Very Light Magnetized Jets on Large Scales- I. Evolution and Magnetic Fields

Abstract

Magnetic fields, which are undoubtedly present in extragalactic jets and responsible for the observed synchrotron radiation, can affect the morphology and dynamics of the jets and their interaction with the ambient cluster medium. We examine the jet propagation, morphology and magnetic field structure for a wide range of density contrasts, using a globally consistent setup for both the jet interaction and the magnetic field. The MHD code NIRVANA is used to evolve the simulation, using the constrained-transport method. The density contrasts are varied between η = 10 −1 and 10 −4 with constant sonic Mach number 6. The jets are supermagnetosonic and simulated bipolarly due to the low jet densities and their strong backflows. The helical magnetic field is largely confined to the jet, leaving the ambient medium nonmagnetic. We find magnetic fields with plasma β ∼ 10 already stabilize and widen the jet head. Furthermore they are efficiently amplified by a shearing mechanism in the jet head and are strong enough to damp Kelvin–Helmholtz instabilities of the contact discontinuity. The cocoon magnetic fields are found to be stronger than expected from simple