Mathematics – Logic
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6426f&link_type=abstract
HST Proposal ID #6426
Mathematics
Logic
Hst Proposal Id #6426 Cosmology
Scientific paper
We propose to obtain high-resolution images of a sample of QSOs with strong damped Ly-alpha absorption in their spectra. The observations would determine whether these QSOs are multiply imaged by gravitational lensing. It is widely believed that damped Ly-alpha absorption occurs in HI disks of spiral galaxies. Gravitational lensing is then inevitable for lines of sight passing close to the centers of these systems. For typical spiral galaxies, the resulting image separation is expected to be in the range 0.1''-0.5'' and can only be detected with HST at optical wavelengths. Because the neutral -hydrogen column density increases on average towards the center of galaxies, strong Ly-alpha absorption provides an efficient ``flag'' for gravitationally lensed QSOs with small image separations. Bartelmann & Loeb {1995, ApJ, in press} have shown that the frequency of multiple images is enhanced by 1-2 orders of magnitude in samples of QSOs with strong damped Ly-alpha absorption {N_ HI> 10^21 cm^-2}, compared to the overall QSO population. For this proposal, we have selected those QSOs most likely to exhibit gravitational lensing, from the published list of QSOs with damped Ly-alpha absorption. If lensing is found in this QSO sample, it will have important cosmological implications. First, it will confirm the association of damped Ly-alpha systems with galactic potential wells. Second, the magnification bias associated with lensing would make the appearance of damped systems more likely, and therefore affect estimates of the neutral-hydrogen fraction in the Universe {Bartelmann & Loeb 1995}. Third, the cosmological history of HI is linked to the process of galaxy formation and evolution {Lanzetta, Wolfe, & Turnshek 1995, ApJ, 440, 435}. Finally, the lensing frequency provides a sensitive probe of the cosmological constant.
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