Numerical Investigation of Dynamical and Morphological Trends in Relativistic Jets

TL;DR

This study uses relativistic hydrodynamic simulations to explore how AGN jet properties and their surrounding environments influence jet morphology and observable features, explaining the FR I/II dichotomy and jet knots.

Contribution

The paper introduces a comprehensive simulation framework linking jet dynamics, morphology, and environment, reproducing observed AGN features and the FR I/II classification.

Findings

Reproduces FR I and FR II morphologies based on jet energy density.

Identifies recollimation shocks as origins of jet knots.

Derives a scaling relation between knots and energy density ratio.

Abstract

Active galactic nuclei (AGN) show a range of morphologies and dynamical properties, which are determined not only by parameters intrinsic to the central engine but also their interaction with the surrounding environment. We investigate the connection of kiloparsec scale AGN jet properties to their intrinsic parameters and surroundings. This is done using a suite of 40 relativistic hydrodynamic simulations spanning a wide range of engine luminosities and opening angles. We explore AGN jet propagation with different ambient density profiles, including $r^{-2}$ (self-similar solution) and $r^{-1}$, which is more relevant for AGN host environments. The Fanaroff-Riley (FR) morphological dichotomy arises naturally in our models. Jets with low energy density compared to the ambient medium produce a center-brightened emissivity distribution, while emissivity from relatively higher energy density jets is dominated by a terminal bright spot. We observe recollimation shocks in our simulations that can generate bright spots along the spine of the jet, providing a possible explanation for "knots" observed in AGN jets. We additionally find a scaling relation between the number of knots and the jet-head-to-surroundings energy density ratio. This scaling relation is generally consistent with the observations of the jets in M87 and Cygnus A. Our model also correctly predicts M87 as FR I and Cygnus A as FR II. Our model can be used to relate jet dynamical parameters such as jet head velocity, jet opening angle, and external pressure to jet power and ambient density estimates.