The Ages of Elliptical Galaxies from Mid‐Infrared Emission

Abstract

The mid-infrared (10-20 μm) 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 μm in 17 early-type galaxies for which stellar ages have been determined from optical spectral indices. The radial surface brightness distributions at 15 μm generally follow the stellar de Vaucouleurs profile, as expected. We find that the surface brightness ratio μ15 μm/μI band is systematically higher in elliptical galaxies with ages 5 Gyr and in galaxies that exhibit evidence of recent mergers. Within the accuracy of our observations, μ15 μm/μI band shows no age dependence for ages 5 Gyr. The corresponding flux ratios F15 μm/FI band within apertures scaled to the effective radius (Re/8) are proportional to the μ15 μm/μI band ratios at larger galactic radii, indicating that no 15 μm emission is detected from central dust clouds visible in optical images in some of our sample galaxies. Emission at 15 μm 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 μm. Several ellipticals have extended regions of 15 μm emission that have no obvious counterparts at other frequencies.