# Dust emission at 8-mic and 24-mic as Diagnostics of HII Region Radiative Transfer

## TL;DR

This study investigates how 8-mic and 24-mic emissions from dust and PAHs relate to the radiative transfer in HII regions within the Magellanic Clouds, revealing metallicity-dependent differences in dust and PAH behavior.

## Contribution

It provides new insights into how metallicity influences dust and PAH emissions and their connection to the optical depth of HII regions, using Spitzer and Herschel data.

## Key findings

- PAH destruction correlates with optically thin conditions in the LMC.
- Higher nebular optical depths are associated with increased dust emission.
- Low-metallicity SMC shows no variation in dust emission due to suppressed PAH and dust formation.

## Abstract

We use the Spitzer SAGE survey of the Magellanic Clouds to evaluate the relationship between the 8-mic PAH emission, 24-mic hot dust emission, and HII region radiative transfer. We confirm that in the higher-metallicity Large Magellanic Cloud, PAH destruction is sensitive to optically thin conditions in the nebular Lyman continuum: objects identified as optically thin candidates based on nebular ionization structure show 6 times lower median 8-mic surface brightness (0.18 mJy arcsec^-2) than their optically thick counterparts (1.2 mJy arcsec^-2). The 24-mic surface brightness also shows a factor of 3 offset between the two classes of objects (0.13 vs 0.44 mJy arcsec^-2, respectively), which is driven by the association between the very small dust grains and higher density gas found at higher nebular optical depths. In contrast, PAH and dust formation in the low-metallicity Small Magellanic Cloud is strongly inhibited such that we find no variation in either 8-mic or 24-mic emission between our optically thick and thin samples. This is attributable to extremely low PAH and dust production together with high, corrosive UV photon fluxes in this low-metallicity environment. The dust mass surface densities and gas-to-dust ratios determined from dust maps using Herschel HERITAGE survey data support this interpretation.