Until the various foreground cosmic sources of diffuse infrared radiation are properly discriminated and subtracted, an effort of the DIRBE team currently under way, the most credible direct observational limits on the CIBR are the minimum observed sky brightnesses. The preliminary DIRBE spectrum toward the south ecliptic pole, one of the darkest directions in the sky at many wavelengths (Hauser et al. 1991, Table 2), showed that the faintest levels of the foreground emissions occur at 3.4 µm (I = 150 ± 60 nW m-2 sr-1) and near 241 µm (I = 70 ± 40 nW m-2 sr-1), confirming these as the most sensitive spectral windows for the CIBR search. The sensitivity of the COBE DIRBE and FIRAS measurements in each of their respective fields-of-view is generally well below these observed sky brightnesses toward the ecliptic pole (Hauser et al. 1991). With the COBE data in hand, discrimination of foreground emission, rather than measurement sensitivity, is clearly the major challenge in searching for the CIBR.
Since publication of the spectrum of the south ecliptic pole, which was based on a quick-look reduction and initial calibration of the DIRBE data, the complete data set acquired while the DIRBE was cryogenically cooled has been processed with an improved calibration. Table 1 lists, for each DIRBE wavelength, a representative darkest sky value observed. At wavelengths where interplanetary dust scattering or emission is strong, the sky is darkest near the ecliptic poles. At wavelengths where the interplanetary cloud signal is rather weak (i.e., at 3.5 µm and longward of 100 µm), the sky is darkest near the galactic poles or in minima of HI column density. A 20% error is shown for each value, representative of the present absolute calibration uncertainties. These darkest sky brightnesses are the current DIRBE upper limits to any isotropic infrared background.
As described by Mather at this Symposium (see also Mather et al. 1994), the CMBR spectrum in the wavelength range 0.5-5 mm deviates from a 2.726 K blackbody shape by less than 0.03% of the peak intensity. Taking this as an upper limit to an additional cosmic infrared background implies I < 340 / (µm) nW m-2 sr-1. This limit does not make allowance for systematic errors in separating the Galactic signal from the FIRAS CMBR signal. A complete systematic error analysis is currently in progress; Table 1 shows twice the above limit as an estimate of the proper magnitude. Analysis of the FIRAS high frequency data, combined with foreground modeling, will limit or provide measurements of the CIBR in the 100 µm to 500 µm range. It should be noted that the COBE DIRBE and FIRAS sky brightness measurements have been compared at 140 and 240 µm as a check on the calibrations of both instruments. The two sets of measurements were found to be consistent within the present calibration uncertainties.
|µm||nW m-2 sr-1|
|1.25||480 ± 96||DIRBE dark sky|
|2.2||190 ± 38||"|
|3.5||83 ± 17||"|
|4.9||260 ± 52||"|
|12||2800 ± 560||"|
|25||2400 ± 480||"|
|60||300 ± 60||"|
|100||90 ± 18||"|
|140||140 ± 28||"|
|240||28 ± 6||"|
|500-5000||680 / (µm)||Mather et al. (1994)|
There have been reports of upper limits on, or possible detections of, isotropic residuals in the infrared sky brightness from several rocket experiments (Matsumoto et al. 1988; Matsumoto 1990; Noda et al. 1992). These investigators have arrived at these limits after attempting to discriminate the various foreground components of emission contributing to their measurements. Because of the limited sky and spectral coverage available to discriminate contributions to the measured signal and the brief time to check possible systematic measurement errors, conclusions from the rocket experiments require confirmation. Where the DIRBE and rocket observations have been compared (Hauser et al. 1991; Noda et al. 1992), the actual sky brightness measurements have been generally similar.
As a complement to the CIBR upper limits which can be set by diffuse infrared background measurements, measurements of galaxies in the infrared allow estimation of lower limits to the total extragalactic infrared background. For example, Cowie et al. (1990) have estimated the integrated contribution of galaxies at 2.2 µm to be I = 5 nW m-2 sr-1 on the basis of deep galaxy counts. Hacking & Soifer (1991) have used galaxy luminosity functions derived from IRAS data to predict minimum diffuse backgrounds (integrated to z = 3) at 25, 60, and 100 µm of 1, 2, and 4 nW m-2 sr-1 respectively. Beichman & Helou (1991) have used synthesized galaxy spectra, also based largely on IRAS data, to estimate the diffuse infrared background due to galaxies. At 300 µm, their minimum estimated brightness (integrated to z = 3) is 2 nW m-2 sr-1. The integrated galaxy far-infared background contribution may exceed these estimates substantially if there has been evolution in galaxy luminosity or space density: deeper counts from future space infrared observatories such as ISO and SIRTF will improve these estimates. Even these minimum extragalactic background contributions should be detectable if the foreground contributions to the COBE measurements can be modeled to about the 1% level, a difficult but perhaps achievable goal based on our initial studies.