Recognizing that the uncertainties in isolating the bright contribution
from the IPD are a major obstacle to CIB detection in the far infrared,
Finkbeiner, Davis,
& Schlegel (2000)
analyzed the DIRBE data using two approaches which avoid the need for a
detailed IPD model. Both methods were based on analysis of the annual
variation of dimensionless ratios of the data after removal of the ISM
contribution. The approach of
Schlegel et
al. (1998)
was used to
determine the ISM contribution. Subtraction of the ISM contribution from
each DIRBE weekly map yielded maps nominally containing only the IPD
signal, which varies over the course of the year, and any
time-independent signals due to other Galactic contributions and the
CIB. They constructed dimensionless ratios of the brightnesses in
opposing directions on the sky, and compared the annual variation or
ecliptic latitude variation of these ratios to models containing
contributions from interplanetary dust plus a temporally constant
background. Though these methods do not require detailed models of the
IPD cloud, they do require assumptions about the spatial symmetries and
temporal invariance of the IPD cloud similar to those of the
Kelsall et
al. (1998)
model. In one approach, they analyzed the annual variation of
[N-S] / [N+S], where N (S) is the brightness toward the North (South)
ecliptic pole. This analysis yielded statistically significant
temporally constant backgrounds at 60 and 100 µm, but does
not demonstrate isotropy of that signal. To explore the ecliptic
latitude dependence of the background, they analyzed a second
dimensionless ratio based upon brightnesses at 90° solar elongation
and symmetric ecliptic latitudes forward and backwards relative to the
Earth's direction of motion. Interpretation of the results of this
method does depend modestly on assumptions about the ecliptic latitude
dependence of the IPD contribution, i.e., on a model for the IPD cloud
at high ecliptic latitudes. These two methods gave comparable results
for the constant backgrounds at 60 and 100 µm. Finkbeiner et
al. tentatively identified these backgrounds with the CIB
(Table 1), but noted that the values
are rather high and not consistent with limits implied by the opacity of
the intergalactic medium to TeV
-rays
(Aharonian et
al. 1999;
Samuelson et
al. 1998).
They concluded that there is not yet a satisfactory explanation for
these constant backgrounds.
A distinctly different approach to studying the CIB is to look for fluctuations attributable to the non-uniform spatial distribution of discrete sources contributing to the background. If one can estimate the expected amplitude of the fluctuations relative to the brightness of the background itself, one can use observations of the fluctuations to determine or set limits upon the CIB. Of course, measuring fluctuations in the extragalactic background brightness presents similar challenges of discriminating contributions from foreground sources and instrumental effects as those that exist for direct detection of the background. However, at mid-infrared wavelengths, the dominant IPD foreground is less of an obstacle to fluctuation studies than to total CIB brightness measurements since the IPD emission varies relatively smoothly over the sky.
Kashlinsky et
al. (1996a)
provided a framework for such analyses. In order to relate fluctuations
to the intensity of the CIB, they evaluated the expected fluctuations
from galaxies, finding that the fluctuations on the 0.7° angular
scale of the DIRBE beam are expected to be of the order of 5-10% of the
CIB.
Kashlinsky, Mather,
& Odenwald (1996b)
analyzed the fluctuations in the DIRBE maps from 12-100 µm
after removing the IPD light using the model of
Kelsall et
al. (1998)
and blanking bright discrete sources. They calculated the zero lag autocorrelation function, C(0), finding
C(0)1/2 (1 -
1.5) nW m-2 sr-1.
Kashlinsky &
Odenwald (2000)
further analyzed the fluctuations in this spectral range, finding a
slightly lower limit
C(0)1/2
(0.5
- 1) nW m-2 sr-1. These analyses imply upper
limits to the 12-100 µm CIB contributed by sources clustered
like galaxies of the order of 10-15 nW m-2 sr-1,
the most restrictive limits based upon infrared measurements in this
spectral range (Table 1).