PAH Feature Ratios Around Stellar Clusters and Associations in 19 Nearby Galaxies

TL;DR

This study compares observed PAH feature ratios in 19 nearby galaxies with theoretical models, revealing that PAH ratios are influenced by radiation field hardness and that small PAHs are destroyed in intense radiation regions.

Contribution

It provides the first extensive comparison of observed PAH feature ratios from JWST data with theoretical models across multiple galaxies, highlighting the impact of radiation fields on PAH properties.

Findings

Observed PAH ratios largely match theoretical models, especially those with ionized and larger PAHs.

Diffuse interstellar medium shows smaller PAH ratios compared to stellar clusters and associations.

PAH ratios are anti-correlated with radiation field hardness, indicating PAH processing or destruction.

Abstract

We present a comparison of observed polycyclic aromatic hydrocarbon (PAH) feature ratios in 19 nearby galaxies with a grid of theoretical expectations for near- and mid-infrared dust emission. The PAH feature ratios are drawn from Cycle 1 JWST observations and are measured for 7224 stellar clusters and 29176 stellar associations for which we have robust ages and mass estimates from HST five-band photometry. Though there are galaxy-to-galaxy variations, the observed PAH feature ratios largely agree with the theoretical models, particularly those that are skewed toward more ionized and larger PAH size distributions. For each galaxy we also extract PAH feature ratios for 200 pc-wide circular regions in the diffuse interstellar medium, which serve as a non-cluster/association control sample. Compared to what we find for stellar clusters and associations, the 3.3um/7.7um and 3.3um/11.3um ratios from the diffuse interstellar medium are $\sim 0.10-0.15$ dex smaller. When the observed PAH feature ratios are compared to the radiation field hardness as probed by the [OIII]/H$\beta$ ratio, we find anti-correlations for nearly all galaxies in the sample. These results together suggest that the PAH feature ratios are driven by the shape intensity of the radiation field, and that the smallest PAHs -- observed via JWST F335M imaging -- are increasingly 'processed' or destroyed in regions with the most intense and hard radiation fields.