The Mid-Infrared Spectral Energy Distribution, Surface Brightness, and Color Profiles in Elliptical Galaxies

We combine 2MASS data and Spitzer archival data to study the emission in mid-infrared passbands (1.2Y24 m) from a sample of 18 elliptical galaxies. In general the surface brightness distributions resemble de Vaucouleurs pro- files, indicating that most of the emission arises from the photospheres or circumstellar regions of red giant stars. The spectral energy distribution peaks near the H band at 1.6 m. The half-light or effective radius has a pronounced minimum near the K band (2.15 m) with a second, less consistent minimum in the 24 m passband. All sample-averaged radial color profiles hki kji, where ki < kj (and j 6¼ 24 m), have positive slopes within about twice the (K-band) effective radius. Evidently this variation arises because of an increase in stellar metallicity toward the galactic cores. Color profiles hK ji all have positive slopes, particularly when j ¼ 5:8 m, although no obvious absorption feature is observed in spectra of elliptical galaxies near 5.8 m. This, and the minimum in Re, suggests that the K band may be anomalously luminous in metal-rich stars in galaxy cores. Unusual radial color profiles involving the 24 m passband may suggest that some 24 m emission comes from interstellar not circumstellar dust grains.