The Dominance of the Magnetic Field in the Central Five Parsecs of the Galaxy

Using polarimetric and photometric data from the HAWC+ instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA), we have estimated the value of the plasma β, the ratio of the thermal-to-magnetic pressure. This value is used traditionally as an indicator of whether magnetic or thermodynamic processes dominate in an environment. If the thermal pressure is greater than the magnetic pressure, β > 1, referred to as a high-β plasma, the gas dynamics will control the structure of the environment, e.g., the solar photosphere. If the thermal pressure is less than the magnetic pressure, β < 1, referred to as a low-β plasma, the magnetic field will control the structure of the environment, e.g., the solar corona. Using values of temperature and density from the literature and the magnetic field value of B = 5 mG obtained from the Davis-Chandrasekhar-Fermi method, we find that β ~ 0.001. Since the widths of all molecular, atomic, and ionized gas lines are quite large in and around this region, we might want to include all forms of kinetic energy, including turbulence, to determine if the magnetic pressure really dominates. Defining β' as the ratio of the turbulent pressure over the magnetic pressure and using an equivalent temperature from the literature, we find that β' ~ 0.03. These values are clearly in the low-β regime where the magnetic pressure dominates. They indicate that, like the solar corona, the magnetic field is channeling the plasma and appears to be a significant force on the matter in this region.