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

¹ I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
e-mail: ossk@ph1.uni-koeln.de
² SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 15 July 2008
Accepted: 7 October 2010

Abstract

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 [C i] emission in an IRDC, comparing it with the ¹³CO 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 × 4 pixel SMART receiver of the KOSMA observatory on Gornergrat to map the [C i] line in the IRDC G48.65-0.29 and compared the resulting spectra with data from the BU-FCRAO ¹³CO 1–0 Galactic Ring Survey.

Results. The [C i]/¹³CO effective beam temperature ratio falls at about 0.3 with local deviations by less than a factor two. All velocity components seen in ¹³CO are also detected in [C i]. We find, however, significant differences in the morphology of the brightest regions seen in the two tracers. While ¹³CO basically follows the column density distribution derived from the near-infrared (NIR) extinction and the submm continuum, the [C i] emission peaks at the locations of the two known NIR point sources. We find C i/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 [C i] 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.