Asymmetries in Extragalactic Double Radio Sources: Clues from 3D Simulations of Jet - Disc Interaction

Observational and theoretical studies of extragalactic radio sources have suggested that an inhomogeneous environment may be responsible for observed arm length asymmetries of jets and the properties of extended emission line regions in high redshift radio galaxies. We perform 3D hydrodynamic simulations of the interaction of a powerful extragalactic bipolar jet with a disc-shaped clumpy interstellar medium of log-normal density distribution and analyze the asymmetry. Furthermore, we compute the relation between jet asymmetry and the ISM properties by means of Monte Carlo simulations based on a 1D propagation model for the jet through the dense medium. We find that the properties of the ISM can be related to a probability distribution of jet arm length asymmetries: Disc density and height are found to have the largest effect on the asymmetry for realistic parameter ranges, while the Fourier energy spectrum of the ISM and turbulent Mach number only have a smaller effect. The hydrodynamic simulations show that asymmetries generally may be even larger than expected from the 1D model due to the complex interaction of the jet and its bow shock with gaseous clumps, which goes much beyond simple energy disposal. From our results, observed asymmetries of medium-sized local radio galaxies may be explained by gas masses of 10^9 to 10^10 solar masses in massive elliptical galaxies. Furthermore, the simulations provide a theoretical basis for the observed correlation that emission line nebulae are generally found to be brighter on the side of the shorter lobe in high redshift radio galaxies (McCarthy et al. 1991). This interaction of jets with the cold gas phase suggests that star formation in evolving high redshift galaxies may be affected considerably by jet activity.

💡 Research Summary

The paper investigates why the two lobes of extragalactic double‑radio sources often have different lengths, focusing on the role of an inhomogeneous interstellar medium (ISM) in the host galaxy. The authors combine three‑dimensional (3‑D) hydrodynamic simulations of a powerful bipolar jet interacting with a clumpy, disc‑shaped ISM and a statistical analysis based on a one‑dimensional (1‑D) jet propagation model. The ISM is modeled as a log‑normal density field with a prescribed Fourier power spectrum, representing turbulent, multiphase gas typical of massive elliptical galaxies at high redshift. Four key parameters are varied: the mean disc density (ρ₀), disc scale height (H), turbulent Mach number (M), and the slope of the Fourier energy spectrum. By running thousands of Monte‑Carlo realizations of the 1‑D model, the authors obtain probability distributions for the arm‑length asymmetry ΔL (the difference in distance reached by the two jets).

The 3‑D simulations reveal that the actual asymmetries can be substantially larger than those predicted by the simple 1‑D model. When the jet’s bow shock encounters dense clumps, the shock is locally slowed, the jet head is compressed, and pressure gradients become highly asymmetric. Individual clumps can be destroyed, compressed, or deflected, leading to episodic delays that accumulate over the jet’s lifetime. This complex interaction produces a broader ΔL distribution and explains why the shorter lobe often coincides with brighter extended emission‑line regions, as observed in high‑z radio galaxies (McCarthy et al. 1991).

Quantitatively, the study finds that the mean disc density and the disc height dominate the asymmetry. Raising ρ₀ by an order of magnitude or doubling H typically increases the median ΔL by 30–50 %. In contrast, variations in the turbulent Mach number and the Fourier spectral slope affect ΔL by less than 10 %. The simulations suggest that realistic gas masses of 10⁹–10¹⁰ M⊙ in massive ellipticals can reproduce the arm‑length differences seen in medium‑sized local radio galaxies. This mass range is consistent with observations of cold molecular and atomic gas in such systems.

The authors discuss the broader astrophysical implications. The jet‑ISM interaction not only shapes radio morphology but also influences the ionisation state and brightness of the surrounding emission‑line nebulae. The bow shock’s compression of cold clouds can trigger localized star formation, while the destruction and heating of other clumps can suppress it. Thus, jet activity may have a dual role in the evolution of high‑redshift galaxies, both quenching and stimulating star formation depending on the local gas conditions.

Finally, the paper acknowledges limitations: the current simulations are purely hydrodynamic and neglect magnetic fields and radiative feedback, which are expected to be significant in real galaxies. Future work should incorporate magnetohydrodynamics (MHD) and detailed radiative transfer to assess how magnetic pressure and radiation pressure modify jet propagation and cloud survival. Such extensions would provide a more complete theoretical framework for interpreting the observed asymmetries, emission‑line properties, and star‑formation histories of powerful radio galaxies across cosmic time.