The Isotropic Interplanetary Dust Cloud and Near-infrared Extragalactic Background Light Observed with COBE/DIRBE

We report observation of isotropic interplanetary dust (IPD) by analyzing the infrared (IR) maps of the Diffuse Infrared Background Experiment (DIRBE) on board the Cosmic Background Explorer (COBE) spacecraft. To search for the isotropic IPD, we perform new analysis in terms of the solar elongation angle (epsilon), because we expect the zodiacal light (ZL) intensity from the isotropic IPD to decrease as a function of epsilon. We use the DIRBE weekly averaged maps covering 64 degrees less than or similar to epsilon less than or similar to 124 degrees and inspect the epsilon dependence of residual intensity after subtracting conventional ZL components. We find the epsilon dependence of the residuals, indicating the presence of the isotropic IPD. However, the mid-IR epsilon dependence is different from that of the isotropic IPD model at epsilon greater than or similar to 90 degrees, where the residual intensity increases as a function of epsilon. To explain the observed epsilon dependence, we assume a spheroidal IPD cloud showing higher density farther away from the Sun. We estimate the intensity of the near-IR extragalactic background light (EBL) by subtracting the spheroidal component, assuming the spectral energy distribution from the residual brightness at 12 mu m. The EBL intensity is derived as 45(-8)(+11), 21(-4)(+3), and 15 +/- 3 nWm(-2) Sr-1 at 1.25, 2.2, and 3.5 mu m, respectively. The EBL is still a few times larger than the integrated light of normal galaxies, suggesting the existence of unaccounted-for extragalactic sources.