Consequences of Dust in Damped Lyman-Alpha Systems

Abstract

Dust has been detected in damped Lyα systems at high redshifts by two independent methods: the reddening of background quasars and the gas-phase depletion of Cr relative to Zn. Both methods give a typical dust-to-gas ratio roughly 10% of that in the Milky Way. Here, we review some of the consequences of these observations. First, the dust in damped Lyα systems causes obscuration and hence incompleteness in optically selected samples of quasars. We estimate that up to 70% of the bright quasars at z = 3 are missing from such samples. Corrections for this bias help to reconcile the ionizing UV background inferred from the proximity effect with that expected from quasars alone. Second, since the existing samples of damped Lyα systems were obtained from the spectra of optically bright quasars, they too must be incomplete. This affects the distribution of HI column densities f (N, z) and, especially, the comoving density Ω hi(z). The bias therefore has implications for the histories of gas consumption, star formation, and metal production in the damped Lyα systems. In particular, it solves a puzzle that has arisen in attempts to understand the chemical evolution in the damped Lyα systems (the “G-dwarf problem”). Third, the dust is likely to absorb most of the Lyα photons produced in the damped Lyα systems and thereby account for the null results of most searches for Lyα emission from them.