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 H₂ 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 H₂ emission, the 11.3 μm feature still provides a valid measure of the star formation rate.