Observational signatures of stellar feedback

Massive stars play a key role in the evolution of the interstellar medium (ISM) in galaxies because they impact the ISM through ionization, stellar winds, and supernova explosions. The gas is heated by UV radiation and cools via far-infrared cooling lines, in particular the CII 158 $\mu$m line and the OI 63 $\mu$m line. The SOFIA legacy project FEEDBACK performs extended maps in these lines in 11 Galactic massive star-forming regions. The major results of the program so far are the discovery of expanding CII shells that are driven by stellar winds, the detection of large amounts of cold carbon that is associated with atomic hydrogen, and the application of CII as a tracer for molecular cloud assembly.