Confusion limit resulting from galaxies: using the Infrared Array Camera on board SIRTF

Recent ISO data have allowed, for the first time, observationally based estimates for source confusion in mid-infrared surveys. We use the extragalactic source counts from ISOCAM in conjunction with K-band counts to predict the confusion resulting from galaxies in deep mid-infrared observations. We specifically concentrate on the near-future Space Infrared Telescope Facility (SIRTF) mission, and calculate expected confusion for the Infrared Array Camera (IRAC) on board SIRTF. A defining scientific goal of the IRAC instrument will be the study of high-redshift galaxies using a deep, confusion-limited wide-field survey at 3–10 μm. A deep survey can reach 3-μJy sources with reasonable confidence in the shorter wavelength IRAC bands. Truly confusion-limited images with the 8 μm will be difficult to obtain because of practical time constraints, unless infrared galaxies exhibit very strong evolution beyond the deepest current observations. We find L* galaxies to be detectable to z=3–3.5 at 8 μm, which is slightly more pessimistic than found in 1999 by Simpson & Eisenhardt.