Dust in high-redshift radio galaxies and the early evolution of spheroidal galaxies

Several lines of evidence suggest that early-type galaxies might have beenvery dusty during their initial evolutionary stages, characterized by intensestar formation activity. The radio selection has provided, by far, the richestsamples of high-$z$ galaxies, most likely of early type, which may thereforeyield crucial direct information on the period when the bulk of stars wereborn. In order to investigate the role of dust in these phases, we haveanalyzed recent observations of a number of high-$z$ radio galaxies in theframework of a self-consistent scenario for the evolution of early typegalaxies. The available data can be fully accounted for by ``opaque'' modelssimilar to that already used by Mazzei and De Zotti (1994a) to fit the spectrumof the hyperluminous galaxy IRAS~F$10214+4724$. However, there is stillconsiderable latitude for models. Good fits can be obtained with galactic agesranging from 0.05 to 2 Gyr. Crucial constraints may be provided by ground-basedsub-mm measurements and by observations with the forthcoming Infrared SpaceObservatory (ISO).Comment: MNRAS accepted; latex file, 10 pages, 7 figures available upon reques