Nonthermal Pressures: Key to Energy Balance and Structure Formation near Sgr A* in the Milky Way

Abstract The circumnuclear region of the Galactic center offers a unique laboratory to study energy balance and structure formation around Sgr A ⋆ . This work investigates thermal and nonthermal processes within a 7 pc distance from Sgr A ⋆ . Using MeerKAT 1.3 GHz radio continuum data and Atacama Large Millimeter/submillimeter Array H40 α radio recombination line emission from the ACES survey, we separate free–free and synchrotron components at ∼0.2 pc resolution. With a thermal fraction of ≃13%, the 1.3 GHz emission shows tight correlations with the Herschel PACS IR data. The correlation between the equipartition magnetic field and molecular gas traced by JCMT 12 CO ( J = 3 → 2) observations reveals a balance between the magnetic field, cosmic rays, and molecular gas pressures south of the circumnuclear disk on ∼0.7 pc scales. Unlike the magnetic field and ionized gas, the molecular gas density declines in the cavity ( R ≤ 2 pc) toward the center, likely due to feedback from Sgr A ⋆ . We find that nonthermal pressure from turbulent gas nearly balances magnetic and cosmic-ray pressures and exceeds the thermal pressure by 2 orders of magnitude. The medium surrounding Sgr A ⋆ is filled by a low- β (thermal-to-magnetic energy ratio), supersonic plasma, with an Alfvén Mach number ≃ 4 (assuming equipartition). Analysis of the mass-to-magnetic flux ratio suggests that the circumnuclear region is mostly subcritical and, therefore, the magnetic field can help stabilize gas clouds against gravitational collapse.