Confusion limit due to galaxies: using SIRTF's Infrared Array Camera

Recent ISO -data has 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 due to galaxies in deep mid-IR observations. We specifically concentrate on the near-future Space Infrared Telescope Facility ( SIRTF ) mission, and calculate expected confusion for the Infrared Array Camera (IRAC) onboard 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 µ m. A deep survey can reach 3 µ Jy sources with reasonable confidence in the shorter wavelength IRAC bands. Truly confusion limited images with the 8 µ m will be difficult to obtain due to 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 µ m, which is slightly more pessimistic than found by Simpson & Eisenhardt (1999).