[Paper Review] Kinematics and Dynamics of kiloparsec-scale Jets in Radio Galaxies with SKA

R. A. Laing|arXiv (Cornell University)|Jan 2, 2015

Radio Astronomy Observations and Technology5 citations

TL;DR

This paper proposes using the Square Kilometre Array (SKA) to measure kinematics and dynamics of kiloparsec-scale jets in radio galaxies by modeling relativistic, intrinsically symmetrical flows from deep, polarized radio images in Stokes I, Q, and U. SKA1-MID will enable large-sample studies of weak-flavor jets, while SKA2 is required for high-resolution, high-fidelity imaging of strong-flavor jets to test for fast spines and magnetic confinement.

ABSTRACT

We explore the use of SKA to deduce the physical parameters of kiloparsec-scale jet flows in radio galaxies. Jets in Active Galactic Nuclei are relativistic where they are first formed, but their speeds and compositions change as they propagate. It has long been known that kiloparsec-scale jets in radio galaxies can be divided into two flavours: strong (found in powerful sources, narrow and terminating in compact hot-spots) and weak (found in low-luminosity sources, geometrically flaring, unable to form hot-spots and terminating in diffuse lobes or tails). We have developed methods to model AGN jets as intrinsically symmetrical, relativistic flows by fitting to deep, well-resolved radio images in Stokes I, Q and U. This has yielded a wealth of information about the brightest few weak-flavour jets. Our first key objective is to observe large samples of weak and transition jets at 0.1 - 0.5 arcsec resolution with SKA1-MID. This would allow us to see how jet propagation depends on power and environment and to quantify the energy and momentum input into the IGM. We will require typical noise levels of 1 microJy/beam, and may be able to exploit survey imaging in some cases. Our second, more challenging, application is to determine the velocity fields in strong-flavour jets. Do they have very fast spines with bulk Lorentz factors of 5 - 10? Is there evidence for magnetic confinement by a toroidal field? What are their energy fluxes? This is a major imaging challenge for SKA2: we need resolution better than 0.05 arcsec, ideally in the 1 - 10 GHz frequency range, with rms noise levels of roughly 10 nJy/beam and extremely high dynamic range, imaging fidelity and polarization purity.

Motivation & Objective

• To extend current modeling of weak-flavor jets to large samples using SKA1-MID, enabling statistical studies of jet propagation and energy input into the IGM.
• To resolve the long-standing uncertainty about the velocity structure of strong-flavor jets, particularly whether they host ultra-relativistic spines (Γ ≈ 5–10) surrounded by slower sheath flows.
• To determine the role of magnetic fields, external environment, and jet composition in shaping jet morphology and evolution on kpc scales.
• To quantify jet energy and momentum fluxes and their impact on the intergalactic medium, including heating and magnetic field modification.
• To test competing models of jet acceleration and confinement using high dynamic range, polarization-sensitive imaging with SKA2.

Proposed method

• Modeling kpc-scale jets as intrinsically symmetrical, relativistic flows by fitting observed Stokes I, Q, and U images to infer velocity fields, geometry, and 3D magnetic field topology.
• Using relativistic aberration effects on polarization to independently measure jet bulk Lorentz factors and inclination angles from the asymmetry in total intensity and linear polarization between approaching and receding jets.
• Applying the method to deep, well-resolved radio data to derive velocity fields, mass fluxes, and entrainment rates, assuming axisymmetric, stationary flows with average symmetry.
• Requiring high dynamic range (≥10⁷:1), sub-0.05 arcsec resolution, and rms noise ≤10 nJy/beam to image faint counter-jets in strong-flavor sources.
• Leveraging survey imaging and deep observations with SKA1-MID (0.1–0.5 arcsec resolution, 1 μJy/beam noise) for weak-flavor jet studies.
• Using VLBI proper-motion data and X-ray observations as constraints to test model predictions for jet Lorentz factors and emission mechanisms.

Experimental results

Research questions

• RQ1 What are the velocity fields in weak-flavor jets, and how do they relate to jet deceleration, brightness evolution, and the Fanaroff-Riley dichotomy?
• RQ2 How does jet composition (mass flux, entrainment rate) evolve with distance from the AGN in different jet types?
• RQ3 What is the magnetic field topology (ordered vs. disordered) on kpc scales, and how does it affect jet collimation and stability?
• RQ4 To what extent is jet propagation and confinement determined by external environmental conditions such as density, pressure, and magnetization?
• RQ5 Do strong-flavor jets host ultra-relativistic spines (Γ ≈ 5–10), and is their collimation maintained by magnetic confinement or external pressure?

Key findings

• SKA1-MID will enable large-sample studies of weak-flavor jets with 0.1–0.5 arcsec resolution and 1 μJy/beam noise, allowing statistical quantification of energy and momentum input into the IGM.
• SKA2 is required to achieve sub-0.05 arcsec resolution and 10 nJy/beam noise to image and model faint counter-jets in strong-flavor sources, enabling dynamic ranges ≥10⁷:1.
• Modeling of NGC 6251’s transition jet shows a strong velocity gradient across the jet, indicated by a sidedness ratio image, suggesting non-uniform bulk motion.
• VLBI proper-motion measurements imply jet Lorentz factors up to Γ ≈ 40 in pc-scale jets, making significant deceleration on kpc scales unlikely without disruption.
• X-ray emission in extended jets may require large bulk Lorentz factors (Γ ≳ 10) if due to inverse Compton scattering, supporting the presence of ultra-relativistic components.
• The counter-jet in 3C 334 requires ≈0.05 arcsec resolution and ≤10 nJy/beam noise to be imaged and modeled in linear polarization, a challenge even for SKA2.