Spectroscopic [C I] mapping of the infrared dark cloud G48.65-0.29

Aims. We report the first spectroscopic mapping of an atomic carbon line in an infrared dark cloud (IRDC). By observing the spatial distribution of the [Ci] emission in an IRDC, comparing it with the (13)CO emission and the known distribution of internal heating sources, we can quantify the role of internal and external UV irradiation in the production of atomic carbon.Methods. We used the 2 x 4 pixel SMART receiver of the KOSMA observatory on Gornergrat to map the [Ci] (3)P(1) - (3)P(0) line in the IRDC G48.65-0.29 and compared the resulting spectra with data from the BU-FCRAO (13)CO 1-0 Galactic Ring Survey.Results. The [CI]/(13)CO effective beam temperature ratio falls at about 0.3 with local deviations by less than a factor two. All velocity components seen in (13)CO are also detected in [CI]. We find, however, significant differences in the morphology of the brightest regions seen in the two tracers. While (13)CO basically follows the column density distribution derived from the near-infrared (NIR) extinction and the submm continuum, the [CI] emission peaks at the locations of the two known NIR point sources. We find Ci/CO abundance ratios between 0.07 and 0.13 matching the lower end of the range previously measured in star-forming regions.Conclusions. Evaluating the relative importance of the irradiation by embedded sources and by the Galactic interstellar UV field, we find that in G48.65-0.29 most [CI] emission can be attributed to externally illuminated surfaces. Embedded sources have a significant impact on the overall abundance distribution of atomic carbon as soon as they are found in an evolved state with noticeable NIR flux.