A NUMERICAL MODEL OF HERCULES A BY MAGNETIC TOWER: JET/LOBE TRANSITION, WIGGLING, AND THE MAGNETIC FIELD DISTRIBUTION

We apply magnetohydrodynamic (MHD) modeling to the radio galaxy Hercules A to investigate the jet-driven shock, jet/lobe transition, wiggling, and magnetic field distribution associated with this source. The model consists of magnetic tower jets in a galaxy cluster environment, which has been discussed in a series of our papers. The profile of the underlying ambient gas plays an important role in the jet/lobe morphology. The balance between the magnetic pressure generated by the axial current and the ambient gas pressure can determine the lobe radius. The jet body is confined jointly by the external pressure and gravity inside the cluster core radius Rc, while outside Rc it expands radially to form fat lobes in a steeply decreasing ambient thermal pressure gradient. The current-carrying jets are responsible for generating a strong, tightly wound helical magnetic field. This magnetic configuration will be unstable against the current-driven kink mode, which visibly grows beyond Rc, where a separation between the jet forward and return currents occurs. The reversed pinch profile of the global magnetic field associated with the jet and lobes produces projected B-vector distributions aligned with the jet flow and the lobe edge. An AGN-driven shock powered by the expanding magnetic tower jet surrounds the jet/lobe structure and heats the ambient ICM. The lobes expand subsonically; no obvious hot spots are produced at the heads of lobes. Several key features in our MHD modeling may be qualitatively supported by observations of Hercules A.