The Ages of Elliptical Galaxies from Infrared Spectral Energy Distributions

The mean ages of early-type galaxies obtained from the analysis of optical spectra give a mean age of 8 Gyr at z = 0, with 40% being younger than 6 Gyr. Independent age determinations are possible by using infrared spectra (5-21 km), which we have obtained with the Infrared Spectrograph on Spitzer. This age indicator is based on the collective mass-loss rate of stars, in which mass loss from AGB stars produces a silicate emission feature at 9-12 km. This feature decreases more rapidly than the shorter wavelength continuum as a stellar population ages, providing an age indicator. From observations of 30 nearby early-type galaxies, 29 show a spectral energy distribution dominated by stars, and one has significant emission from the ISM and is excluded. The infrared age indicators for the 29 galaxies show them all to be old, with a mean age of about 10 Gyr and a standard deviation of only a few Gyr. This is consistent with the ages inferred from the values of M/L sub(B), but is inconsistent with the ages derived from the optical line indices, which can be much younger. All of these age indicators are luminosity weighted and should be correlated, even if multiple-age components are considered. The inconsistency indicates that there is a significant problem with either the infrared and the M/L sub(B) ages, which agree, or with the ages inferred from the optical absorption lines.