Letter to the Editor

Unravelling the dynamics of cosmic vortices: Probing a Kelvin-Helmholtz instability in the jet of 3C 84

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
² Department of Physics, National and Kapodistrian University of Athens, Panepistimiopolis, 15783 Zografos, Greece
³ Institut für Theoretische Physik, Goethe Universität Frankfurt, Max-von-Laue-Str.1, 60438 Frankfurt am Main, Germany

^⋆ Corresponding author; gfparaschos@mpifr-bonn.mpg.de

Received: 20 February 2025
Accepted: 18 March 2025

Abstract

Understanding the creation of relativistic jets originating from active galactic nuclei (AGNs) requires a thorough understanding of the accompanying plasma instabilities. Our high-sensitivity, high-resolution, global, very-long-baseline-interferometry observations of the jet in the radio galaxy 3C 84 have enabled us to study its inner morphology. We find that its thread-like pattern can be described by a Kelvin-Helmholtz instability, consisting of four instability modes. Our model favours a jet described by a Mach number of M_j = 5.0 ± 1.7 and a sound speed of α_j = 0.14 ± 0.06. On this basis, we are able to describe the internal structure of 3C 84 and tentatively connect the origin of the instability to disc accretion activity.