The Ages of Elliptical Galaxies from Infrared Spectral Energy Distributions

The mean ages of early-type galaxies obtained from the analysis of opticalspectra, 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 (5-21microns), which we have obtained with the Infrared Spectrograph on the SpitzerObservatory. This age indicator is based on the collective mass loss rate ofstars, where mass loss from AGB stars produces a silicate emission feature at9-12 microns. This feature decreases more rapidly than the shorter wavelengthcontinuum as a stellar population ages, providing an age indicator. Fromobservations of 30 nearby early-type galaxies, 29 show a spectral energydistribution dominated by stars and one has significant emission from the ISMand is excluded. The infrared age indicators for the 29 galaxies show them allto be old, with a mean age of about 10 Gyr and a standard deviation of only afew Gyr. This is consistent with the ages inferred from the values of M/L_B,but is inconsistent with the ages derived from the optical line indices, whichcan be much younger. All of these age indicators are luminosity-weighted andshould be correlated, even if multiple-age components are considered. Theinconsistency indicates that there is a significant problem with either theinfrared and the M/L_B ages, which agree, or with the ages inferred from theoptical absorption lines.Comment: Accepted for publication in Ap