The Resolved Structure of a Low-metallicity Photodissociation Region
Journal article, 2025
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 0 . ″ 146 ± 0 . ″ 042 ( stat. ) ± 0 . ″ 012 ( syst. ) 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.
Author
Ilyse Y. Clark
University of California
Karin Sandstrom
University of California
M.G. Wolfire
University of Maryland
A. Bolatto
University of Maryland
Jérémy Chastenet
Ghent university
Daniel A. Dale
College of Engineering and Physical Sciences
Brandt Gaches
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
University of Duisburg-Essen
S. C.O. Glover
Heidelberg University
J.R. Goicoechea
Spanish National Research Council (CSIC)
K. Gordon
Ghent university
Space Telescope Science Institute (STScI)
Brent Groves
University of Western Australia
Lindsey Hands
University of California
R. S. Klessen
Heidelberg University
Harvard University
Harvard-Smithsonian Center for Astrophysics
I. De Looze
Ghent university
J. D.T. Smith
University of Toledo
Dries Van De Putte
Western University
Stefanie Walch
University of Cologne
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 990 2 209Subject Categories (SSIF 2025)
Atom and Molecular Physics and Optics
Astronomy, Astrophysics, and Cosmology
DOI
10.3847/1538-4357/adef38