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Mid-IR Observations of Mass Loss in Elliptical Galaxies
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机译：椭圆星系质量损失的中红外观测
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摘要
Early-type galaxies exhibit thermal and molecular resonance emission fromdust that is shed and heated through stellar mass loss as a subset of thepopulation moves through the AGB phase of evolution. Because this emission cangive direct insight into stellar evolution in addition to galactic stellar massloss and ISM injection rates, we conducted a program to search for thissignature emission with CAM on ISO. We obtained 6-15 micron imagingobservations in six narrow bands for nine elliptical galaxies; every galaxy isdetected in every band. For wavelengths shorter than 9 microns, the spectra arewell matched by a blackbody, originating from the K and M stars that dominatethe integrated light of elliptical galaxies. However, at wavelengths between 9and 15 microns, the galaxies display excess emission relative to the stellarphotospheric radiation. Additional data taken with the fine resolution circularvariable filter on one source clearly shows broad emission from 9 to 15microns, peaking around 10 microns. This result is consistent with the known,broad silicate feature at 9.7 microns, originating in the circumstellarenvelopes of AGB stars. This emission is compared with studies of Galactic andLMC AGB stars to derive cumulative mass loss rates. In general, these mass lossrates agree with the expected ~0.8 solar masses per year value predicted bystellar evolutionary models. Both the photospheric and circumstellar envelopeemission follow a de Vaucouleurs' R^{1/4} law, supporting the conclusion thatthe mid-infrared excess emission originates in the stellar component of thegalaxies and acts as a tracer of AGB mass loss and mass injection into the ISM.
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