The ages of elliptical galaxies from mid-infrared emission

The mid-infrared (10-20 mu m) luminosity of elliptical galaxies is dominated by the integrated emission from circumstellar dust in red giant stars. As a single stellar population evolves, the rate of dustymass loss from red giant stars decreases with time, so the mid-infrared luminosity should also decline with stellar age. To seek such a correlation, we have used archival Infrared Space Observatory (ISO) observations to determine surface brightness profiles and central fluxes at 15 mu m in 17 early-type galaxies for which stellar ages have been determined from optical spectral indices. The radial surface brightness distributions at 15 mu m generally follow the stellar de Vaucouleurs profile, as expected. We find that the surface brightness ratio mu(15 mu m)/mu(I band) is systematically higher in elliptical galaxies with ages less than or similar to 5 Gyr and in galaxies that exhibit evidence of recent mergers. Within the accuracy of our observations, mu(15 mu m)/mu(I band) shows no age dependence for ages greater than or similar to 5 Gyr. The corresponding flux ratios F-15 mu m/F-I band within apertures scaled to the effective radius (R-e/8) are proportional to the mu(15 mu m)/mu(I band) ratios at larger galactic radii, indicating that no 15 mu m emission is detected from central dust clouds visible in optical images in some of our sample galaxies. Emission at 15 mu m is observed in noncentral massive clouds of dust and cold gas in NGC1316, an elliptical galaxy that is thought to have had a recent merger. Recent Spitzer Space Telescope data also indicate the presence of polycyclic aromatic hydrocarbon (PAH) emission at 8 mu m. Several ellipticals have extended regions of 15 mu m emission that have no obvious counterparts at other frequencies.