Confirmation of dust in damped Lyman-alpha systems

As part of a search for dust at high redshifts we have acquired new spectra of quasars in the Wolfe et al. sample: 13 with damped Lyα systems along the lines of sight and 15 without. Our spectra cover the region 3950-6900 A at a resolution of 15 A with signal-to-noise ratios of 1-50. We have determined spectral indices over a common range of wavelengths in the rest frames of the quasars between Lyα and C IV emission. Several internal and external checks indicate that the typical errors in the spectral indices are +/- 0.1. We find that quasars with damped Lyα systems in the foreground are stochastically redder than those without damped Lyα systems in the foreground. This difference, significant at the 3 σ level, confirms our previous detection in the sample of Sargent, Steidel, & Boksenberg. Combining the Wolfe et al. and Sargent et al. samples increases the significance of the reddening to at least 4 σ. We compute dust-to-gas ratios from the spectral indices on the assumption that the extinction curve in the damped Lyα systems has the same shape as that in the Milky Way or the Large or Small Magellanic Cloud. Galactic-type dust appears to be ruled out by the absence of strong extinction near 2175 A in the rest frames of several damped Lyα systems. Our best estimates of the typical dust-to-gas ratio in the damped Lyα systems are then 5%-20% of that in the Milky Way. This result, together with recent measurements of the gas-phase abundances of Zn and Cr in three damped Lyα systems, indicates that the overall abundances of heavy elements (in both the gas and solid phases) are about one order of magnitude lower than the Galactic value.