How do recollimation-induced instabilities shape the propagation of hydrodynamic relativistic jets?

¹ DiSAT, Università dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
² INAF – Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
³ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
⁴ Department of Astronomy, Yale University, PO Box 208101 New Haven, CT, 06520-8101 USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 March 2025
Accepted: 22 October 2025

Abstract

Context. Recollimation is a phenomenon of particular importance in the dynamical evolution of jets and in the emission of high-energy radiation. Additionally, the full comprehension of this phenomenon provides insights into fundamental properties of jets in the vicinity of the active galactic nucleus (AGN). Three-dimensional (3D)(magneto)hydrodynamic simulations revealed that the jet conditions downstream of recollimation shocks favor the growth of strong instabilities, challenging the traditional view – supported by two-dimensional (2D) simulations – of confined jets undergoing a series of recollimation and reflection shocks.

Aims. In order to investigate the stability of relativistic jets in AGNs at recollimation sites, we performed a set of long duration 3D relativistic hydrodynamic simulations, to focus on the development of hydrodynamical instabilities. We explored the nonlinear growth of the instabilities and their effects on the physical jet properties as a function of the initial jet parameters.

Methods. We performed 2D and 3D relativistic hydrodynamic simulations using the state-of-the-art PLUTO code. We assumed that an initially free-expanding jet is collimated by the external medium, and we explored the role of the jet Lorentz factor, temperature, opening angle, and jet-environment density-contrast in the jet deceleration and entrainment. The parameter space was designed to describe low-power, weakly magnetized jets at small distances from the core (around the parsec scale).

Results. All of the collimating jets that we simulate develop instabilities. Recollimation instabilities decelerate the jet, heat it, entrain external material, and move the recollimation point to shorter distances from the core. This is true for both conical and cylindrical jets. The instabilities, which are first triggered by the centrifugal instability, appear to be less disruptive in the case of narrower, denser, warmer, and more relativistic jets. These results provide valuable insights into the complex processes governing AGN jets and could be used to model the properties of low-power, weakly magnetized jetted AGNs.