The physical structure of radio galaxies explored with three-dimensional simulations

TL;DR

This study uses 3D hydrodynamic simulations to analyze how jet parameters affect the structure and energy transfer in radio galaxies, revealing new insights into cocoon formation and precession effects.

Contribution

It provides a comprehensive analysis of jet density, Mach number, and precession angle impacts on radio galaxy morphology and energetics, with novel findings on energy transfer and bow shock structures.

Findings

Jet density has limited influence on structure at fixed Mach number.

Energy transfer to ambient medium can reach 80%, mainly as thermal energy.

Wide cocoons can result from slow precession at low Mach numbers.

Abstract

We present a large systematic study of hydrodynamic simulations of supersonic adiabatic jets in three dimensions to provide a definitive set of results on exploring jet density, Mach number and precession angle as variables. We restrict the set-up to non-relativistic pressure-equilibrium flows into a homogeneous environment. We first focus on the distribution and evolution of physical parameters associated with radio galaxies. We find that the jet density has limited influence on the structure for a given jet Mach number. The speed of advance varies by a small factor for jet densities between 0.1 and 0.0001 of the ambient density while the cocoon and cavity evolution change from narrow pressure balanced to wide over-pressure as the ratio falls. We also find that the fraction of energy transferred to the ambient medium increases with decreasing jet-ambient density ratio, reaching approx 80%. This energy is predominantly in thermal energy with almost all the remainder in ambient kinetic form. The total energy remaining in the lobe is typically under 5\%. We find that radio galaxies with wide transverse cocoons can be generated through slow precession at low Mach numbers. We explore a slow precession model in which the jet direction changes very slowly relative to the jet flow dynamical time. This reveals two separated bow shocks propagating into the ambient medium, one associated with the entire lobe expansion and the other with the immediate impact zone. The lobes generated are generally consistent with observations, displaying straight jets but asymmetric lobes.