LUMINOSITY EVOLUTION AND DUST EFFECTS IN DISTANT GALAXIES - IMPLICATIONS FOR THE OBSERVABILITY OF THE EARLY EVOLUTIONARY PHASES

We explore effects of luminosity evolution in normal galaxies by means of a spectrophotometric model treating in a self-consistent way the e nergy emitted by various stellar generations at different metallicitie s, the opacity of the enriched interstellar gas, and the flux reradiat ed by dust in the far-infrared. The very wide spectral coverage of the model, ranging from UV to far-IR and radio wavelengths, allows us for the first time to relate so diverse observational facts as the counts of galaxies in optical and K bands, the absence of high-redshift (z > 1) galaxies in faint optical samples, and the galaxy counts at far-IR and radio wavelengths. A consistent picture obtains assuming that dur ing the main phases of energy production by stellar nucleosynthesis mo st of the optical radiation might have been obscured by an enriched IS M and reradiated at longer wavelengths. We suggest that signs of this can already be read in deep IRAS and possibly also submillijansky radi o counts: observational techniques-including optical identifications a nd spectroscopy of samples selected at longer wavelengths and measurem ents of the background radiation in the IR and submillimeter domains-a re proposed to check this possibility. If this view is correct, the se arch for primeval objects and distant evolving galaxies (of which the recently discovered IRAS F10214+4724 may be a prototype) would have be tter chances if performed in the IR through radio spectral domain rath er than in the optical: it should concentrate, in particular, on faint IR and radio-selected sources with very faint or undetected optical c ounterparts.