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

By Keith Cowing
Press Release
December 27, 2022
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PHANGS-JWST First Results: Destruction of the PAH molecules in HII regions probed by JWST and MUSE
JWST/MIRI images in the F770W, F1000W and F1130W bands (red, green and blue channels, respectively) of the four galaxies analyzed in this letter. Contours show the borders of individual H ii regions selected from MUSE data, which meet the criteria in Sec. 3.1. Blue contours correspond to the H ii regions with hydrogen-ionizing photons rate Q 0 < 1050 s −1 , and red contours to those with Q 0 > 1050 s −1. — astro-ph.GA

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 Hα 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.

Oleg V. Egorov, Kathryn Kreckel, Karin M. Sandstrom, Adam K. Leroy, Simon C. O. Glover, Brent Groves, J. M. Diederik Kruijssen, Ashley. T. Barnes, Francesco Belfiore, F. Bigiel, Guillermo A. Blanc, Médéric Boquien, Yixian Cao, Jérémy Chastenet, Mélanie Chevance, Enrico Congiu, Daniel A. Dale, Eric Emsellem, Kathryn Grasha, Ralf S. Klessen, Kirsten L. Larson, Daizhong Liu, Eric J. Murphy, Hsi-An Pan, Ismael Pessa, Jérôme Pety, Erik Rosolowsky, Fabian Scheuermann, Eva Schinnerer, Jessica Sutter, David A. Thilker, Elizabeth J. Watkins, Thomas G. Williams

Comments: 16 pages, 7 figures, 1 table. Accepted for publication in ApJL as part of a PHANGS-JWST First Results Focus issue
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2212.09159 [astro-ph.GA] (or arXiv:2212.09159v1 [astro-ph.GA] for this version)
Submission history
From: Oleg Egorov Dr.
[v1] Sun, 18 Dec 2022 20:25:22 UTC (12,065 KB)
Astrobiology, Astrochemistry

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