Data from The Resolved Structure of a Low Metallicity Photodissociation Region

JWST: NIRSpec and MIRI-MRS, HST: Photometry.
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 \hii regions and molecular clouds, where far-ultraviolet (FUV) radiation from massive stars heats gas and dissociates molecules. Photochemical models predict that the C/CO transition occurs deeper in the PDR compared to the \htohtwo transition in low-metallicity environments, increasing the extent of CO-dark \htwo gas. This prediction has been difficult to test outside the Milky Way due to the lack of high spatial resolution observations tracing \htwo 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 \htwo dissociation front, and the C/CO transition. Using $^{12}$CO J=2$-$1, 3$-$2 and [CI] (1-0) observations from the Atacama Large Millimeter/sub-mm Array (ALMA) and near-infrared spectroscopy of \htwo vibrational lines from the James Webb Space Telescope (JWST), we resolve the layers of the PDR in N13. Our analysis shows that the separation between the H/\htwo and C/CO boundaries is approximately \seppc (equivalent to \separc at the distance of the SMC), defining the spatial extent of the CO-dark \htwo 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 \htwo layer in N13. This study represents the first resolved benchmark for low metallicity PDRs.