THE EXTRAGALACTIC INFRARED BACKGROUND

Current limits on the intensity of the extragalactic IR background are consistent with the expected contribution from evolving galaxies. Depending on the behaviour of the star formation rate and of the initial mass function, we can expect that dust extinction during early evolutionary phases ranges from moderate to strong. An example of the latter case may be the ultraluminous galaxy IRAS F10214 + 4724. The remarkable lack of high red-shift galaxies in faint optically selected samples may be indirect evidence that strong extinction is common during early phases. Testable implications of different scenarios are discussed, the ISO can play a key role in this context. Estimates of possible contributions of galaxies to the background under different assumptions are presented. The COBE/FIRAS limits on deviations from a black-body spectrum at sub-millimetre wavelengths already set important constraints on the evolution of the far-IR emission of galaxies and on the density of obscured (''Type 2'') AGNs. Major progress in the field is expected on the completion of the analysis of COBE/DIRBE data.