Dust in Galaxies

Abstract

The subject of dust in galaxies, if covered comprehensively, is so vast that it would require an entire book for adequate discussion. In this review we concentrate on a few topics related primarily to dust as a constituent of systems in the Universe having masses comparable to galaxies. We have intentionally omitted discussing characteristics of dust as interstellar matter within galaxies or associated with stars. Several previous review articles have been devoted to the physical properties of dust within the Milky Way, and we will avoid repetition of material covered earlier. Our discussion focuses on observable characteristics and the physical importance of dust in galaxies. We have included active nuclei galaxies and quasars in this discussion, interpreting galaxies in the broadest sense. In the subjects we have covered, we have attempted to be up to date as of approximately June 1982. The observable characteristics of dust that generally lead to significant information about the solid grains are extinction, reddening (as deduced from continuum alteration or anomalous line ratios), scattering (such as in reflection nebulae), polarization produced by extinction and scattering, and thermal emission in the infrared. All of these processes may play a role in determining the characteristics of galaxies inferred from observations. From an understanding of the observational effects due to the presence of dust, we can better understand the important physical processes at work in galaxies. In the following sections we discuss the manifestations of dust, first in our Galaxy, then in normal galaxies, and finally in the active nuclei galaxies and quasars.

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