Annu. Rev. Astron. Astrophys. 2001. 39: 249-307
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3.10. Summary of Extragalactic Background Observations

As the preceding Sections show, there is now a great deal of direct and indirect observational evidence providing CIB upper limits, lower limits, and tentative or probable detections. Figure 5 summarizes this evidence, together with the data at UV and optical wavelengths to provide a more complete picture of the EBL. References for the direct infrared background measurements and the limits from infrared fluctuation measurements are in Tables 1 and 2, respectively. References for the UV-optical measurements are in Table 4. Figure 5 also shows the lower limits on the EBL found from the integrated light of resolved extragalactic sources. References for these limits are in Table 3. Missing from this figure for the sake of clarity are the CIB upper limits deduced from analysis of TeV gamma-ray data, which are shown in Figure 4. The dotted line in Figure 5 indicates the nominal measurement where detections are reported, or a somewhat arbitrary intermediate value between upper and lower limits where there is no claimed detection.

Figure 5

Figure 5. Summary of extragalactic background light measurements and limits. Error bars for detections are 1sigma. (Squares) Lower limits obtained by integrating the light of detected sources; (×) 2sigma lower limits on integrated resolved sources from Bernstein (1999); (diamonds) upper limits from fluctuation measurements; (remaining symbols) absolute background measurements (1sigma error bars) or limits (2sigma). (Shaded region) Current observational limits for the EBL spectrum; (dotted line) the nominal measurement where detections are reported, or a somewhat arbitrary intermediate value between upper and lower limits where there is no claimed detection (for a discussion, see Section 3.10). The black line (CMB) shows the cosmic microwave background radiation.

Not surprisingly, the most convincing detections of the diffuse infrared background are in the spectral windows in the local foregrounds near 3.5 µm and longward of 100 µm. At other wavelengths, the most certain and constraining results come from integrated galaxy light. The limits inferred from the less direct measures, including fluctuations in the infrared sky brightness and attenuation of TeV gamma-rays, are somewhat model dependent and therefore less certain. However, they are particularly useful in the 5 to 100 µm range, where direct measurements are so difficult.

It is notable that present detections of the EBL and the independently determined upper and lower limits are not in conflict with each other and, at some wavelengths, are not very far apart. In particular, the deepest SCUBA counts at 850 µm (Blain et al. 1999b) yield an integrated source light comparable to the CIB brightness determined from the FIRAS data. Thus, the discovery of the submillimeter background and resolution of its sources seem to have occurred virtually simultaneously. In the 7 to 180 µm range, the counts from the IRAS and ISO instruments are rising too steeply for the light to have converged, so the integrated light is necessarily below the actual CIB. However, the lowest TeV gamma-ray limits on the CIB are only modestly greater than the integrated light from the ISO 7 and 15 µm number counts, which suggests that the counts may be resolving a significant fraction of the CIB at these wavelengths.

The situation in the optical and near-infrared (0.3-2.2 µm) is somewhat different. Although the galaxy counts have been extended to sufficiently faint levels that the integrated light has converged (Madau & Pozzetti 2000), there remains an apparent gap between the integrated galaxy light and the reported EBL measurements. However, there may be systematic underestimates by a factor of ~ 2 in the integrated galaxy light due to photometry incompleteness at low surface brightness, as argued by Bernstein (1999). Furthermore, the claimed EBL measurements in this range are less than 4sigma positive and are at most only slightly more than 2sigma above the integrated galaxy light. In view of the potentially large systematic errors in both the integrated galaxy light and the extragalactic background determinations, one cannot confidently conclude that there is a significant difference between these two measures. Such a difference would imply the existence of unidentified discrete or diffuse sources contributing to the background light.

The shaded region in Figure 5 indicates conservative upper and lower limits on the spectral energy distribution of the EBL based on all available measurements and their uncertainties. We use these limits in the discussions of implications (Section 4) and models (Section 5).

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