ARE DAMPED Ly SAMPLES BIASED BY DUST?
Surveys for damped Ly systems have the greatest impact if they represent a fair sample of the neutral gas in the Universe, allowing a clear probe of the evolution with redshift of the neutral hydrogen content and the metallicity of neutral gas. However, it has long been a major concern that the sample of damped Ly systems suffers from “dust bias,” i.e., the absence from a magnitude-limited QSO sample of those QSOs that suffer obscuration from dusty foreground damped Ly systems, leading to underrepresentation of dusty damped Ly systems in the overall sample. The easiest way to probe the existence and abundance of dust in damped Ly systems would be to find the 2175 Å bump feature superimposed in absorption on background QSO spectra. While Junkkarinen et al. (2004) found at least one strong example, this does not appear to be the rule (Pei, Fall & Bechtold 1991). Without such sharp features to look for and given the wide range of intrinsic QSO spectral slopes, reddening from dust in dampe
