NANTEN Observations of the Southern Coalsack in 13CO and C18O J = 1−0 Emission

We made observations of the 13CO and C18O J = 1−0 emission toward the Southern Coalsack. The 13CO observations covered the whole extent of the main cloud with a 2′.7 beam at an 8′ or 4′ grid spacing. The 13CO emitting gas is highly clumpy, consisting of small “cloudlets” of ≲ 0.4 pc diameter. The densest and most massive cloudlet, whose mass is ∼ 200 M⊙ , is coincident with the region of the heaviest visual extinction, located at the western edge of the cloud. The total mass of the 13CO emitting gas amounts to ∼ 600 M⊙ , corresponding to only ∼ 17% of the total cloud mass estimated from 12CO observations. In addition, toward the densest region in 13CO we made C18O J = 1−0 observations. We found five C18O dense cores, in agreement with well-known optically dark globules, and identified these dense cores to be potential sites of star formation, as indicated by their high column density, which is typical of star-forming cores. On the other hand, a comparison of the 13CO and C18O distributions with young stellar objects detected as IRAS point sources indicates no sign of recent star formation in the Southern Coalsack.