Mid-infrared Diagnostics of Polycyclic Aromatic Hydrocarbon Processing and Radiation Fields in M33 H II Regions with JWST/MIRI

Abstract We present James Webb Space Telescope (JWST) Mid-Infrared Instrument (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 to 20.9 μ m, enabling detailed analysis of polycyclic aromatic hydrocarbon (PAH) emission, ionic fine-structure lines, and warm molecular hydrogen. The spectra exhibit prominent PAH bands at 7.7, 8.6, and 11.3 μ m, multiple H 2 pure rotational transitions, and hydrogen recombination lines. From H 2 rotational diagrams, we derive warm molecular gas temperatures of ∼316–407 K. The detection of [Ne ii ], [Ne iii ], [S iii ], and [S iv ] enables measurements of ionic abundances and radiation-field hardness, yielding ionization indices of −1.41 to +0.98. We find that the PAH ionization fraction shows no clear dependence on radiation hardness, consistent with previous Spitzer-based results. However, unlike previous work, we find no clear correlation between the 7.7/11.2 and 8.6/11.2 μ m PAH ratios and a moderate negative correlation between 7.7/11.2 μ m and metallicity, possibly due to our limited sample size ( N = 5). The PAH-to-very small grain ratio decreases with increasing radiation hardness. We derive the ionization parameter γ , far-ultraviolet radiation field, and PAH charge state, finding a predominantly cationic PAH population (∼75%) despite [Ne III ]/[Ne II ] < 1. The ionization parameter γ shows no one-to-one correspondence with the 7.7/11.2 μ m ratio, indicating that PAH ionization is influenced by additional environmental factors. Ionic abundances exhibit both positive and negative correlations with radiation hardness, depending on the species. The limited sample size renders these trends suggestive rather than statistically definitive.