[Paper Review] Bridging Bondi and Event Horizon Scales: 3D GRMHD Simulations Reveal X-Shaped Radio Galaxy Morphology

Aretaios Lalakos, Ore Gottlieb+7 more|arXiv (Cornell University)|Feb 16, 2022

Astrophysical Phenomena and Observations58 references50 citations

TL;DR

This study presents the first 3D general-relativistic magnetohydrodynamic (GRMHD) simulation spanning a dynamic range of RB/Rg = 10³, showing that X-shaped radio galaxy morphology can emerge spontaneously from turbulent, magnetized accretion flows without requiring preexisting conditions like binary black holes or large-scale asymmetries. The simulation reveals that intermittent, wobbling jets—driven by variable magnetic flux accumulation and ram pressure from infalling gas—inflate misaligned cavities, forming X-shaped jet structures that resemble observed XRGs, with jet power reaching 2×10⁴⁴ erg s⁻¹ and accretion efficiency peaking at 150%.

ABSTRACT

X-shaped radio galaxies (XRGs) produce misaligned X-shaped jet pairs and make up $\lesssim10\%$ of radio galaxies. XRGs are thought to emerge in galaxies featuring a binary supermassive black hole ($ m SMBH$), $ m SMBH$ merger, or large-scale ambient medium asymmetry. We demonstrate that XRG morphology can naturally form without such special, preexisting conditions. Our 3D general-relativistic magnetohydrodynamic (GRMHD) simulation for the first time follows magnetized rotating gas from outside the $ m SMBH$ sphere of influence of radius $R_{ m B}$ to the $ m SMBH$ of gravitational radius $R_{ m g}$, at the largest scale separation $R_{ m B}/R_{ m g} = 10^3$ to date. Initially, our axisymmetric system of constant-density hot gas contains weak vertical magnetic field and rotates in an equatorial plane of a rapidly spinning $ m SMBH$. We seed the gas with small-scale $2\%$-level pressure perturbations. Infalling gas forms an accretion disk, and the $ m SMBH$ launches relativistically-magnetized collimated jets reaching well outside $R_{ m B}$. Under the pressure of the infalling gas, the jets intermittently turn on and off, erratically wobble, and inflate pairs of cavities in different directions, resembling an X-shaped jet morphology. Synthetic X-ray images reveal multiple pairs of jet-powered shocks and cavities. Large-scale magnetic flux accumulates on the $ m SMBH$, becomes dynamically important, and leads to a magnetically arrested disk state. The $ m SMBH$ accretes at $2\%$ of the Bondi rate ($\dot{M}\simeq2.4 imes10^{-3}M_{\odot}\,{ m yr}^{-1}$ for M87*), and launches twin jets at $\eta=150\%$ efficiency. These jets are powerful enough ($P_{ m jets}\simeq2 imes10^{44}\,{ m erg\,s}^{-1}$) to escape along the spin axis and end the short-lived jets state whose transient nature can account for the rarity of XRGs

Motivation & Objective

• . The study aims to investigate whether X-shaped radio galaxy (XRG) morphology can form spontaneously without requiring special initial conditions such as binary SMBHs or ambient medium asymmetries.
• The research addresses the challenge of bridging the vast scale separation between the Bondi radius (RB) and the gravitational radius (Rg) in GRMHD simulations of accretion flows.
• It seeks to determine whether transient, misaligned jet activity can naturally produce the X-shaped morphology observed in XRGs.
• The objective includes quantifying jet power, accretion efficiency, and cavity inflation dynamics in a self-consistent 3D GRMHD framework.

Proposed method

• . The simulation uses 3D general-relativistic magnetohydrodynamics (GRMHD) to model magnetized, rotating gas from outside the Bondi radius (RB) down to the event horizon scale (Rg), with RB/Rg = 10³.
• Initial conditions include a constant-density, rotating, weakly magnetized hot gas disk with 2%-level pressure perturbations to seed turbulence.
• The simulation evolves the system from t ≈ 0 to t ≳ 10⁵ Rg/c, tracking accretion, jet formation, and cavity inflation using relativistic MHD equations with GR effects.
• Synthetic X-ray images are generated from the simulation to compare with observations, identifying shock fronts and cavities from jet interactions.
• The model tracks magnetic flux accumulation on the black hole, identifying the onset of the magnetically arrested disk (MAD) state when ⟨φBH⟩ = 50.
• Jet power and accretion rate are calculated using the Blandford-Znajek (BZ) mechanism, with efficiency η = Pjets / (˙M c²) and accretion rate ˙M normalized to the Bondi rate ˙MB.

Experimental results

Research questions

• RQ1 . Can X-shaped radio galaxy morphology emerge spontaneously from turbulent, magnetized accretion flows without requiring preexisting conditions like binary black holes or ambient medium asymmetries?
• RQ2 What physical mechanisms drive the intermittent, wobbling jet activity that leads to misaligned cavity inflation?
• RQ3 How does the accumulation of large-scale magnetic flux on the black hole influence jet power and accretion efficiency?
• RQ4 To what extent do ram pressure from infalling gas and variable magnetic flux cause jet deflection and disruption?
• RQ5 Does the simulated jet morphology, including cavity inflation and shock structures, match observational features seen in XRGs like M87*?

Key findings

• . The simulation successfully bridges a dynamic range of RB/Rg = 10³, the largest to date in 3D GRMHD, enabling study of accretion from Bondi to event horizon scales.
• Intermittent, wobbling jets form due to variable magnetic flux on the black hole and high ram pressure from infalling gas, leading to misaligned cavity inflation.
• Synthetic X-ray images reveal multiple pairs of shocks and cavities at different orientations, mimicking the X-shaped morphology of observed XRGs.
• The black hole achieves a magnetically arrested disk (MAD) state with ⟨φBH⟩ = 50, leading to peak jet power of Pjets ≃ 2×10⁴⁴ erg s⁻¹ and accretion efficiency η = 150%.
• The mass accretion rate on the black hole is 2% of the Bondi rate (˙M ≃ 2.4×10⁻³ M⊙ yr⁻¹ for M87*), with 98% of infalling gas ejected in outflows.
• The transient nature of the intermittent jet phase—lasting ~10⁴–10⁵ Rg/c—provides a natural explanation for the rarity of XRGs, which constitute <10% of radio galaxies.