Effects of Ambient Medium Profiles on the Evolution of Relativistic Magnetized Jets

Abstract

Relativistic radio-galaxy jets are conventionally divided into two morphological classes: FR~I and FR~II. Among them, FR~I jets have relatively low radio luminosities and edge-darkened morphologies, indicating they undergo substantial deceleration and disruption on sub-kiloparsec to kiloparsec scales, often developing a turbulent and plume-like morphology. Here, we investigate the evolution of relativistic magnetized jets in stratified ambient media using 2D and 3D relativistic magnetohydrodynamic simulations with several ambient profiles. When the jet and ambient pressures are initially balanced, a Bondi-like atmosphere confines the jet to a broad parabolic shape ($R_j\propto z^{0.649\pm0.003}$) and produces a series of recollimation shocks. Over-pressured jets instead produce a conical shape and allow the development of the current-driven kink instability in the downstream. Based on the simulations, we derive a kink-stability criterion that depends primarily on the jet power and ambient pressure. We also calculate multi-frequency emission maps using a special relativistic radiative transfer code. Each recollimation shock appears as a bright knot, resembling the knot complex downstream of HST-1 of the M87 jet, whereas a jet with a single recollimation shock develops a bright feature close to its base and thus exhibits an FR~I-like morphology.