FAR-INFRARED PROPERTIES OF EARLY-TYPE GALAXIES

We have investigated the IRAS colors and the far-IR to optical luminosity ratios of a complete sample of elliptical and SO galaxies brighter than B(T) = 12. On the average, elliptical galaxies emit in the far-IR 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 photospheric emission of red giant stars can account for 50%-60% of the 12 mum flux from early-type galaxies; the contribution from diffuse dust at this wavelength is < 10% in the case of elliptical galaxies and may amount to 20%-40% for SO galaxies. An additional, approximately 30%-40%, contribution from circumstellar emission from evolved giants with mass loss (particularly OH/IR stars) seems to be required in the case of elliptical galaxies. This suggests a small but significant star-formation activity in these galaxies at a look-back time of 1-2 Gyr, corresponding to about 10% of that typical of a disk galaxy having the same Y-band luminosity. As for SO galaxies, the larger diffuse dust emission may swamp to some extent that of circumstellar dust, which is indicated to comprise, on the average, less-than-or-similar-to 20% of the far-IR emission. The weak emission from diffuse interstellar dust, detected mostly at 60 mum and 100 mum, has color temperatures similar to those of disk galaxies; as in the case of the latter, a warm dust component is suggested, associated to star-forming regions, in addition to a cooler component, heated by the general interstellar radiation field. The implied star formation would be a few percent of that of disk galaxies of similar V-band luminosity and could account for a fraction of the observed UV branch of early-type galaxies.