Dust Absorption and Emission in Galaxies at High and Low Redshifts

This article reviews three related topics: the extragalactic background light and its sources, evolution models for the dust absorption and emission in galaxies, and empirical constraints on these transfer processes in nearby starburst galaxies. It is intended that the material presented here will serve as an introduction to this Joint Discussion. 1. Extragalactic Background Light A good place to begin our discussion of the effects of dust on the radiation from galaxies is with the extragalactic background light (EBL). We denote the intensity of this isotropic radiation per unit frequency ν by Jν and the integral of this over all ν, the bolometric intensity, by Jbol. By definition, Jν excludes foreground radiation from the Milky Way and other nearby galaxies and the cosmic microwave background radiation from the Big Bang, but it includes the radiation from everything in between. A few years ago, we had only rough estimates and upper or lower limits on Jν at most wavelengths, but this situation has improved dramatically, thanks largely to HST and COBE. We now know Jν to an accuracy of a factor of two or better over four decades in wavelength, from about 0.2 to 2000 μm. This is shown in Figure 1 below. Two interesting results follow directly from Figure 1. First, emission by dust is important. There is at least as much energy in the long-wavelength hump of the EBL spectrum (λ ∼ > 10 μm) as in the short-wavelength hump (λ ∼ < 10 μm). Second, integrations under the smooth curves in Figure 1 give Jbol ≈ 50 nWm sr. (1) This is a summary record of the bolometric emissivity Ebol (power radiated per unit comoving volume) integrated over the age of Universe t0: