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A.E. Athey, J.N. Bregman (Univ. of Michigan), J.D. Bregman, P. Temi (NASA Ames Research Center)
Early-type galaxies should exhibit thermal emission from the dust that is shed through stellar mass loss and heated by internal friction. Because this emission can give us direct insight into galactic stellar mass loss and ISM injection rates, we conducted a program to search for this emission with CAM on ISO. We obtained 6-15um imaging observations (3.2' FOV and 6" pixels) in six narrow bands (lw4-9) for the galaxies NGC 1344, NGC 1395, NGC 1399, NGC 1404, NGC 1407, NGC 4636, NGC 5102, NGC 5846, and NGC 7507. Every galaxy is detected in every band. For wavelengths shorter than 9um, the spectra are well matched by a cool (T~3500K) blackbody, originating from the K and M stars that dominate elliptical galaxies' integrated light. However, in wavelengths longer than 9um (and up to 15um), every galaxy displays excess emission over stellar blackbody emission. This emission cannot be attributed to PAHs because emission in the stronger 7.7um band is not detected. To study this feature in detail, we obtained CVF data from ISOCAM on one source, NGC 1404, which clearly shows broad emission from 9um to 15um, peaking in the 9.5-11.3um region. This result is consistent with emission from silicates or SiC originating in the circumstellar envelopes of AGB stars. We compare our emission with Galactic AGB stars and derive a cumulative mass loss rate for each of these ellipticals. We place additional constraints on the origin of this emission by comparing the spatial distribution of the emission with the distribution of the stellar light.
The author(s) of this abstract have provided an email address for comments about the abstract: aathey@umich.edu