Far-IR Properties of Early Type Galaxies

Abstract

We have investigated the IRAS colours and the far-IR to optical luminosity ratios of a complete sample of elliptical&#39;s and S0&#39;s brighter than $B_T = 12$. On the average, elliptical galaxies emit in the far-IR less than 1\% of their bolometric luminosity, while S0&#39;s are about a factor of three brighter in the far-IR. On the average, the photospheric emission of red giant stars can account for 50--60\% of the $12\mic$ flux from early type galaxies; the contribution from diffuse dust at this wavelength is $< 10\%$ in the case of ellipticals,and may amount to 20--40\% for S0&#39;s. An additional, $\sim 30--40\%$, contribution from circumstellar emission from evolved giants with mass loss (particularly OH/IR stars) seems to be required in the case of ellipticals. 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 V-band luminosity. As for S0&#39;s, the larger diffuse dust emission may swamp to some extent that of circumstellar dust, which is indicated to be, on the average, $\lsim 20\%$. The weak emission from diffuse interstellar dust, detected mostly at $60\mic$ and $100\mic$, has color temperatures similar to those of disk galaxies; as in that case of the latter, a warm dust component is suggested, associated to star-forming regions. 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.