Dr Thomas Williams

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 29,176 stellar associations for which we have robust ages and mass estimates from Hubble Space Telescope 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 noncluster/association control sample. Compared to what we find for stellar clusters and associations, the 3.3 μm/7.7 μm and 3.3 μm/11.3 μm ratios from the diffuse interstellar medium are ∼0.10–0.15 dex smaller. When the observed PAH feature ratios are compared to the radiation field hardness as probed by the [O iii]/Hβ ratio, we find anticorrelations for nearly all galaxies in the sample. These results together suggest that the PAH feature ratios are driven by the shape and 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.

Abstract:

The structure and chemistry of the dusty interstellar medium (ISM) are shaped by complex processes that depend on the local radiation field, gas composition, and dust grain properties. Of particular importance are polycyclic aromatic hydrocarbons (PAHs), which emit strong vibrational bands in the mid-infrared, and play a key role in the ISM energy balance. We recently identified global correlations between PAH band and optical line ratios across three nearby galaxies, suggesting a connection between PAH heating and gas ionization throughout the ISM. In this work, we perform a census of the PAH heating–gas ionization connection using ∼700,000 independent pixels that probe scales of 40–150 pc in 19 nearby star-forming galaxies from the PHANGS survey. We find a universal relation between log PAH(11.3 μm/7.7 μm) and log ([S ii]/Hα) with a slope of ∼0.2 and a scatter of ∼0.025 dex. The only exception is a group of anomalous pixels that show unusually high (11.3 μm/7.7 μm) PAH ratios in regions with old stellar populations and high starlight-to-dust emission ratios. Their mid-infrared spectra resemble those of elliptical galaxies. Active galactic nucleus hosts show modestly steeper slopes, with a ∼​​​​​​10% increase in PAH(11.3 μm/7.7 μm) in the diffuse gas on kiloparsec scales. This universal relation implies an emerging simplicity in the complex ISM, with a sequence that is driven by a single varying property: the spectral shape of the interstellar radiation field. This suggests that other properties, such as gas-phase abundances, gas ionization parameter, and grain charge distribution, are relatively uniform in all but specific cases.