The Resolved Structure of a Low-metallicity Photodissociation Region
Artikel i vetenskaplig tidskrift, 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.
Författare
Ilyse Y. Clark
University of California
Karin Sandstrom
University of California
M.G. Wolfire
University of Maryland
A. Bolatto
University of Maryland
Jérémy Chastenet
Universiteit Gent
Daniel A. Dale
College of Engineering and Physical Sciences
Brandt Gaches
Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik
Universität Duisburg-Essen
S. C.O. Glover
Universität Heidelberg
J.R. Goicoechea
Consejo Superior de Investigaciones Científicas (CSIC)
K. Gordon
Universiteit Gent
Space Telescope Science Institute (STScI)
Brent Groves
University of Western Australia
Lindsey Hands
University of California
R. S. Klessen
Universität Heidelberg
Harvard University
Harvard-Smithsonian Center for Astrophysics
I. De Looze
Universiteit Gent
J. D.T. Smith
University of Toledo
Dries Van De Putte
Western University
Stefanie Walch
Universität zu Köln
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 990 2 209Ämneskategorier (SSIF 2025)
Atom- och molekylfysik och optik
Astronomi, astrofysik och kosmologi
DOI
10.3847/1538-4357/adef38