Resonant Kelvin-Helmholtz modes in sheared relativistic flows

Abstract

We announce the discovery of a new kind of high‐order resonant Kelvin‐Helmholtz modes acting on relativistic sheared flows which have the largest linear growth rates and crucially dominate the global stability properties of the flow in the non‐linear regime. These modes were overlooked by previous studies that focused on non‐relativistic relative flow speeds, infinitely thin transition layers, or very‐low‐order perturbation modes. The new modes grow up very fast in the linear regime, form small‐scale shock fronts and then dissipate forming a relativistically hot sheath surrounding the jet core. The modification of the background flow by these modes leads to the stabilization of other disruptive modes of Kelvin‐Helmholtz instability. Numerical simulations show that high Lorentz‐factor jets developing these modes are exceptionally stable in the non‐linear regime. The potential connection of the development of these stabilizing modes in relativistic jets with the FRI/FRII morphological dichotomy of extragala...