The Behavior of the Aromatic Features in M101 HII Regions: Evidence for Dust Processing

TL;DR

This study investigates how aromatic features in M101's HII regions vary with ionization and metallicity, revealing dust processing effects that weaken aromatic emission in high-ionization environments, supported by spectroscopic and photometric data.

Contribution

It demonstrates that aromatic feature weakening correlates primarily with ionization index, indicating dust processing rather than formation differences, expanding understanding of dust behavior in star-forming regions.

Findings

Aromatic feature equivalent widths correlate better with ionization index than metallicity.

Weakening of aromatic emission occurs above a threshold ionization level (~1 in [NeIII]/[NeII]).

Behavior observed in M101 HII regions is consistent with starburst galaxy data.

Abstract

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 [NeIII]/[NeII] and [SIV/SIII], which we probe over the ranges 0.03-20 and 0.044-15 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 7 HII regions and the nucleus were found to correlate better with ionization index than with 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 (although formation could still be important on a secondary basis). The behavior of the correlation between the aromatic feature equivalent widths and ionization index can be described as a constant equivalent width until a threshold in ionization index is reached ([NeIII]/[NeII] ~ 1), above which the equivalent widths decrease with a power law dependence. This behavior for M101 HII regions is also seen for the sample of starburst galaxies presented in the companion study of Engelbracht et al. (2008) which expands the range of [NeIII]/[NeII] 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. [abridged]