Probing the formation of megaparsec-scale giant radio galaxies

II. Continuum and polarization behavior from magneto-hydrodynamic simulations

¹ Istituto Nazionale di Astrofisica (INAF) – Istituto di Radioastronomia (IRA), via Gobetti 101, 40129 Bologna, Italy
² Department of Physics, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa
³ Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
⁴ Department of Physics and Electronics, Christ University, Hosur Road, Bangalore 560029, India
⁵ Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Pune 411007, India
⁶ Wits Centre for Astrophysics, School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg 2000, South Africa
⁷ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa

^⋆ Corresponding author: g.giri@ira.inaf.it

Received: 30 March 2025
Accepted: 3 October 2025

Abstract

Context. The persistence of radiative signatures in giant radio galaxies (GRGs ≳ 700 kpc) remains a frontier topic of research, with contemporary telescopes revealing intricate features that require investigation.

Aims. This study aims to examine the emission characteristics of simulated GRGs, and correlate them with their underlying three-dimensional dynamical properties.

Methods. Sky-projected continuum and polarization maps at 1 GHz were computed from five 3D relativistic magnetohydrodynamical (RMHD) simulations by integrating the synthesized emissivity data along the line of sight, with the integration path chosen to reflect the GRG evolution in the sky plane. The emissivities were derived from these RMHD simulations, featuring FR-I and FR-II jets injected at different locations of the large-scale environment and with propagation along varying jet frustration paths.

Results. Morphologies, such as widened lobes from low-power jets and collimated flows from high-power jets, are strongly shaped by the triaxiality of the environment, resulting in features such as wings and asymmetric cocoons, thereby making morphology a crucial indicator of GRG formation mechanisms. The decollimation of the bulk flow in GRG jets gives rise to intricate cocoon features, most notably filamentary structures–magnetically dominated threads with lifespans of a few mega-year. High jet power cases frequently display enhanced emission zones at mid-cocoon distances (alongside warmspots around the jet head), contradicting the interpretations of the GRG as a restarting source. In such cases, examining the lateral intensity variation of the cocoon may reveal the source’s state, with a gradual decrease in emission suggesting a low active stage. This study highlights that applying a simple radio power–jet power relation to a statistical GRG sample is unfeasible, as it depends on growth conditions of individual GRGs. Effects such as inverse-Compton cooling due to cosmic microwave background photons and matter entrainment significantly impact the long-term emission persistence of GRGs. The diminishing fractional polarization with GRG evolution reflects increasing turbulence, underscoring the importance of modeling this characteristic further, particularly for even larger-scaled sources.