The Coalsack near and far

Abstract

Context: The large Coalsack dark cloud 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, we identified additional molecular gas components - in particular a second extended molecular cloud at a velocity of ~-30km/s and an estimated distance of ~3.1kpc - that dominate the column density and visual extinction distributions in the northeastern part of the Coalsack. Although comprising ~2600Msun, the mass of this distant cloud is distributed over an extent of ~73pc, 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 0.4km/s, 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.