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 energy emitted by various stellar generations at different metallicities, the opacity of the enriched interstellar gas, and the flux reradiated 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 during the main phases of energy production by stellar nucleosynthesis most of the optical radiation might have been obscured by an enriched ISM and reradiated at longer wavelengths. We suggest that signs of this can already be read in deep IRAS and possibly also submillijansky radio counts: observational techniques-including optical identifications and spectroscopy of samples selected at longer wavelengths and measurements of the background radiation in the IR and submillimeter domains-are proposed to check this possibility. If this view is correct, the search for primeval objects and distant evolving galaxies (of which the recently discovered IRAS F10214+4724 may be a prototype) would have better chances if performed in the IR through radio spectral domain rather than in the optical: it should concentrate, in particular, on faint IR and radio-selected sources with very faint or undetected optical counterparts.