The Effect of Active Galactic Nuclei on the Mid-Infrared Aromatic Features

TL;DR

This study uses Spitzer data to analyze how active galactic nuclei influence mid-infrared aromatic features, revealing suppression of certain features due to shocks and confirming aromatic emissions as reliable star-formation indicators in AGN hosts.

Contribution

It provides new insights into the impact of AGN activity on aromatic features and their relation to star formation, highlighting shock effects and validating aromatic-based star-formation estimates.

Findings

Suppression of 6.2, 7.7, and 8.6 micron features in Seyferts

Anti-correlation between 7.7/11.3 micron ratio and H2 emission

Most Seyferts follow star-forming galaxy aromatic emission relationships

Abstract

We present Spitzer measurements of the aromatic (also known as PAH) 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 micron are suppressed relative to the 11.3 micron feature in Seyferts. Furthermore, we find an anti-correlation between the L(7.7 micron)/L(11.3 micron) ratio and the strength of the rotational H2 (molecular hydrogen) 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 in AGN host galaxies are generally reasonable. For the outliers from this relationship, which have small L(7.7 micron)/L(11.3 micron) ratios and strong H2 emission, the 11.3 micron feature still provides a valid measure of the star-formation rate.