The JWST-NIRCam View of Sagittarius C. II. Evidence for Magnetically Dominated H <scp>ii</scp> Regions in the Central Molecular Zone

Abstract We present JWST-NIRCam narrowband, 4.05 μ m Br α images of the Sgr C H ii region, located in the central molecular zone (CMZ) of the Galaxy. Unlike any H ii region in the solar vicinity, the Sgr C plasma is dominated by filamentary structure in both Br α and the radio continuum. Some bright filaments, which form a fractured arc with a radius of about 1.85 pc centered on the Sgr C star-forming molecular clump, likely trace ionization fronts. The brightest filaments form a “ π -shaped” structure in the center of the H ii region. Fainter filaments radiate away from the surface of the Sgr C molecular cloud. The filaments are emitting optically thin free–free emission, as revealed by spectral index measurements from 1.28 GHz (MeerKAT) to 97 GHz (Atacama Large Millimeter/submillimeter Array). But, the negative in-band 1 to 2 GHz spectral index in the MeerKAT data alone reveals the presence of a nonthermal component across the entire Sgr C H ii region. We argue that the plasma flow in Sgr C is controlled by magnetic fields, which confine the plasma to ropelike filaments or sheets. This results in the measured nonthermal component of low-frequency radio emission plasma, as well as a plasma β (thermal pressure divided by magnetic pressure) below 1, even in the densest regions. We speculate that all mature H ii regions in the CMZ, and galactic nuclei in general, evolve in a magnetically dominated, low plasma β regime.