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.
|
| I
| Reference |
| 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.