On measuring the infrared luminosity of distant galaxies with the Space Infrared Telescope Facility

The Space Infrared Telescope Facility (SIRTF) will revolutionize the study of dust-obscured star formation in distant galaxies. Although deep images from the Multiband Imaging Photometer for SIRTF (MIPS) will provide coverage at 24, 70, and 160 mum, the bulk of MIPS-detected objects may only have accurate photometry in the shorter wavelength bands because of the confusion noise. Therefore, we have explored the potential for constraining the total infrared (IR) fluxes of distant galaxies solely with the 24 mum flux density and for the combination of 24 and 70 mum data. We also discuss the inherent systematic uncertainties in making these transitions. Under the assumption that distant star-forming galaxies have IR spectral energy distributions (SEDs) that are represented somewhere in the local universe, the 24 mum data (plus optical and X-ray data to allow the redshift estimation and AGN rejection) constrain the total IR luminosity to within a factor of 2.5 for galaxies with. 0.4 less than or similar to z less than or similar to 1.6 Incorporating the 70 mum data substantially improves this constraint by a factor less than or similar to6. Lastly, we argue that if the shape of the IR SED is known (or well constrained, e. g., because of a high IR luminosity or a low ultraviolet/IR flux ratio), then the IR luminosity can be estimated with more certainty.