2025ApJ...990..209C

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 $0\mathop{.}\limits^{^{\prime\prime} }146\pm 0\mathop{.}\limits^{^{\prime\prime} }042({\rm{stat.}})\pm 0\mathop{.}\limits^{^{\prime\prime} }012({\rm{syst.}})$ 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.