The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background. II. Model of the Interplanetary Dust Cloud

Abstract

The COBE Diffuse Infrared Background Experiment (DIRBE) was designed to search for the cosmic infrared background (CIB) radiation. For an observer confined to the inner solar system, scattered light and thermal emission from the interplanetary dust (IPD) are major contributors to the diffuse sky brightness at most infrared wavelengths. Accurate removal of this zodiacal light foreground is a necessary step toward a direct measurement of the CIB. The zodiacal light foreground contribution in each of the 10 DIRBE wavelength bands ranging from 1.25 to 240 μm is distinguished by its apparent seasonal variation over the whole sky. This contribution has been extracted by fitting the brightness calculated from a parameterized physical model to the time variation of the all-sky DIRBE measurements over 10 months of liquid He cooled observations. The model brightness is evaluated as the integral along the line of sight of the product of a source function and a three-dimensional dust density distribution function. The dust density distribution is composed of multiple components: a smooth cloud, three asteroidal dust bands, and a circumsolar ring near 1 AU. By using a directly measurable quantity that relates only to the IPD cloud, we exclude other contributors to the sky brightness from the IPD model. High-quality maps of the infrared sky with the zodiacal foreground removed have been generated using the IPD model described here. Imperfections in the model reveal themselves as low-level systematic artifacts in the residual maps that correlate with components of the IPD. The most evident of these artifacts are located near the ecliptic plane in the mid-IR and are less than 2% of the zodiacal foreground brightness. Uncertainties associated with the model are discussed, including implications for the CIB search.