Obscuration of quasars by dust in damped Lyman-alpha systems

Abstract

A largely analytical method for computing the obscuration of quasars by dust in damped Ly-alpha absorption systems is presented. Between 10 and 70 percent of the bright quasars at z of 3 are missing from optical samples. This is not enough obscuration to explain the flattening or turnover in the observed comoving density of bright quasars in the z interval of 2-3. It does help to account for the UV background radiation implied by the proximity effect at z of about 3. It is estimated that quasars produce at least 10 percent and possibly all of this radiation. The obscuration increases so rapidly with redshift that samples of optically selected quasars may be only 10 percent complete at z of 4. The limits from COBE on the FIR background radiation imply that the dust of the damped Ly-alpha systems cannot be much hotter than the dust in the Milky Way.