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

TL;DR

This study uses JWST/MIRI spectroscopy to analyze PAH emission in H II regions of NGC 5457, revealing how PAH ionization and processing vary with local radiation fields and indicating potential PAH destruction in intense environments.

Contribution

First detailed JWST/MIRI spectroscopic analysis of PAH evolution in multiple H II regions, linking PAH ionization states to radiation field intensity and processing.

Findings

PAH spectral diversity correlates with ionization levels.

Anti-correlation between F(7.7)/F(8.6) and F(8.6)/F(11.3) ratios.

Evidence for PAH processing and selective destruction in harsh radiation fields.

Abstract

Aims: To investigate 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 micron 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). 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 micron feature.