The Ages of Elliptical Galaxies from Mid‐Infrared Emission

The mid-infrared (10-20 um) luminosity of elliptical galaxies is dominated bythe integrated emission from circumstellar dust in red giant stars. As a singlestellar population evolves, the rate of dusty mass loss from red giant starsdecreases with time, so the mid-infrared luminosity should also decline withstellar age. To seek such a correlation, we have used archival ISO observationsto determine surface brightness profiles and central fluxes at 15 um in 17early-type galaxies for which stellar ages have been determined from opticalspectral indices. The radial surface brightness distributions at 15 umgenerally follow the stellar de Vaucouleurs profile as expected. We find thatthe surface brightness ratio mu_{15um}/mu_{I-band} is systematically higher inelliptical galaxies with ages < 5 Gyrs and in galaxies that exhibit evidence ofrecent mergers. Within the accuracy of our observations, mu_{15um}/mu_{I-band}shows no age dependence for ages > 5 Gyrs. The corresponding flux ratiosF_{15um}/F_{I-band} within apertures scaled to the effective radius (R_e/8) areproportional to the mu_{15um}/mu_{I-band} ratios at larger galactic radii,indicating that no 15 um emission is detected from central dust clouds visiblein optical images in some of our sample galaxies. Emission at 15 um is observedin non-central massive clouds of dust and cold gas in NGC1316, an ellipticalgalaxy that is thought to have had a recent merger. Recent {\it Spitzer SpaceTelescope} data also indicate the presence of PAH emission at 8 um. Severalellipticals have extended regions of 15 um emission that have no obviouscounterparts at other frequencies.Comment: 13 pages, 5 figures and 2 tables. Accepted for publication in Ap