G0.18-0.04: Interaction of Thermal and Nonthermal Radio Structures in the Arc Near the Galactic Center

Using a high-resolution radio-continuum image made at a frequency of 4.8 GHz, the authors discuss the radio source G0.18-0.04 located at the intersection of the galactic center Arc with the galactic plane. In this direction, the radio Arc is composed of a set of linear, nonthermal filamentary structures superimposed on a narrow, sickle-shaped, thermal feature which crosses them. The radio image suggests a physical interaction between the thermal and nonthermal structures. The authors argue that the magnetic field is dynamically important and estimate that its strength is very large, ≡10^-3G. Furthermore, they suggest that the ionization of G0.18-0.04 is caused by collisional ionization rather than ultraviolet photons from massive stars. The discussion presents possible ways in which some of the energy of the relativistic particles or the magnetic field in the nonthermal filaments might be extracted to ionize ambient gas in the galactic plane.