FAR-INFRARED PROPERTIES OF EARLY-TYPE GALAXIES

We have investigated the IRAS colors and the far-IR to optical luminos ity ratios of a complete sample of elliptical and SO galaxies brighter than B(T) = 12. On the average, elliptical galaxies emit in the far-I R less than 1% of their bolometric luminosity; SO galaxies are about a factor of 3 brighter in the far-IR. There is a considerable spread in the far-IR properties of individual galaxies. On the average, the pho tospheric emission of red giant stars can account for 50%-60% of the 1 2 mum flux from early-type galaxies; the contribution from diffuse dus t at this wavelength is < 10% in the case of elliptical galaxies and m ay amount to 20%-40% for SO galaxies. An additional, approximately 30% -40%, contribution from circumstellar emission from evolved giants wit h mass loss (particularly OH/IR stars) seems to be required in the cas e of elliptical galaxies. This suggests a small but significant star-f ormation activity in these galaxies at a look-back time of 1-2 Gyr, co rresponding to about 10% of that typical of a disk galaxy having the s ame Y-band luminosity. As for SO galaxies, the larger diffuse dust emi ssion may swamp to some extent that of circumstellar dust, which is in dicated to comprise, on the average, less-than-or-similar-to 20% of th e far-IR emission. The weak emission from diffuse interstellar dust, d etected mostly at 60 mum and 100 mum, has color temperatures similar t o those of disk galaxies; as in the case of the latter, a warm dust co mponent is suggested, associated to star-forming regions, in addition to a cooler component, heated by the general interstellar radiation fi eld. The implied star formation would be a few percent of that of disk galaxies of similar V-band luminosity and could account for a fractio n of the observed UV branch of early-type galaxies.