Extragalactic jets with helical magnetic fields: relativistic MHD simulations

Context. Extragalactic jets are inferred to harbor dynamically important, organized magnetic fields which presumably aid in the collimation of the relativistic jet flows. We here explore by means of grid-adaptive, high resolution numerical simulations the morphology of AGN jets pervaded by helical field and flow topologies. We concentrate on morphological features of the bow shock and the jet beam behind the Mach disk, for various jet Lorentz factors and magnetic field helicities. Aims. We investigate the influence of helical magnetic fields on jet beam propagation in overdense external medium. We adopt a special relativistic magnetohydrodynamic (MHD) viewpoint on the shock-dominated AGN jet evolution. Due to the Adaptive Mesh Refinement (AMR), we can concentrate on the long term evolution of kinetic energy dominated jets, with beam-averaged Lorentz factor Γ ' 7, as they penetrate into denser clouds. These jets have near-equipartition magnetic fields (with the thermal energy), and radially varying Γ(R) profiles within the jet radius R < R j maximally reaching Γ ∼ 22. Methods. We use the AMRVAC code, employing a novel hybrid block-based AMR strategy, to compute ideal plasma dynamics in special relativity. We combine this with a robust second-order shock-capturing scheme and a diffusive approach for controlling magnetic monopole errors. Results. We find that the propagation speed of the bow shock systematically exceeds the value expected from estimates using beamaverage parameters, in accord with the centrally peaked Γ(R) variation. The helicity of the beam magnetic field is effectively transported down the beam, with compression zones in between diagonal internal cross-shocks showing stronger toroidal field regions. In comparison with equivalent low-relativistic jets (Γ ' 1.15) which get surrounded by cocoons with vortical backflows filled by mainly toroidal field, the high speed jets demonstrate only localized, strong toroidal field zones within the backflow vortical structures. The latter are ring-like due to our axisymmetry assumption and may further cascade to smallscale in 3D. We find evidence for a more poloidal, straight field layer, compressed between jet beam and backflows. This layer decreases the destabilizing influence of the backflow on the jet beam. In all cases, the jet beam contains rich cross-shock patterns, across which part of the kinetic energy gets transferred. For the high speed reference jet considered here, significant jet deceleration only occurs beyond distances exceeding O(100R j), as the axial flow can reaccelerate downstream to the internal cross-shocks. This reacceleration is magnetically aided, due to field compression across the internal shocks which pinch the flow.