The Spitzer First Look Survey—Ecliptic Plane Component: Asteroids and Zodiacal Background

The Spitzer First Look Survey (FLS) provided an initial characterization of the infrared sky at Spitzer wavelengths and sensitivities. The ecliptic plane component (EPC) of the FLS concentrated on two 0.13 deg 2 fields at a solar elongation of 115° and ecliptic latitudes (β) of 0° and +5°. The FLS-EPC explored the small asteroid counts at 8 and 24 μm, with a detection limit down to ∼0.08 and 0.8 mJy, respectively, and a completeness limit almost twice as deep as the 8 μm equivalent flux density of the previous deepest mid-IR survey. The FLS-EPC also provided initial characterization of the zodiacal light near the ecliptic plane. Fifteen known and 19 unknown asteroids were identified, and asteroids detected at both wavelengths displayed similar 8 to 24 μm flux ratios of ∼0.1. Comparing number counts for the β = 0 ° and +5° fields indicates a slower-than-anticipated drop-off in contrast to predicted scale heights, possibly due to the presence of higher inclination objects in the small population sampled by Spitzer . The measured zodiacal light background was found to be within 5% of Spitzer model predictions at 24 μm.