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 spectral energy distribution of elliptical galaxies over the whole frequency range from UV to far-IR. Internal extinction and far-IR reemission by interstellar dust have been taken into account in a self-consistent way. Diffuse dust emission bag been modeled in terms of two components: warm dust, located in regions of high radiation intensity, and cold dust, heated by the general radiation field. Emission from circumstellar dust clouds at different galactic ages was also taken into account. The models reproduce well the present average broad-band spectrum of nearby ellipticals over similar to 4 decades in frequency. Under the assumption of a dust-to-gas ratio proportional to the metallicity, the fraction of bolometric luminosity coming out at far-IR wavelengths strongly evolves during the galaxy lifetime, ranging from very low local values (less than or similar to 0.5%) to similar to 30% or more in the first billion years. Some models even imply that early phases are optically thick; these models provide an excellent fit of the observed spectral energy distribution of the high-redshift galaxy IRAS F10214+4724. 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.