A Numerical Model of Hercules A by Magnetic Tower: Jet/Lobe Transition, Wiggling, and the Magnetic Field Distribution

TL;DR

This paper uses magnetohydrodynamic modeling to explore the complex jet and lobe structures, magnetic fields, and shock interactions in the radio galaxy Hercules A, revealing insights into jet stability and magnetic configurations.

Contribution

It introduces a detailed MHD model of Hercules A's jet/lobe transition, magnetic field distribution, and shock dynamics, advancing understanding of magnetic tower jets in galaxy clusters.

Findings

Magnetic pressure balances ambient gas pressure to determine lobe size.

Jet confinement is influenced by external pressure and gravity within the cluster core.

Magnetic kink instability occurs beyond the core radius, affecting jet stability.

Abstract

We apply magnetohydrodynamic (MHD) modeling to the radio galaxy Hercules A for investigating 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 underlying ambient gas plays an important role in jet-lobe morphology. The balance between the magnetic pressure generated by 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 R_c, while outside R_c 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 and it visibly grows beyond R_c where a separation between the jet forward and return currents occurs. The reversed pinch profile of global magnetic field associated with the jet and lobes produces projected B-vector distributions aligned with the jet flow and the lobe edge. 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 the observations of Hercules A.