Probing the formation of megaparsec-scale giant radio galaxies II. Continuum & polarization behavior from MHD simulations

TL;DR

This study advances the understanding of giant radio galaxy GRG emission by linking 3D RMHD dynamics to observable continuum and polarization signatures at 1 GHz. Using five diverse 3D RMHD simulations, it demonstrates how environment-induced morphologies (including wings and asymmetric cocoons) and jet power govern the evolution of radio brightness and magnetic-field structure, revealing filamentary, magnetized cocoons and complex backflow dynamics. The work shows that inverse-Compton cooling off the CMB and ambient-matter entrainment significantly affect long-term emission persistence, and that simple radio power–jet power relations fail to capture the diversity of GRG growth histories. By combining radiative maps with velocity and magnetic-field diagnostics, the paper highlights the potential of polarization and lateral-intensity metrics to infer GRG evolutionary states and motivates future, higher-resolution modeling with more realistic particle acceleration and cooling processes.

Abstract

The persistence of radiative signatures in giant radio galaxies remains a frontier topic of research, with contemporary telescopes revealing intricate features that require investigation. This study aims to examine the emission characteristics of simulated GRGs, and correlate them with their underlying 3D dynamical properties. Sky-projected continuum and polarization maps at 1 GHz were computed from five 3D-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 from different locations of the large-scale environment. The jet-cocoon morphologies are strongly shaped by the triaxiality of the environment, resulting in features like 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 Myr. 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 CMB cooling and matter entrainment significantly impact the long-term emission persistence of GRGs. The diminishing fractional polarization with GRG evolution reflects increasing turbulence in the cocoon.

Figures (16)

• Figure 1: Schematic representation of the jet-environment interaction framework developed to investigate the influence of factors on the development of giant radio galaxies. The setup simulates a one-sided relativistic jet propagating up to $\sim 700$ kpc through a stratified medium representing a galaxy-group environment, capturing the varying influence of the environment on the jet's motion. We also used two different jet powers to mimic Fanaroff-Riley type I and II jets, allowing us to examine the role of the jet's primary thrust in the development of such radio galaxies. This parameter space yields a total of five simulation scenarios, discussed in detail in Giri2025 and also concisely in Section \ref{['Sec:Revisiting Paper I']}.
• Figure 2: Sliced velocity maps ($x-y$ plane, $z=0$) for the resultant giant radio galaxy morphologies from five simulation cases (labels indicated in each plot) are shown, illustrating the underlying matter-transport mechanisms responsible for these structures. These maps are displayed at dynamical ages of $\sim$ 167 Myr (GRG_lp_min), 196 Myr (GRG_lp_maj), 49 Myr (GRG_hp_edge), 137 Myr (GRG_hp_maj), and 69 Myr (GRG_hp_min), as also highlighted in Giri2025. The colormap represents the $x-$component of velocity, capturing the bulk-flow (in blue) and back-flow (in red) dynamics, while the contours depict the $y-$component of velocity, with solid lines indicating $0.005c$ and dashed lines indicating $-0.005c$, to highlight the behavior of secondary structures forming within the domain. To analyze the impact of jet power on morphology and the influence of the tri-axial environment on the resulting structure, comparisons are made column-wise and row-wise, respectively.
• Figure 3: Initial density configuration of the ambient environment, illustrating a $10^{\circ}$ rotation of the ambient medium's principal axis, in order to introduce an asymmetry to the jet-flow, while the jet injection axis remains fixed along the negative x-direction. The density distribution is shown using colormap, with the corresponding $\log \rho [\rm amu/cc]$ values annotated along the contour lines. This represents one of the simulation scenarios employed, as shown in Fig. \ref{['Fig:Setup']}.
• Figure 4: A. Intensity maps (log $I_{\nu}$) at 1 GHz radio frequency are presented for two simulation cases: a low-power jet ("GRG_lp_min"; top) and a high-power jet ("GRG_hp_min"; bottom), both propagating along the minor axis of the ambient medium. While the overall structures resemble the dynamical maps from Giri2025, the emission maps offer deeper insights, highlighting crucial aspects of the observability of these simulated giant radio structures at their evolved stages ($\sim$ 167 Myr: top; $\sim$ 69 Myr: bottom).
• Figure 5: B. Continued. Intensity maps (log $I_{\nu}$) at 1 GHz radio frequency depicting the X-shaped radio galaxy in their giant stages, resulting from jet propagation along the direction of highest jet frustration (i.e., along the major axis direction). The low jet power case ("GRG_lp_maj"; top) and the high jet power case ("GRG_hp_maj"; bottom) exhibit distinct decollimation patterns, with both cases generating internal regions of emission valleys and enhanced emission ridges. The wings are luminous, indicating their active evolutionary stages ($\sim$ 196 Myr: top; $\sim$ 137 Myr: bottom).