Extended atomic carbon around molecular clouds

Abstract: Models predict that atomic carbon occurs at the surface and in the process of the formation of molecular clouds, making its fine structure transitions a diagnostic of cloud formation. We study the distribution of atomic carbon in a small inconspicuous region towards 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. We observed a small strip of 5 arcminutes in the ``Forgotten Quadrant'', the third quadrant of the Milky Way, with the APEX telescope in the $^3P_1-^3P_0$ [CI] transition of atomic carbon and the $J=2-1$ transition of the three most abundant CO isotopologues and compared their distribution with existing measurements of gas column density and of ionized carbon. 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 $^{13}$CO and in one velocity-component even brighter than CO. In contrast to observations of many star-forming regions, the [CI] emission seems to extend beyond the molecular gas, in line with the models of photon-dominated regions (PDRs). However, a standard PDR model fit to the observations fails because the models either predict more molecular gas, traced through C$^{18}$O, or more diffuse gas, traced through [CII], than observed. The carbon-budget in the gas phase does not add up to the same column seen through dust emission. To understand the [CI] emission from galaxies it is necessary to get the full statistics for the quiescent gas outside of the star-forming regions that behaves significantly different from dense gas exposed to high ultraviolet fields.