MODELS FOR THE EVOLUTION OF THE SPECTRAL ENERGY-DISTRIBUTION OF ELLIPTIC GALAXIES FROM ULTRAVIOLET TO FAR-INFRARED WAVELENGTHS

We have worked out evolutionary synthesis models of the broad-band spe ctral energy distribution of elliptical galaxies over the whole freque ncy range from UV to far-IR. Internal extinction and far-IR reemission by interstellar dust have been taken into account in a self-consisten t way. Diffuse dust emission bag been modeled in terms of two componen ts: warm dust, located in regions of high radiation intensity, and col d dust, heated by the general radiation field. Emission from circumste llar dust clouds at different galactic ages was also taken into accoun t. The models reproduce well the present average broad-band spectrum o f nearby ellipticals over similar to 4 decades in frequency. Under the assumption of a dust-to-gas ratio proportional to the metallicity, th e fraction of bolometric luminosity coming out at far-IR wavelengths s trongly evolves during the galaxy lifetime, ranging from very low loca l values (less than or similar to 0.5%) to similar to 30% or more in t he first billion years. Some models even imply that early phases are o ptically thick; these models provide an excellent fit of the observed spectral energy distribution of the high-redshift galaxy IRAS F10214+4 724. Far-IR observations may thus play a key role in investigating the early evolution of elliptical galaxies. A strong far-IR evolution of early-type galaxies might be a crucial ingredient to explain the deep 60 mu m IRAS counts.