Ionized Gas Filamentation in Sgr C: Evidence for Magnetically Dominated HII Regions in the CMZ
We present evidence that the Sagittarius C (Sgr C) HII region, located in the Central Molecular Zone (CMZ) and spatially associated with the Sgr C star-forming region, is evolving under magnetically dominated conditions. Unlike any HII region in the Solar vicinity, the Sgr C plasma displays a remarkably filamentary structure in JWST-NIRCam Brα observations. The brightest of these filaments are also visible in the radio continuum with ALMA and MeerKAT, and 1 to 100 GHz spectral index measurements indicate thermal free-free emission. However, in-band 1 to 2 GHz spectral index measurements from MeerKAT alone imply the presence of a non-thermal synchrotron component across the entire HII region. We argue that the strong (~1 mG) CMZ magnetic fields have confined the plasma flow in Sgr C to rope-like filaments or sheets. This results in the measured non-thermal component of low-frequency radio emission, as well as a plasma β (thermal pressure divided by magnetic pressure) below 1, even in the densest regions. Corroborating this claim, we observe a statistically significant peak in the distribution of Brα filament orientations perpendicular to the galactic plane, or along the poloidal component of the global CMZ magnetic field. We speculate that all mature HII regions in the CMZ, and galactic nuclei in general, evolve in a magnetically dominated, low plasma β regime. Therefore, there is a pressing need to incorporate the impact of strong magnetic fields into the ‘standard’ models of HII region evolution, with potential consequences for our understanding of stellar feedback in extreme environments.
