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The resolved structure of a low-metallicity photodissociation region

Photodissociation regions (PDRs) are key to understanding the feedback processes that shape interstellar matter in galaxies. One important type of PDR is the interface between H ii regions and molecular clouds, where far-ultraviolet radiation from massive stars heats gas and dissociates molecules. P...

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Main Authors: Clark, Ilyse (Author) , Sandstrom, Karin (Author) , Wolfire, Mark (Author) , Bolatto, Alberto D. (Author) , Chastenet, Jérémy (Author) , Dale, Daniel A. (Author) , Gaches, Brandt A. L. (Author) , Glover, Simon (Author) , Goicoechea, Javier R. (Author) , Gordon, Karl D. (Author) , Groves, Brent (Author) , Hands, Lindsey (Author) , Klessen, Ralf S. (Author) , De Looze, Ilse (Author) , Smith, J. D. T. (Author) , Van De Putte, Dries (Author) , Walch, Stefanie (Author)
Format: Article (Journal)
Language:English
Published: 2025 September 10
In: The astrophysical journal
Year: 2025, Volume: 990, Issue: 2, Pages: 1-17
ISSN:1538-4357
DOI:10.3847/1538-4357/adef38
Online Access:Verlag, kostenfrei, Volltext: https://doi.org/10.3847/1538-4357/adef38
Verlag, kostenfrei, Volltext: https://iopscience.iop.org/article/10.3847/1538-4357/adef38
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Author Notes:Ilyse Y. Clark, Karin Sandstrom, Mark Wolfire, Alberto D. Bolatto, Jérémy Chastenet, Daniel A. Dale, Brandt A.L. Gaches, Simon C.O. Glover, Javier R. Goicoechea, Karl D. Gordon, Brent Groves, Lindsey Hands, Ralf Klessen, Ilse De Looze, J.D.T. Smith, Dries Van De Putte, and Stefanie K. Walch
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Summary:Photodissociation regions (PDRs) are key to understanding the feedback processes that shape interstellar matter in galaxies. One important type of PDR is the interface between H ii regions and molecular clouds, where far-ultraviolet radiation from massive stars heats gas and dissociates molecules. Photochemical models predict that as metallicity decreases, the C/CO transition occurs at greater depths in the PDR compared to the H/H2 transition, increasing the extent of CO-dark H2 gas in low-metallicity environments. This prediction has been difficult to test outside the Milky Way due to the lack of high-spatial-resolution observations tracing H2 and CO. This study examines a low-metallicity PDR in the N13 region of the Small Magellanic Cloud (SMC), where we spatially resolve the ionization front, the H2 dissociation front, and the C/CO transition using 12CO J = 2−1, 3−2, and [C I] 1-0 observations from the Atacama Large Millimeter/submillimeter Array and near-infrared spectroscopy of the H2 2.12 1-0 S(1) vibrational line, and H recombination lines from the James Webb Space Telescope. Our analysis shows that the separation between the H/H2 and C/CO boundaries is approximately 0.043 ± 0.013(stat.) ± 0.0036(syst.) pc (equivalent to at the SMC’s distance of 62 kpc), defining the spatial extent of the CO-dark H2 region. Compared to our plane-parallel PDR models, we find that a constant-pressure model matches the observed structure better than a constant-density one. Overall, we find that the PDR model does well at predicting the extent of the CO-dark H2 layer in N13. This study represents the first resolved benchmark for low-metallicity PDRs.
Item Description:Online verfügbar: 09. September 2025
Gesehen am 05.02.2026
Physical Description:Online Resource
ISSN:1538-4357
DOI:10.3847/1538-4357/adef38

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