Annu. Rev. Astron. Astrophys. 2005. 43: 861-918
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On the other hand, Prochaska et al. (2005) find that biasing due to gravitational lensing could be important. They compared the full SDSS sample of damped Lyalpha systems with subsamples comprising the brightest 33% of background quasars and the faintest 33% of background quasars. While the incidence of damped Lyalpha systems, dN / dX, was found to be insensitive to quasar magnitude, the mass density, Omegag(z), was found to vary significantly. Specifically, the bright subsample showed sysematically higher values of Omegag(z) than the faint subsample. To explain the independence of dN / dX on quasar magnitude, the difference must lie in the incidence of systems with large N(H I), which is observed to be larger in the bright subsample.

Prochaska et al. (2005) argue that gravitational magnification of the background quasars by massive halos or disks associated with the foreground damped Lyalpha systems could account for this systematic effect, which was first detected at the 2sigma level by Murphy & Liske (2004). Whereas obscuration by dust would cause Omegag(z) to be lower in the bright subsample, magnification by lensing due to exponential disks is greatest for damped Lyalpha systems with large values of N(H I) (Bartelmann & Loeb 1996; Maller et al. 1997).

Consequently, the values of Omegag(z) in Figure 5 may be 10-20% too high, but the evolution of Omegag(z) with z is still likely to be correct. However, the effect could be more important for damped Lyalpha system samples with z < 2 (Rao & Turnshek 2000).

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