Optical number count estimation of IRIS far-infrared survey of galaxies

Infrared Imaging Surveyor (IRIS) is a satellite which will be launched at t he beginning of 2003. One of the main purposes of the IRIS mission is an al l-sky survey in the far-infrared region with a flux limit much deeper than that of IRAS. The detection of a large number of galaxies (similar to sever al x 10(6) in the whole sky) is expected in this survey. We investigated th e expected optical and near-infrared (NIR) number counts of galaxies detect ed by the far-infrared scanner (FIS) of IRIS thereafter, IRIS galaxies) and the possibility of their optical and NIR follow-up. The Spectral energy di stribution and the luminosity function of the IRIS galaxies are modeled bas ed on the properties of galaxies observed by IRAS. The IRIS galaxies are di vided into two populations according to their infrared luminosities (L-IR): normal spirals (L-IR < 10(10) L.) and starbursts (L-IR > 10(10) L.). The e xpected number counts of IRIS galaxies for both of the populations are calc ulated in B and H bands. We show that about 60 normal galaxies and about 80 starburst galaxies are detected per square degree in both of the two bands , when galaxy evolution is not taken into account. All of the normal popula tion of IRIS galaxies are located at the redshift z less than or similar to 0.1. As for the starburst population, we also calculated the number of gal axies with a simple model of evolution. The total number of starburst popul ation predicted by the evolution model is larger by 20% than that expected from the non-evolution model. In the evolution model, the numbers of low-z (z < 1), intermediate-z (1 < z < 3), and high-z (z > 3) galaxies are 100, 2 0, and 0.2 per square degree, respectively.