THE EFFECT OF ACTIVE GALACTIC NUCLEI ON THE MID-INFRARED AROMATIC FEATURES

We present Spitzer measurements of the aromatic (also known as polycyclic aromatic hydrocarbon) features for 35 Seyfert galaxies from the revised Shapley-Ames sample and find that the relative strengths of the features differ significantly from those observed in star-forming galaxies. Specifically, the features at 6.2, 7.7, and 8.6 mu m are suppressed relative to the 11.3 mu m feature in Seyferts. Furthermore, we find an anti-correlation between the L(7.7 mu m)/L(11.3 mu m) ratio and the strength of the rotational H-2 emission, which traces shocked gas. This suggests that shocks suppress the short-wavelength features by modifying the structure of the aromatic molecules or destroying the smallest grains. Most Seyfert nuclei fall on the relationship between aromatic emission and [ Ne II] emission for star-forming galaxies, indicating that aromatic-based estimates of the star formation rate are generally reasonable in galaxies hosting active galactic nuclei. For the outliers from this relationship, which have small L(7.7 mu m)/L(11.3 mu m) ratios and strong H-2 emission, the 11.3 mu m feature still provides a valid measure of the star formation rate.