The JWST-NIRCam View of Sagittarius C. II. Evidence for Magnetically Dominated H ii Regions in the Central Molecular Zone
Journal article, 2025
We present JWST-NIRCam narrowband, 4.05 μm 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.
Author
J. Bally
University of Colorado at Boulder
S. Crowe
University of Virginia
R. Fedriani
Spanish National Research Council (CSIC)
A. Ginsburg
College of Liberal Arts and Sciences
Rainer Schödel
Spanish National Research Council (CSIC)
Morten Andersen
European Southern Observatory (ESO)
Jonathan Tan
University of Virginia
Chalmers, Space, Earth and Environment
Z. Y. Li
University of Virginia
Francisco Nogueras-Lara
European Southern Observatory (ESO)
Yu Cheng
National Astronomical Observatory of Japan
Chi-Yan Law
Arcetri Astrophysical Observatory
Q. D. Wang
University of Massachusetts
Yichen Zhang
Shanghai Jiao Tong University
University of Virginia
Suinan Zhang
Chinese Academy of Sciences
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 983 1 20Subject Categories (SSIF 2025)
Atom and Molecular Physics and Optics
Fusion, Plasma and Space Physics
Astronomy, Astrophysics, and Cosmology
DOI
10.3847/1538-4357/ad9d0b