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

1. INTRODUCTION

One of the outstanding challenges in modern cosmology is to explain the formation of structure in the universe. The assembly of matter into stars and galaxies and the subsequent evolution of such systems is accompanied by the release of radiant energy powered by gravitational and nuclear processes. Cosmic expansion and the absorption of short-wavelength radiation by dust and reemission at long wavelengths will shift a significant part of this radiant energy into infrared background radiation. A cosmic infrared radiation background is therefore an expected relic of structure formation processes, and its measurement provides new insight into those processes. Until a few years ago, there had been no definite measurements of the infrared background radiation.

For perspective, we show in Figure 1 the spectrum of the extragalactic background radiation over ~ 20 decades of energy, from radio waves (10^-7 eV) to high-energy -ray photons of a few hundred GeV. The cosmic microwave background (CMB), the fossil blackbody radiation from the Big Bang, is the dominant form of electromagnetic energy. Figure 1 gives only a schematic representation of the spectrum at wavelengths from the ultraviolet (UV) to the far infrared, roughly based on the energy released in producing the heavy elements. The extragalactic background light (EBL) from UV to far-infrared wavelengths is likely to be the dominant radiant energy in the universe aside from the CMB. The background light in the more limited spectral range from 1-1000 µm, excluding the CMB, is referred to as the cosmic infrared background (CIB). As we discuss, most radiant backgrounds shown in Figure 1 other than the CMB are causally connected.

Figure 1. Spectrum of the cosmic background radiations. The radio background (CRB) is represented by a I 0.3 spectrum, normalized to the Bridle (1967) value at 170 cm. The cosmic microwave background (CMB) is represented by a blackbody spectrum at 2.725 K. The UV-optical (CUVOB) and infrared (CIB) backgrounds are schematic representations of the work summarized in this review (see Figure 5). The data for the X-ray background (CXB) are taken from Wu et al. (1991), and the curves are analytical representations summarized by Fabian & Barcons (1992). The -ray background (CGB) is represented by the power law given by Sreekumar et al. (1998).

In spite of the recognized significance of the CIB, its measurement has remained elusive because of the bright foreground radiations from which it must be distinguished. The observational evidence has changed dramatically in the past few years, with the first direct measurements of this radiation provided by NASA's Cosmic Background Explorer (COBE) satellite, and additional evidence coming from Japan's Infrared Telescope in Space (IRTS). Indirect evidence constraining the CIB is coming from the rapidly developing ability to measure intergalactic attenuation of -rays at TeV energies. Coincidentally, rapid advances in ground and space observations are resolving at least some of the sources of the CIB. Because this field is now very dynamic, the emphasis of this review is on the observational advances since the mid-1990s and their implications. Other summaries of both observational and theoretical work may be found in papers presented at numerous conferences and references therein (Lawrence 1988, Bowyer & Leinert 1990, Holt et al. 1991, Rocca-Volmerange et al. 1991, Longair 1995, Calzetti et al. 1995, Kafatos & Kondo 1996, Dwek 1996, Lemke et al. 2000, Franceschini 2001, Harwit & Hauser 2001). We treat sources within the solar system and Milky Way galaxy as undesired foregrounds to be discriminated from the CIB and do not describe them in any detail [for a recent compilation, see Leinert et al. (1998)].

The plan of this review is as follows. In Section 2 we sketch the history of the growing interest in the infrared background radiation. Section 3 provides a description of the observational evidence for the CIB and a summary of the UV-optical background. In Section 4 we address direct implications of the measurements. Section 5 deals with how the measurements constrain evolutionary models. In Section 6 we summarize the main conclusions and address future prospects.

We uniformly present photometric results in terms of I, where I is the spectral intensity at frequency . A unit of convenient size is the nW m^-2 sr^-1. Conversion to I in MJy sr^-1 (1 MJy 1 megajansky = 10^-20 W m^-2 Hz^-1) can be done with the relation

Conversion to energy density, ² n, is given by

where is the photon energy in eV and n is the photon spectral number density in photons cm^-3 eV^-1. Throughout this review we express the Hubble constant, H₀, as H₀ = 100 h km s^-1 Mpc^-1.