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 (1.2-24 mu m) from a sample of 18 elliptical galaxies. In general the surface brightness distributions resemble de Vaucouleurs profiles, 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 mu m. The half-light or effective radius has a pronounced minimum near the K band (2.15 mu m) with a second, less consistent minimum in the 24 mu m passband. All sample-averaged radial color profiles ,where lambda(i) < lambda(j) (and j not equal 24 mu 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 < K - j > all have positive slopes, particularly when j = 5.8 mu m, although no obvious absorption feature is observed in spectra of elliptical galaxies near 5.8 mu m. This, and the minimum in R-e, suggests that the K band may be anomalously luminous in metal-rich stars in galaxy cores. Unusual radial color profiles involving the 24 mu m passband may suggest that some 24 mu m emission comes from interstellar not circumstellar dust grains.