JWST/MIRI Observations of PAH Emission and Evolution in H II Regions of NGC 5457

Abstract

Background: PAH emission features in a single observation, allowing for a robust and systematic study of their relative variations without the systematic uncertainties inherent in stitching together data from different instruments or observing modes. This provides a powerful and self-consistent lever for untangling their complex excitation mechanisms and environmental dependencies. Aims: The present study investigates the physical processing of PAHs in high-mass star-forming environments. This study aims to characterize how the PAH ionization fraction varies across different H II regions and to search for spectroscopic evidence of the processing or destruction of PAH molecules in intense radiation fields. Methodology: We utilized mid-infrared integral field spectroscopic data from the JWST's Mid-Infrared Instrument (MIRI) in its Medium Resolution Spectroscopy (MRS) mode. One-dimensional spectra were extracted for each of the four H II regions. We measured the integrated fluxes of the prominent PAH emission features at 7.7, 8.6, and 11.3 µm and calculated key diagnostic flux ratios to probe the physical state of the PAH population. Results: The four H II regions in NGC 5457 exhibit significant diversity in their PAH spectral characteristics. The diagnostic F(8.6)/F(11.3) ratio, a tracer of PAH ionization, varies from 0.23 to 0.62 across the sample. The region with the highest ionization (highest F(8.6)/F(11.3)) shows the lowest F(7.7)/F(8.6) ratio (1.13), while the region with the lowest ionization has the highest F(7.7)/F(8.6) ratio (2.38).  The inclusion of the 6.2 µm feature would provide an additional probe of the cation population, while the 17 µm feature would trace the largest, most stable PAHs, providing a more robust anchor for size distribution models. This study demonstrates the immense potential of JWST spectroscopy for dissecting the microphysics of the ISM and motivates such future investigations. Conclusion: The observed spectral diversity is primarily driven by variations in the PAH ionization state, which is governed by the local radiation field intensity. The strong anti-correlation between the F(7.7)/F(8.6) and F(8.6)/F(11.3) ratios provides a new, powerful diagnostic for PAH processing. This trend is interpreted as an evolutionary sequence where increasingly harsh radiation fields not only ionize the PAH population but also alter the surviving cations, consistent with the selective destruction of the carriers of the 7.7 µm feature.