COSMOLOGICAL EVOLUTION OF EXTRAGALACTIC SOURCES IN THE INFRARED AND CONTRIBUTIONS TO THE BACKGROUND-RADIATION

Infrared surveys provide essential insights on galaxy evolution. If ne ar-IR studies suggest a mild evolution of stellar populations with cos mic time, indications of a substantial evolution have been seen in the far IR, although the available information is largely insufficient to delineate precise evolutionary properties. A consistent picture encom passing all the currently available data may be obtained assuming that dust extinction hides the ear ly evolutionary phases of spheroidal ga laxies in the optical band, while the corresponding dust re-radiation in the far IR may have been, during the early evolutionary phases, ord ers of magnitude larger than today Hyperluminous IRAS galaxies might b e extreme examples of this situation. Additional indications that at l east some spheroidal galaxies may have been very dusty during their ea rly evolution ape provided by recent data on high redshift radio galax ies and quasars. Galaxies are the likely dominant contributors to the IR background. However, in the framework of unified models for Active Galactic Nuclei, a large number of nuclei hidden by a dusty torus may be expected. Implications for the IR background are discussed.