2023NatAs...7..546S

Molecular hydrogen clouds are a key component of the interstellar medium because they are the birthplaces for stars. They are embedded in atomic gas that pervades the interstellar space. However, the details of how molecular clouds assemble from and interact with the atomic gas are still largely unknown. As a result of new observations of the 158 µm line of ionized carbon [CII] in the Cygnus region within the FEEDBACK program on SOFIA (Stratospheric Observatory for Infrared Astronomy), we present compelling evidence that [CII] unveils dynamic interactions between cloud ensembles. This process is neither a head-on collision of fully molecular clouds nor a gentle merging of only atomic clouds. Moreover, we demonstrate that the dense molecular clouds associated with the DR21 and W75N star-forming regions and a cloud at higher velocity are embedded in atomic gas, and all components interact over a large range of velocities (roughly 20 km s-1). The atomic gas has a density of around 100 cm-3 and a temperature of roughly 100 K. We conclude that the [CII] 158 µm line is an excellent tracer to witness the processes involved in cloud interactions and anticipate further detections of this phenomenon in other regions.