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 μm are suppressed relative to the 11.3 μm feature in Seyferts. Furthermore, we find an anti-correlation between the L (7.7 μm)/ L (11.3 μm) 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 μm)/ L (11.3 μm) ratios and strong H 2 emission, the 11.3 μm feature still provides a valid measure of the star formation rate.