Galaxy evolution at low redshift? II. Number counts and optical identifications of faint IRAS sources

We analyse a Far InfraRed (FIR) catalogue of galaxies at 60 microns with a flux limit of approx. 110 mJy, extracted from a deep subsample of the IRAS Faint Source Survey. Monte-Carlo simulations and optical identification statistics are used to put constraints on the evolution of galaxies in the FIR. We find evidence for strong evolution of IRAS galaxies, in luminosity prop. to (1+z)^3.2 or density prop. to (1+z)^6.0 for f60 > 150 mJy, in agreement with previous claims. An excess of rather red faint optical counterparts with B>18, consistent with the above evolution, is detected. We interpret these results as strong evolution at recent times among the starburst (or dusty AGN) population of merging/interacting galaxies. Most of these objects at moderate redshifts may pass unnoticed among the population of massive spirals in broad-band optical surveys, because of large amounts of dust extinction in their central regions. A possible link with the strong evolution observed in the optical for blue sub-M* galaxies is discussed.