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

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 pressureequilibrium 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 ≈ 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.

The morphological classification of distant radio galaxies explored with three-dimensional simulations

“…6. The relevant time scale to generate these images for a GRG as described by Donohoe & Smith (2016) is 290 Myr (for a GRG at 10 simulation time units).…”

“…The changing of the jet direction is implemented through a smooth precession. The precession is calculated in the same way as covered in Donohoe & Smith (2016) by adding in a periodic function of the global time into the velocity components of the jet input. As seen from Donohoe & Smith (2016), the precession spreads the jet energy perpendicular to the propagation axis.…”

“…The precession is calculated in the same way as covered in Donohoe & Smith (2016) by adding in a periodic function of the global time into the velocity components of the jet input. As seen from Donohoe & Smith (2016), the precession spreads the jet energy perpendicular to the propagation axis. There is also a significant increase in the number of secondary shocks generated as the precessing jet catches up to the previous shock impact region.…”

“…However, to be able to relate intrinsic dynamical properties to the diverse shapes of radio galaxies requires a large systematic exploration rather than one-off comparisons. In this programme, we have sought to achieve this by first generating systematically a large set of simulations using E-mail: m.d.smith@kent.ac.uk † E-mail: donohoejustin@googlemail.com three dimensional hydrodynamics (see Paper 1: Donohoe & Smith 2016). We continue now by deriving the observational properties, covering a range in Mach number, density contrast and precession properties.…”

The flickering nuclear activity of Fornax A

“…The central emission may have been generated by precessing jets. S-symmetric bent jets are a typical signature for this mechanism (Monceau- Baroux et al 2014;Donohoe & Smith 2016;Krause et al 2019).…”