Limits on Dust in Damped Lyman-Alpha Systems and the Obscuration of Quasars

Abstract

The damped Lyα systems discovered in the spectra of quasars at high red-shifts are natural places to search for dust. They have column densities greater than 1020 cm−2, contain most of the neutral hydrogen in the Universe, and may be protogalaxies or galactic disks in an early, gas-rich phase of evolution. We compare the spectra of quasars in the Wolfe et al (1986) survey that have damped Lyα with those that do not have damped Lyα to obtain statistical information about the reddening by dust. Our results are given in terms of the dimensionless dust-to-gas ratio k = 1021 (τ B /N H ) cm−2, where τ B is the optical depth in the B band in the rest frame of an absorber and N H is the column density of neutral hydrogen. Using non-parametric tests, we find, at the 95% confidence level, k ≤ 0.41 (GAL), k ≤ 0.29 (LMC) and k ≤ 0.19 (SMC), depending on whether the extinction curve is assumed to have the same shape as that in the Milky Way or the Large or Small Magellanic Clouds. Our upper limits on the dust-to-gas ratio in the damped Lyα systems are half the observed value in the Milky Way but are several times larger than the observed values in the Magellanic Clouds.