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

TL;DR

This study analyzes mid-infrared photometry of 18 elliptical galaxies, revealing that their surface brightness profiles follow de Vaucouleurs profiles and highlighting metallicity effects and dust contributions in their spectral energy distributions.

Contribution

It provides detailed mid-infrared surface brightness and color profiles of elliptical galaxies, emphasizing metallicity gradients and potential dust emission sources, which were not previously characterized in this wavelength range.

Findings

Surface brightness profiles follow de Vaucouleurs law.

Color profiles indicate increasing metallicity toward galaxy centers.

Possible interstellar dust contribution at 24 microns.

Abstract

We describe photometry at mid-infrared passbands (1.2 - 24 microns) for a sample of 18 elliptical galaxies. All 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 1.6 microns, but the half-light or effective radius has a pronounced minimum near the K band (2.15 microns). Apart from the 24 micron passband, all sample-averaged radial color profiles have measurable slopes within about twice the (K band) effective radius. Evidently this variation arises because of an increase in stellar metallicity toward the galactic cores. For example, the sampled-averaged color profile (K - 5.8 microns) has a positive slope although no obvious absorption feature is observed in spectra of elliptical galaxies near 5.8 microns. This, and the minimum in the effective radius, suggests that the K band may be anomalously luminous in metal-rich stars in galaxy cores. Unusual radial color profiles involving the 24 micron passband may suggest that some 24 micron emission comes from interstellar not circumstellar dust grains.