How Plasma Properties of the Fanaroff-Riley Jet can Shape its Morphology

TL;DR

This study investigates the FR I/II dichotomy of extragalactic jets by performing large-scale 3D MHD simulations of low-power, subrelativistic jets with a toroidal magnetic field injected into a stratified ambient medium and governed by a multispecies, relativistically correct equation of state. Six models (REF,HYP,HOT,MAG,CMp5,CMp2) explore how injection velocity, ambient temperature, magnetization, and plasma composition affect large-scale morphology and the associated synchrotron emission, revealing that composition and Mach number strongly influence whether jets develop diffuse FR I-type structures or terminal-hot-spot FR II features. A kink-instability criterion and the advection- versus kink-growth timescales are used to interpret the temporal evolution of jet heads, linking physical conditions to observed FR I/II transitions. The findings imply that jet composition and environment can drive morphological transitions even at kpc scales, with potential observational signatures in synthetic radio maps.

Abstract

Extragalactic jets are broadly classified into two categories based on radio observations: core-brightened jets, known as Fanaroff-Riley Type I (FR I), and edge-brightened jets, classified as Type II (FR II). This FR dichotomy may arise due to variation in the ambient medium and/or the properties of the jet itself, such as injection speed, temperature, composition, magnetization, etc. To investigate this, we perform large-scale three-dimensional magnetohydrodynamic (3D-MHD) simulations of low-power, supersonic jets extending to kiloparsec scales. We inject a jet beam carrying an initially toroidal magnetic field into a denser, unmagnetized, and stratified ambient medium through a cylindrical nozzle. Our simulations explore jets with varying injection parameters to investigate their impact on morphology and emission properties. Furthermore, we examine jets with significantly different plasma compositions, such as hadronic and mixed electron-positron-proton configurations, to study the conditions that may drive transitions between FR I and FR II morphologies. We find that, under the same injection parameters, mixed plasma composition jets tend to evolve into FR I structures. In contrast, electron-proton jets exhibit a transition between FR I and FR II morphologies at different stages of their evolution.

Figures (11)

• Figure 1: Jet morphology in X-Z plane for Model-REF at various times $t=10.5,25.5,38.5,49.5\times10^3$ in different row panels. The plots are tracer distributions on the left and log density contours on the right, overlaid with velocity vectors (black arrows). The black dashed line in the left panel shows the Mach-1 surface. The length is measured in units of the jet radius $r_j$.
• Figure 2: Synthetic synchrotron $I(x,z)$ map for Model-REF at $t=38.5,49.5\times10^3$
• Figure 3: Top: Jet morphology in X-Z plane for Model-HYP at various times $t=6.0,9.0\times10^3$ in different row panels. The plot variables are the same as in Figure \ref{['fig:REFframes']}. Bottom: Synthetic synchrotron $I(x,z)$ map for Model-HYP at $t=9.0\times10^3$
• Figure 4: Volume rendering of the tracer distributions for Model-HOT jet at time $t=83.0\times10^3$. (An animation of the figure is available via https://youtu.be/QakcH_pg8U8) The animation shows the propagation of the jet up to $t=85.0\times10^3$, including a camera rotation around the z-axis at $t=41.0\times10^3$ that highlights the development of the kink instability, followed by a spiral zoom at $t=85.0\times10^3$ showing the diffused jet beam.
• Figure 5: Jet morphology in X-Z plane for Model-HOT at various times $t=15.0,35.0,53.5,85.0\times10^3$ in different panels. The plots in the upper two rows are tracer distributions showing the diffusion of the jet beam overlaid with magnetic field streamlines (in cyan color). The bottom row is the synthetic synchrotron $I(x,z)$ map showing an FR I morphology.