Clustering of the diffuse infrared light from the COBE DIRBE maps. III. Power spectrum analysis and excess isotropic component of fluctuations

The cosmic infrared background (CIB) radiation is the cosmic repository for energy release throughout the history of the universe. The spatial fluctuations of the CIB resulting from galaxy clustering are expected to be at least a few percent on scales of a degree, depending on the luminosity and clustering history of the early universe. Using the all-sky data from the COBE DIRBE instrument at wavelengths 1.25-100 mu m, we attempt to measure the CIB fluctuations. In the near-IR, foreground emission is dominated by small-scale structure due to stars in the Galaxy. There we find a strong correlation between the amplitude of the fluctuations and Galactic latitude after removing bright foreground stars. Using data outside the Galactic plane (\ b \ > 20 degrees) and away from the center (90 degrees < l < 270 degrees), we extrapolate the amplitude of the fluctuations to csc \ b \ = 0. We find positive intercepts of delta F-rms = 15.5(-7.0)(+3.7), 5.9(-3.7)(+1.6), 2.4(-0.9)(+0.5), and 2.0(-0.5)(+0.25) nW m(-2) sr(-1) at 1.25, 2.2, 3.5, and 4.9 mu m, respectively, where the errors are the range of 92% confidence limits. For color subtracted maps between band 1 and 2 we find the isotropic part of the fluctuations at 7.6(-2.4)(+1.2) nW m(-2) sr(-1). Based on detailed numerical and analytic models, this residual is not likely to originate from the Galaxy, our clipping algorithm, or instrumental noise. We demonstrate that the residuals from the lit used in the extrapolation are distributed isotropically and suggest that this extra variance may result from structure in the GIB. We also obtain a positive intercept from a linear combination of maps at 1.25 and 2.2 mu m. For 2 degrees < theta < 15 degrees, a power-spectrum analysis yields firm upper limits of (theta/5 degrees) x delta F-rms(theta) < 6, 2.5, 0.8, and 0.5 nW m(-2) sr(-1) at 1.25, 2.2, 3.5, and 4.9 mu m, respectively. From 10 to 100 mu m, the dominant foregrounds are emission by dust in the solar system and the Galaxy. The upper limits on the CIB fluctuations are below 1 nW m-2 sr-l there and are lowest ( less than or equal to 0.5 nW m(-2) sr(-1)) at 25 mu m.