The Resolved Structure of a Low-metallicity Photodissociation Region

Abstract 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/H 2 transition, increasing the extent of CO-dark H 2 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 H 2 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 H 2 dissociation front, and the C/CO transition using 12 CO J = 2−1, 3−2, and [C I ] 1–0 observations from the Atacama Large Millimeter/submillimeter Array and near-infrared spectroscopy of the H 2 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/H 2 and C/CO boundaries is approximately 0.043 ± 0.013(stat.) ± 0.0036(syst.) pc (equivalent to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0</mml:mn> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>″</mml:mi> </mml:mrow> </mml:mover> <mml:mn>146</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0</mml:mn> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>″</mml:mi> </mml:mrow> </mml:mover> <mml:mn>042</mml:mn> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">stat.</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>±</mml:mo> <mml:mn>0</mml:mn> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>″</mml:mi> </mml:mrow> </mml:mover> <mml:mn>012</mml:mn> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">syst.</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> at the SMC’s distance of 62 kpc), defining the spatial extent of the CO-dark H 2 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 H 2 layer in N13. This study represents the first resolved benchmark for low-metallicity PDRs.