2025A&A...696A.148O

Context. Models predict that atomic carbon occurs at the surface and in the process of the formation of molecular clouds. This makes its fine-structure transitions a diagnostic of cloud formation.
Aims. We study the distribution of atomic carbon in a small inconspicuous region toward the outer Galaxy that might be representative for a large fraction of the molecular gas of the Milky Way that is not directly affected by star formation.
Methods. We observed a small strip of 5 arcminutes in the so-called Forgotten Quadrant, the third quadrant of the Milky Way, with the APEX telescope in the ³P₁-³P₀ [C I] transition of atomic carbon and the J = 2-1 transition of the three most abundant CO isotopologs. We compared their distribution with existing measurements of the gas column density and of ionized carbon.
Results. The atomic carbon shows a very smooth distribution with the smallest gradient along the strip compared to the other lines. It is always brighter than ¹³CO, and in one velocity component, it is even brighter than CO. In contrast to observations of many star-forming regions, the [C I] emission seems to extend beyond the molecular gas. This is in line with models of photon-dominated regions (PDRs). However, a standard PDR model fit to the observations fails because the models either predict more molecular gas than observed, traced through C¹⁸O, or more diffuse gas than observed, traced through [C II]. The carbon budget in the gas phase does not add up to the same column seen through dust emission.
Conclusions. To understand the [C I] emission from galaxies, it is necessary to obtain the full statistics for the quiescent gas outside of the star-forming regions that behaves significantly different from dense gas that is exposed to high-ultraviolet fields.