|Annu. Rev. Astron. Astrophys. 2001. 39:
Copyright © 2001 by . All rights reserved
In this review we have shown that knowledge of the CIB, its sources, and its implications has advanced dramatically in the past few years. We now have claimed detections of the background at near-infrared and far-infrared/submillimeter wavelengths based on data from the COBE and IRTS missions. Useful upper limits at other wavelengths come from these direct measurements and indirectly from observations of TeV -rays, whereas lower limits from ground-based galaxy counts and counts from ISO and SCUBA surveys further limit the observational uncertainty in the CIB (Section 3.10). Although observational knowledge of the CIB still has much room for improvement, the detections and limits presented in this review already have many important implications.
We can anticipate that study of the CIB and its implications will continue to be vigorous for some time to come. Although there are no new space instruments under development for the specific purpose of diffuse infrared background measurements, one can expect there to be further improvements in limits or detections of the extragalactic infrared background based on the sky brightness measurements already in hand and better determinations of the foregrounds. Completion of the 2MASS survey at J, H, and K bands will permit accurate removal of the stellar foreground over the substantial sky areas needed for convincing demonstration of isotropy. The Space Infrared Telescope Facilty (SIRTF) has the potential of providing direct measurements of foreground sources at 3.5 µm, although clearly over limited sky areas (Werner et al. 2001). More extensive maps of the distribution of H+ will be available as the WHAM survey progresses, facilitating CIB discrimination particularly in the far infrared.
The largest obstacle to direct EBL measurements over much of the infrared spectrum and at UV-optical wavelengths remains the bright foreground due to scattering and emission from the interplanetary dust. This major problem could be substantially eliminated by making measurements with instruments located several astronomical units from the Sun in the ecliptic plane, or in a location out of the ecliptic plane. We are not aware of any current plans to accomplish such measurements.
Indirect constraints on the CIB from measurements of TeV -rays will continue to improve as more sources over a greater range of distances are observed. Next-generation TeV -ray telescopes, such as the Very Energetic Radiation Imaging Telescope Array System (VERITAS), will be able to detect fainter sources at higher redshift, providing needed confirmation of the intergalactic absorption signature in the source spectrum (Catanese & Weekes 1999). Space missions such as the Gamma-ray Large Area Space Telescope (GLAST) will help to clarify the intrinsic spectrum of the TeV -ray sources from 20-200 GeV.
There promises to be continued rapid advance in measurements of discrete extragalactic sources contributing to the background. Such measurements will provide both increasingly robust lower limits to the CIB and essential information on the distance and character of these sources. Analyses of ISO data in the thermal and far-infrared are still in progress. Source counts will be carried to much fainter levels by missions such as SIRTF (Werner et al. 2001), the Far Infrared Space Telescope (FIRST; Pilbratt 2001), and the Infrared Imaging Surveyor (IRIS; Shibai 2001). The Atacama Large Millimeter Array (ALMA) will provide the sensitivity and angular resolution at submillimeter and millimeter wavelengths needed to clarify the nature of the sources being revealed in SCUBA observations. Deep, wide-field near infrared imagery from the Next Generation Space Telescope (NGST; Stockman & Mather 2001), combined with spectroscopy from NGST and large ground-based telescopes, will dramatically advance our understanding of the nature and distribution of sources at high redshift and their contribution to the infrared background. Within the next decade or so it should become clear whether the CIB arises entirely from discrete sources.
As observational knowledge of the cosmic histories of star formation and metal formation, and of the nature of the systems (AGN, starburst galaxies) making substantial contributions to the CIB improves, uncertainties in the models of background light generation associated with these processes will be reduced. This offers the prospect of a consistent, comprehensive history of the growth of cosmic structure and accompanying energy releases.
During the preparation of this manuscript we have greatly benefited from helpful and enlightening discussions with Rebecca Bernstein, Andrew Blain, Doug Finkbeiner, Dale Fixsen, Alex Kashlinsky, Richard Mushotzky, Sten Odenwald, Ray Protheroe, and Ned Wright. We thank Felix Aharonian, Rick Arendt, Chuck Dermer, Mike Fall, Jim Felten, Michel Fioc, Alex Konopelko, and Bob Silverberg for their helpful comments on sections of the manuscript. We thank Prof. T. Matsumoto for permission to reproduce the IRTS background spectrum. We thank Andrew Blain, James Bullock, Daniela Calzetti, Julien Devriendt, Alberto Franceschini, Guilain Lagache, Matt Malkan, Jonathan Tan, and Cong Xu for communicating their model results in digital form. Finally, we thank the scientific editor, Allan Sandage, for his careful review and valuable comments. Preparation of this review was partially supported by NASA grant NAG5-3899 and NASA contract NAS 5-26555 to the Association of Universities for Research in Astronomy, Inc. (MGH), and by NASA's Astrophysical Theory Program (NRA 99-OSS-01) (ED).