Mid-Infrared Diagnostics of PAH Processing and Radiation Fields in M33 H II Regions with JWST/MIRI

Abstract

We present JWST/MIRI (Proposal ID 4297) medium-resolution integral field spectroscopy of five H II regions in the nearby spiral galaxy M33, obtained from the Mikulski Archive for Space Telescopes (MAST). The fully calibrated Level-3 spectral cubes provide continuous wavelength coverage from 6.5 - 20.9~$\mu$m, enabling analysis of polycyclic aromatic hydrocarbon (PAH) emission, ionic fine-structure lines, and warm molecular hydrogen. The spectra show prominent PAH bands at 7.7, 8.6, and 11.3~$\mu$m, multiple H${}_2$ rotational transitions, and hydrogen recombination lines. H${}_2$ rotational diagrams yield warm molecular gas temperatures of $\sim 316$ - $407$~K. The detected [Ne II], [Ne III], [S III], and [S IV] lines yield ionization indices of $-1.41$ to $+0.98$. The PAH ionization fraction shows no clear dependence on radiation hardness, consistent with previous Spitzer results. Unlike previous work, we find no clear correlation between the 7.7/11.2 and 8.6/11.2~$\mu$m PAH ratios, while the 7.7/11.2~$\mu$m ratio shows a moderate negative correlation with metallicity, possibly due to our small sample ($N=5$). The PAH-to-VSG ratio decreases with increasing radiation hardness. We derive the ionization parameter $\gamma$, far-ultraviolet radiation field, and PAH charge state, finding a predominantly cationic PAH population ($\sim 75$) despite [Ne III]/[Ne II] $<1$. The lack of a one-to-one correspondence between $\gamma$ and the 7.7/11.2~$\mu$m ratio suggests that PAH ionization is influenced by additional environmental factors. Given the limited sample, these trends are suggestive rather than statistically definitive.

arXiv comment25 page, 15 figures, Accepted for publication in The Astrophysical Journal (ApJ)