Letter to the Editor

FR0 jets and recollimation-induced instabilities

¹ DiSAT, Università dell’ Insubria, Via Valleggio 11, 22100 Como, Italy
e-mail: agnese.costa@inaf.it
² INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
³ INAF – Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
⁴ Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
⁵ Dipartimento di Fisica, Università degli Studi di Genova, Via Dodecaneso 33, 16146 Genova, Italy
⁶ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁷ Dipartimento di Fisica e Astronomia, Università di Bologna, Via P. Gobetti 93, 40129 Bologna, Italy

Received: 14 December 2023
Accepted: 22 January 2024

Abstract

Context. The recently discovered population of faint Fanaroff-Riley type 0 (FR0) radio galaxies has been interpreted as the extension to low power of the classical FRI sources. Their radio emission appears to be concentrated in very compact parsec scale cores, any extended emission is very weak or absent, and very-long-baseline interferometry (VLBI) observations show that jets are already mildly or sub-relativistic at parsec scales. Based on these observational properties, we propose here that the jets of FR0s are strongly decelerated and disturbed at the parsec scale by hydrodynamical instabilities.

Aims. With the above scenario in mind, we studied the dynamics of a low-power relativistic jet propagating into a confining external medium, focusing on the effects of entrainment and mixing promoted by the instabilities developing at the jet-environment interface downstream of a recollimation shock.

Methods. We performed a 3D relativistic hydrodynamical simulation of a recollimated jet by means of the state-of-the-art code PLUTO. The jet was initially conical, relativistic (with an initial Lorentz factor Γ = 5), cold, and light with respect to the confining medium, whose pressure is assumed to slowly decline with distance. The magnetic field is assumed to be dynamically unimportant.

Results. The 3D simulation shows that, after the first recollimation and reflection shock system, a rapidly growing instability develops, as a result of the interplay between recollimation-induced instabilities and Richtmyer-Meshkov modes. In turn, the instabilities promote strong mixing and entrainment that rapidly lead to the deceleration of the jet and spread its momentum to slowly moving, highly turbulent external gas. We argue that this mechanism could account for the peculiarities of the low-power FR0 jets. For outflows with a higher power, Lorentz factor, or magnetic field, we expect that the destabilizing effects are less effective, allowing the survival of the jet up to the kiloparsec scale, as observed in FRIs.