Probing the formation of megaparsec-scale giant radio galaxies

I. Dynamical insights from magnetohydrodynamic simulations

¹ Department of Physics, University of Pretoria, Private Bag X20, Hatfield 0028 South Africa
² South African Radio Astronomy Observatory, 2 Fir St, Black River Park, Observatory 7925 South Africa
³ Department of Physics and Electronics, Christ University, Hosur Road, Bangalore 560029 India
⁴ Wits Centre for Astrophysics, School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg 2000 South Africa
⁵ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701 South Africa
⁶ Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Pune 411007 India

^⋆ Corresponding author; gourab.giri@up.ac.za

Received: 6 August 2024
Accepted: 15 November 2024

Abstract

Context. Constituting a relatively small fraction of the extended-jetted population, giant radio galaxies (GRGs) form in a wide range of jet and environment configurations. This observed diversity complicates the identification of the growth factors that facilitate their attainment of megaparsec scales.

Aims. This study aims to numerically investigate the hypothesized formation mechanisms of GRGs extending ≳1 Mpc in order to assess their general applicability.

Methods. We employed tri-axial ambient medium settings to generate varying levels of jet frustration and simulated jets with a low and a high power from different locations in the environment. This approach formulated five representations evolving under a relativistic magnetohydrodynamic framework.

Results. The emergence of distinct giant phases in all five simulated scenarios suggests that GRGs may be more common than previously believed. This prediction can be verified with contemporary and forthcoming radio telescopes. We find that different combinations of jet morphology, power, and evolutionary age of the formed structure hold the potential to elucidate different formation scenarios. In all of these cases, the lobes are overpressured, prompting further investigation into pressure profiles when jet activity ceases, potentially distinguishing between relic and active GRGs. We observed a potential phase transition in GRGs marked by differences in lobe expansion speed and pressure variations compared to their smaller evolutionary phases. This suggests the need for further investigation across a broader parameter space to determine if lobe evolution in GRGs fundamentally differs from smaller radio galaxies. The axial ratio analysis reveals self-similar expansion in rapidly propagating jets, while there is a notable deviation when the jet forms wider lobes. Overall, this study emphasizes that multiple growth factors simultaneously at work can better elucidate the current-day population of GRGs, including scenarios such as the growth of GRGs in dense environments, GRGs extending several megaparsecs, development of GRGs in low-powered jets, and the formation of morphologies such as GRG-XRGs.