Ionized carbon as a tracer of the assembly of interstellar clouds

TL;DR

This paper shows that ionized carbon (CII) emission at 158 micrometers effectively traces the dynamic interactions and assembly processes of molecular clouds within atomic gas in the interstellar medium, revealing complex cloud interactions.

Contribution

It provides new observational evidence that CII emission unveils the interactions between atomic and molecular clouds, highlighting a complex assembly process not solely due to collisions or gentle merging.

Findings

CII traces dynamic cloud interactions in the Cygnus region.

Molecular clouds are embedded in atomic gas interacting over large velocity ranges.

Atomic gas involved has a density of 100 cm$^{-3}$ and temperature of 100 K.

Abstract

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~$\mu$m 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 ofonly 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 (20 km/s). The atomic gas has a density of 100 cm$^{-3}$ and a temperature of 100 K. We conclude that the CII 158 $\mu$m line is an excellent tracer to witness the processes involved in cloud interactions and anticipate further detections of this phenomenon in other regions