IRAS SOURCE COUNTS AND COSMOLOGICAL EVOLUTION OF ACTIVELY STAR-FORMING GALAXIES

Exploiting the tight correlation between IR and radio emissions of disc galaxies, we find that the cosmological evolution of starburst/interacting galaxies, called forth by a recent model by Danese et al. to account for the mJy and sub-mJy radio source counts, is expected to begin to show up in the IRAS survey data at S60μm≃0.5 Jy. Hints of cosmological evolution consistent with the model predictions are indeed found in the luminosity distribution of IRAS sources brighter than this limit. Very good agreement is also found between predicted and observed deep 60-µm counts, which may provide some direct evidence for cosmological evolution of far-IR sources. Detailed predictions of the contributions to the 60-µm counts down to 1 mJy, and to the far-IR background light, by normal spirals and irregulars, starburst/interacting galaxies, Seyferts, ellipticals and S0S, radio-loud and radio-quiet QSOs are worked out. Tentative estimates of the contributions to the counts and to the background by primeval galaxies are presented and briefly discussed.