Mid-IR Observation of Mass Loss in Elliptical Galaxies

– 2 – Early-type galaxies exhibit thermal and molecular resonance emission from dust that is shed and heated through stellar mass loss as a subset of the population moves through the AGB phase of evolution. Because this emission can give direct insight into stellar evolution in addition to galactic stellar mass loss and ISM injection rates, we conducted a program to search for this signature emission with CAM on ISO. We obtained 6-15µm imaging observations in six narrow bands for nine elliptical galaxies; every galaxy is detected in every band. For wavelengths shorter than 9µm, the spectra are well matched by a blackbody, originating from the K and M stars that dominate the integrated light of elliptical galaxies. However, at wavelengths between 9µm and 15µm, the galaxies display excess emission relative to the stellar photospheric radiation. Additional data taken with the fine resolution circular variable filter on one source clearly shows broad emission from 9µm to 15µm, peaking around 10µm. This result is consistent with the known, broad silicate feature at 9.7µm, originating in the circumstellar envelopes of AGB stars. This emission is compared with studies of Galactic and LMC AGB stars to derive cumulative mass loss rates. In general, these mass loss rates agree with the expected ∼0.8 M ⊙ yr −1 value predicted by stellar evolutionary models. Both the photospheric and circumstellar envelope emission follow a de Vaucouleurs' R 1/4 law, supporting the conclusion that the mid-infrared excess emission originates in the stellar component of the galaxies and acts as a tracer of AGB mass loss and mass injection into the ISM.