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

Recent ISO data have allowed, for the first time, observationally based est imates for source confusion in mid-infrared surveys. We use the extragalact ic source counts from ISOCAM in conjunction with K-band counts to predict t he confusion resulting from galaxies in deep midinfrared observations. We s pecifically concentrate on the near-future Space Infrared Telescope Facilit y (SIRTF) mission, and calculate expected confusion for the Infrared Array Camera (IRAC) on board SIRTF. A defining scientific goal of the IRAC instru ment will be the study of high-redshift galaxies using a deep, confusion-li mited wide-field survey at 3-10 mum. A deep survey can reach 3-mu Jy source s with reasonable confidence in the shorter wavelength IRAC bands. Truly co nfusion-limited images with the 8 mum will be difficult to obtain because o f practical time constraints, unless infrared galaxies exhibit very strong evolution beyond the deepest current observations. We find L* galaxies to b e detectable to z = 3-3.5 at 8 mum, which is slightly more pessimistic than found in 1999 by Simpson & Eisenhardt.