Extended atomic carbon around molecular clouds

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 <ns0:sc>I</ns0:sc>] 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 <ns0:sc>I</ns0:sc>] 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 <ns0:sc>II</ns0:sc>]. The carbon budget in the gas phase does not add up to the same column seen through dust emission. Conclusions. To understand the [C <ns0:sc>I</ns0:sc>] 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.