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 redshifts are natural places to search for dust. They have column densities of neutral hydrogen greater than 10^20^ cm^-2^ and may be protogalaxies or galactic disks in an early, gas-rich phase. We compare the spectra of quasars in the Wolfe et al. 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 = 10^21^ (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 nonparametric 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. We also develop a new method to set limits on the mean and the variance of the optical depth along random lines of sight. This includes a correction for the effect, emphasized by Ostriker and Heisler, that highly obscured quasars are less likely to be included in optically selected samples than quasars with little dust in the foreground. Our results for the mean optical depth in the B band of the observer can be approximated by the formula τ^bar^_B_(z) =0.4τ_*_[(1 + z)^5/2^ - 1]; using the upper limits on the dust-to-gas ratio in the damped Lyα systems, we obtain τ_*_ <= 0.07 (GAL), τ_*_ <= 0.06 (LMC), and τ_*_ <= 0.05 (SMC). For comparison, the first models by Ostriker and Heisler predict τ_*_ = 0.4 or 0.8, while their more recent models predict τ_*_ = 0.16. We conclude that neither set of models is consistent with our limits and that the apparent cutoff in the counts of quasars at z ~ 3 is probably not caused by dust in the damped Lyα systems. All the limits derived in this paper could be reduced significantly or a positive detection could be made by determining more accurately the spectral indices of the quasars in the Wolfe et al. survey.

Limits on dust in damped Lyman-alpha systems and the obscuration of quasars

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