Dynamics of powerful radio galaxies

TL;DR

The paper surveys analytical models for the dynamics of powerful FR-II radio galaxy lobes, identifying two central driving mechanisms: forward jet momentum flux and internal lobe pressure, and it emphasizes transitions between jet- and lobe-dominated regimes. It extends early homogeneous-environment models to non-uniform ambient media with density profiles $\rho = k r^{-eta}$, and it analyzes how jet collimation and adiabatic lobe expansion shape source growth, axis ratio, and jet-head pressure. The authors compare four main model classes—Scheuer 1974, Falle 1991, Hardcastle 2018, and RAiSE variants—against 3D hydrodynamic simulations to determine regimes of agreement and where relativistic jet dynamics are essential, providing open-source implementations and guidance for SKA-era population modeling. The work highlights that realistic ambient-density profiles and multi-phase jet–ISM interactions are crucial for accurate predictions of source morphology and luminosity, and it outlines a path toward next-generation analytical models that incorporate these complexities and energy losses for cosmic radio-population studies.

Abstract

Analytical models describing the dynamics of lobed radio sources are essential for interpretation of the tens of millions of radio sources that will be observed by the Square Kilometre Array and pathfinder instruments. We propose that historical models can be grouped into two classes in which the forward expansion of the radio source is driven by either the jet momentum flux or lobe internal pressure. The most recent generation of analytical models combines these limiting cases for a more comprehensive description. We extend the mathematical formalism of historical models to describe source expansion in non-uniform environments, and directly compare different model classes with each other, and with hydrodynamic numerical simulations. We quantify differences in predicted observable characteristics for lobed radio sources due to the different model assumptions for their dynamics. We make our code for the historical models analysed in this review openly available to the community.

Figures (8)

• Figure S1: Schematic of the dynamical model for the Scheuer+1974 Model A. We show a thin shocked gas shell between the contact discontinuity and bow shock as in Figure \ref{['fig:scheuer_dynamics']} of the original paper; however, the shocked gas is not explicitly considered in their model.
• Figure S2: Schematic of the dynamical model proposed by Falle+1991, and subsequently refined by others, including KA+1997 and Alexander+2006. The bow shock is assumed to expand in a self-similar manner (i.e., constant scaling to the lobe) but the energy associated with the shocked gas is not explicitly considered.
• Figure S3: Schematic of the Turner+2015 dynamical model for the lobe and shocked shell. This framework is also used by Turner+2023 for both their jet- and lobe-dominated expansion phases, albeit the lobe (shown in red) only forms once a critical length scale is reached. Taken from Figure 1 of Turner+2023.
• Figure S4: Schematic of the dynamical model proposed by Hardcastle+2018. We depict a conical jet, noting that in this model, the cross-sectional area at the jet head is related to the lobe volume/radius by a constant scaling factor $\kappa_1$, and hence, the dynamics of the jet are not critical to model behaviour.
• Figure S5: Comparison of the four classes of analytical models to the hydrodynamic simulation (grey shading) used by Turner+2023 (specifically, their Figure 3) to assess the success of their model near the commencement of lobe formation. Top panel: source length evolution. Middle panel: lobe axis ratio. Bottom panel: jet head pressure. The Scheuer+1974 and Falle+1991 class models are shown for a range of jet half-opening angles $\theta_j$, while the Hardcastle+2018 model is shown for a range of jet-head cross-sections $\varepsilon$. The RAiSE model Turner+2023 is shown for its optimised set of parameters.