Very Light Extragalactic Jets with Magnetic Fields

We explore the global structure and evolution of powerful radio sources located in clusters of galaxies and their interaction with the ambient gas, in particular with respect to the effects of magnetic fields. Recent observations of inverse-Compton emission from their cocoons at X-ray energies indicate that magnetic fields are present on a considerable (near-equipartition) level. To investigate the impact of magnetic fields on dynamics and morphology, we performed a series of magnetohydrodynamical simulations of bipolar jets, considering a wide range of density contrasts between the jet and the ambient gas and employing a globally consistent setup of the magnetic field and the jet-environment interaction. We find that already sub-equipartition fields (beta ~ 10) stabilize the contact surface between the jet plasma and the ambient gas, resulting in pronounced jet heads and considerably suppressed entrainment. We identify a new shearing mechanism in the jet head, which efficiently amplifies magnetic fields and transfers part of the huge kinetic jet power to magnetic energy. We compare the propagation and shapes of the bow shocks and cocoons with self-similar models, finding deviations for the cocoon width evolution for sources approaching pressure balance with the environment. The simulations exhibit round and weak bow shocks for low jet densities, consistent with X-ray observations in galaxy clusters. Turbulent motion in the cocoon produces waves and ripples in the shocked ambient gas, and hereby provides a physical explanation for those recently found in Perseus A. We compute emission maps for synchrotron, inverse-Compton and bremsstrahlung emission for our simulation data, yielding overall agreement with observed sources within the assumed simplifications. Furthermore, two models for the emission-line nebulae in high-redshift radio galaxies are applied to the simulations, finding that none of them in their considered versions can explain all observed properties yet