The behavior of the aromatic features in M101 HII regions: Evidence for dust processing

The aromatic features in M101 were studied spectroscopically and photometrically using observations from all three instruments on the Spitzer Space Telescope. The global SED of M101 shows strong aromatic feature (commonly called PAH feature) emission. The spatially resolved spectral and photometric measurements of the aromatic feature emission show strong variations with significantly weaker emission at larger radii. We compare these variations with changes in the ionization index (as measured by [Ne III]/[Ne II] and [S IV/S III], which range from 0.03 to 20 and 0.044 to 15, respectively) and metallicity [expressed as log (O/H) + 12, which ranges from 8.1 to 8.8]. Over these ranges, the spectroscopic equivalent widths of the aromatic features from seven H II regions and the nucleus were found to correlate better with ionization index than metallicity. This implies that the weakening of the aromatic emission in massive star-forming regions is due primarily to processing of the dust grains in these environments, not to differences in how they form. The behavior of the aromatic feature versus ionization index correlation can be described as a constant equivalent width until a threshold in ionization index is reached ([Ne III]/[Ne II] similar to 1), above which the equivalent widths decrease with a power law dependence. This behavior is also seen for the star burst galaxy sample presented in the companion study by Engelbracht and coworkers, which expands the range of [Ne III]/[Ne II] ratios to 0.03-25 and log (O/H) + 12 values to 7.1-8.8. The form of the correlation explains seemingly contradictory results present in the literature. The behavior of the ratios of different aromatic features versus ionization index does not follow the predictions of existing PAH models of the aromatic features, implying a more complex origin of the aromatic emission in massive star-forming regions.