Interaction between the Northern Coalsack in the Cygnus OB 7 cloud complex and multiple supernova remnants including HB 21

We report on possible interaction between multiple supernova remnants (SNRs) and the Northern Coalsack (NCS), which is a massive clump (∼1 × 103 M⊙) in the Cygnus OB 7 cloud complex and is forming a massive Class 0 object. We performed molecular observations of the 12CO(J = 1–0), 13CO(J = 1–0), and C18O(J = 1–0) emission lines using the 45 m telescope at the Nobeyama Radio Observatory, and we found that there are four main-velocity components at VLSR ≃ −20, −6, −4, and 10 km s−1. The −6 and −4 km s−1 components correspond to the systemic velocities of NCS and the Cyg OB 7 complex, respectively, and the other velocity components originate from distinct smaller clouds. Interestingly, there are apparent correlations and anticorrelations among the spatial distributions of the four components, suggesting that they are physically interacting with one another. On a larger scale, we find that a group of small clouds belonging to the −20 and 10 km s−1 components are located along two different arcs around some SNRs including HB 21, which has been suggested to be interacting with the Cyg OB 7 cloud complex, and we also find that NCS is located right at the interface of the arcs. The small clouds are likely to be the gas swept up by the stellar wind of the massive stars that created the SNRs. We suggest that the small clouds aligned along the two arcs recently encountered NCS, and the massive star formation in NCS was triggered by the strong interaction of the small clouds.