing #Astronomy and Astrophysical Research #Astrophysics (astro-ph) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena
paper · pdf · doi:10.48550/arxiv.astro-ph/9701100
openalex publication_date 1997/01/15 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Gravitational lensing by a spiral galaxy occurs when the line-of-sight to a background quasar passes within a few kpc from the center of the galactic disk. Since galactic disks are rich in neutral hydrogen, the quasar spectrum will likely be marked by a damped Lyman-alpha absorption trough at the lens redshift. Therefore, the efficiency of searches for gravitational lensing with sub-arcsecond splitting can be enhanced by 1-2 orders of magnitude by focusing on a subset of all bright quasars which show low-redshift (z<1) strong Lyman-alpha absorption (N>1021 cm-2). The double-image signature of lensing could, in principle, be identified spectroscopically and without the need for high-resolution imaging. The absorption spectrum of a spiral lens would show a generic double-step profile due to the superposition of the two absorption troughs of the different images. Finally, we note that searches for microlensing signatures of quasars with damped Lyman-alpha absorption could calibrate the fraction of MACHOs in galactic halos at high redshift.