Annu. Rev. Astron. Astrophys. 2001. 39: 249-307
Copyright © 2001 by . All rights reserved

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The integrated background intensity provides only limited constraints on the history of energy releases in the universe. The total intensity alone does not allow us to address detailed issues, such as the nature and evolution of the CIB sources, the relative contributions of AGN and star-forming galaxies at various wavelengths, and the history of star and element formation. These issues can be addressed, in part, by considering the spectral energy distribution of the background.

The spectral intensity Inu(nu0) of the EBL at the observed frequency nu0 is given by the integral over its sources (Peebles 1993):


where Lnu(nu , z) is the spectral luminosity density of all luminous objects and radiating particles in a comoving volume element at redshift z, nu = nu0(1 + z) is the frequency in the rest frame of the luminous objects, and |dt / dz| was given in Equation 6 (Section 4.2).

In a dust-free universe, the spectral luminosity density, Lnu(nu , z), can in principle be simply derived from a knowledge of the spectrum of the emitting sources and the cosmic history of their energy release. In a dusty universe, the total intensity remains unchanged, but the energy is redistributed over the entire spectrum. Predicting this spectrum poses a significant challenge since the frequency distribution of the reradiated emission depends on a large number of factors (Dwek 2001). On a microscopic level, the emitted spectrum depends on the wavelength dependence of the absorption and scattering properties of the dust, which in turn depend on the dust composition and size distribution. The reradiated spectrum also depends on the dust abundance and the relative spatial distribution of energy sources and absorbing dust. Finally, the cumulative spectrum from all sources depends on evolutionary factors, including the history of dust formation and processes that destroy the dust, modify it, or redistribute it relative to the radiant sources.

Cosmic expansion reduces the overall weight of the contribution to the EBL from sources with z > 2 (Harwit 1999). However, luminous infrared sources at high redshift can dominate the CIB at submillimeter wavelengths because of the negative K-correction. Nonnuclear sources, while not contributing significantly to the total EBL, may still be major contributors at specific wavelengths. Models of the EBL must therefore be able to identify the contribution of the distinct energy sources to all parts of the spectrum.

In the following, we first discuss the contribution of AGN and other nonnuclear sources to the EBL (Section 5.1). We then describe cosmic evolution models that provide estimates of the nuclear contribution to the EBL (Section 5.2).

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