PHANGS–JWST First Results: Destruction of the PAH Molecules in H ii Regions Probed by JWST and MUSE
Oleg V. Egorov, Kathryn Kreckel, Karin Sandström, Adam K. Leroy, Simon C. O. Glover, Brent Groves, J. M. Diederik Kruijssen, Ashley T. Barnes, Francesco Belfiore, Frank Bigiel, Guillermo A. Blanc, M. Boquien, Yixian Cao, Jérémy Chastenet, Mélanie Chevance, Enrico Congiu, Daniel A. Dale, Éric Emsellem, Kathryn Grasha, Ralf S. Klessen, Kirsten L. Larson, Daizhong Liu, E. J. Murphy, Hsi-An Pan, Ismael Pessa, J. Pety, Erik Rosolowsky, Fabian Scheuermann, Eva Schinnerer, Jessica Sutter, David A. Thilker, Elizabeth J. Watkins, Thomas G. Williams
Abstract
Abstract Polycyclic aromatic hydrocarbons (PAHs) play a critical role in the reprocessing of stellar radiation and balancing the heating and cooling processes in the interstellar medium but appear to be destroyed in H ii 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 H ii regions across four nearby star-forming galaxies (NGC 628, NGC 1365, NGC 7496, and IC 5332). We find a strong anticorrelation between the PAH fraction and the ionization parameter (the ratio between the ionizing photon flux and the hydrogen density) of H ii regions. This relation becomes steeper for more luminous H ii regions. The metallicity of H ii regions has only a minor impact on these results in our galaxy sample. We find that the PAH fraction decreases with the H α equivalent width—a proxy for the age of the H ii 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.