PHANGS-JWST First Results: Destruction of the PAH molecules in HII regions probed by JWST and MUSE

TL;DR

This study uses JWST and MUSE observations to analyze how PAH molecules are destroyed in HII regions, finding that ionizing UV radiation significantly reduces PAH fractions, especially in more luminous regions.

Contribution

It provides new observational evidence linking PAH destruction to ionization parameters and UV radiation in HII regions across multiple galaxies.

Findings

PAH fraction strongly anti-correlates with ionization parameter

Destruction of PAHs is more pronounced in luminous HII regions

Metallicity has minor impact on PAH destruction in the studied galaxies

Abstract

Polycyclic aromatic hydrocarbons (PAHs) play a critical role in the reprocessing of stellar radiation and in balancing the heating and cooling processes in the interstellar medium (ISM), but appear to be destroyed in HII regions. However, the mechanisms driving their destruction are still not completely understood. Using PHANGS-JWST and PHANGS-MUSE observations, we investigate how the PAH fraction changes in about 1500 HII regions across four nearby star-forming galaxies (NGC 628, NGC 1365, NGC 7496, IC 5332). We find a strong anti-correlation between the PAH fraction and the ionization parameter (the ratio between the ionizing photon flux and the hydrogen density) of HII regions. This relation becomes steeper for more luminous HII regions. The metallicity of HII regions has only a minor impact on these results in our galaxy sample. We find that the PAH fraction decreases with the H$\alpha$ equivalent width - a proxy for the age of the HII regions - although this trend is much weaker than the one identified using the ionization parameter. Our results are consistent with a scenario where hydrogen-ionizing UV radiation is the dominant source of PAH destruction in star-forming regions.