The IRIS far-infrared galaxy survey: Expected number count, redshift, and perspective

Infrared Imaging Surveyor (IRIS) is a satellite that will be launched in th e beginning of 2003. One of the main purposes of the IRIS mission is an all -sky survey at far-infrared (FIR) wavelengths with a flux limit much deeper than that of IRAS. In order to examine the performance of the survey, we e stimated the FIR galaxy counts in four (50, 70, 120, and 150 mu m) bands ba sed on some models. We adopted a multicomponent model that consists of cirr us and starburst components for galaxy spectra and the nearby FIR luminosit y function derived from that of IRAS galaxies. We derived the number counts , redshift distributions, and infrared diffuse background radiation spectra for (1) no evolution, (2) pure luminosity evolution, and (3) pure density evolution with q(0) = 0.1 and 0.5. We found that a large number of galaxies (similar to a few x 10(6) in the whole sky) will be detected in this surve y. With the aid of a vast number of detections, we will detect the effect o f galaxy evolution and evaluate the amplitude of evolution at least in the nearby universe in the IRIS survey, though it will be still difficult to co nstrain which type of evolution takes place from the number count alone. We also studied the estimation of redshifts of detected galaxies by their inf rared colors alone. Although significant contamination takes place among ne arby faint galaxies and high-z ones, we found that rough estimation of gala xy redshift can be practicable by jointly using present and future optical surveys.