Séminaire : The Role of Magnetic Fields in the Atomic and Molecular Gas of the Galaxy

If magnetic fields are strong enough, they can control the formation and evolution of molecular clouds and the cores within them. If the fields are weak, they play only an incidental role.  In this latter case, turbulence, perhaps involving colliding flows, drives the formation and evolution of molecular clouds on a shorter timescale than magnetic fields. 

Which is it?  The Zeeman effect in radio frequency spectral lines offers important clues.  The Zeeman effect is the only observational technique to measure field strengths directly in localized regions of the ISM.  Despite the difficulties of Zeeman effect observations, several decades of effort have led to measurements in well over 100 locations in the Galaxy. 

We can now assess the role of the field in the formation of molecular clouds out of diffuse atomic gas, and in the evolution of molecular clouds to star formation.   Results to date are not definitive.  However, it appears that the field plays an important role, especially in the early phases of molecular cloud formation and evolution.  Future Zeeman effect observations, especially at high spatial resolution, will help to refine this picture.