The Coalsack near and far

Abstract

Context. The large Coalsack dark cloud is one of the most prominent southern starless clouds, which is even visible to the naked eye. Furthermore, it is one of the rare molecular clouds without clear signs of star formation. Aims. We investigate the dynamical properties of the gas within the Coalsack. Methods. The two highest extinction regions were mapped with the APEX telescope in 13CO(2-1) comprising a region of ~1  square degree. Results. In addition to the well-known, nearby gas component around-4km s -1, we identified additional molecular gas components-in particular a second extended molecular cloud at a velocity of ∼-30 km s -1 and an estimated distance of ∼3.1 kpc-that dominate the column density and visual extinction distributions in the northeastern part of the Coalsack. Although comprising ∼2600 M â, the mass of this distant cloud is distributed over an extent of ∼73 pc, much larger than typical high-mass infrared dark clouds. Its filamentary structure is consistent with a compressible gaseous self-gravitating cylinder, and its low mass per length indicates that it may be stable against gravitational collapse. We find barely any mid-infrared emission in archival MSX data, which is indicative of almost no star-formation activity in the near and far cloud complexes. The nearby clouds have narrow, almost thermal velocity dispersions with median values between 0.2 and 0.4 km s -1, which is also consistent with low star-formation activity. Only Tapia's Globule 2 exhibits a velocity dispersion increase toward the extinction peak and peak-velocity gradients over the core, which is indicative of a state of elevated dynamical properties. Conclusions. The Coalsack is not one single coherent structure, but consists of several cloud complexes nearby as well as at several kpc distance. All studied clouds appear as starless low-turbulence regions that may not even collapse in the future. Only one globule exhibits more dynamical signatures and is a good candidate for present/future star formation. © 2011 ESO.