The Ages of Elliptical Galaxies from Mid-Infrared Emission

The mid-infrared (10–20 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 dusty mass 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 m in 17 early-type galaxies for which stellar ages have been determined from optical spectral indices. The radial surface brightness distributions at 15 m generally follow the stellar de Vaucouleurs profile, as expected. We find that the surface brightness ratio 15 m=I band is systematically higher in elliptical galaxies with ages P5 Gyr and in galaxies that exhibit evidence of recent mergers. Within the accuracy of our observations, 15 m=I band shows no age dependence for ages k5 Gyr. The corresponding flux ratios F15 m=FI band within apertures scaled to the effective radius (Re=8) are proportional to the 15 m=I band ratios at larger galactic radii, indicating that no 15 m emission is detected from central dust clouds visible in optical images in some of our sample galaxies. Emission at 15 m is observed in noncentral massive clouds of dust and cold gas in NGC 1316, 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 m. Several ellipticals have extended regions of 15 m emission that have no obvious counterparts at other frequencies.