RadioAstron reveals a change in the jet collimation profile of 3C 84

¹ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, D–53121 Bonn, Germany
² Julius Maximilians University Würzburg, Faculty of Physics and Astronomy, Institute for Theoretical Physics and Astrophysics, Chair of Astronomy, Emil-Fischer-Str. 31, D-97074 Würzburg, Germany
³ Aalto University Department of Electronics and Nanoengineering, PL 15500, FI–00076 Aalto, Finland
⁴ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, FI–02540 Kylmälä, Finland
⁵ Dipartimento di Fisica e Astronomia, Università di Bologna, via Gobetti 93/2, I–40129 Bologna, Italy
⁶ INAF–IRA, Via Gobetti 101, I-40129 Bologna, Italy
⁷ INAF – Institute for Space Astrophysics and Planetology, via del Fosso del Cavaliere, 100, I-00133 Rome, Italy
⁸ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059 Valparaíso, Chile

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

Received: 22 January 2025
Accepted: 24 March 2026

Abstract

Context. Due to its brightness and proximity, the radio galaxy 3C 84 (optical counterpart NGC 1275 in the Perseus cluster) has been the target of extensive studies investigating the central parsec region of its active galactic nucleus. In 2003, its most recent active phase resulted in a plasma ejection visible in the southern jet, which presented a unique opportunity to study jet formation and evolution at high angular resolution with very long baseline interferometry (VLBI).

Aims. We aim to study the morphology, evolution, and spectral properties of the restarted jet three years after the first ultra-high angular resolution observations with the RadioAstron space-VLBI satellite in September 2013.

Methods. To study 3C 84, we used space-VLBI observations carried out in September 2016 at 22 GHz with a global VLBI network and the 10 m Spektr-R radio telescope in orbit as well as quasi-simultaneous multifrequency observations at 4.8, 8, 15, and 43 GHz from the Very Long Baseline Array, including the Effelsberg 100 m telescope.

Results. We present the 22 GHz RadioAstron image of 3C 84 from 2016, which reveals the source’s central region at a 58 μas effective resolution. During the three years that elapsed between the first and second space-VLBI observations, the source underwent significant morphological changes. We confirm the existence of the limb-brightened jet and counter-jet reported earlier as well as a flip in the position of the hotspot discovered recently via VLBI monitoring at 43 GHz. Based on measuring the collimation profile, we find that it has evolved from being quasi-cylindrical to parabolic. This is most likely the result of the decreased pressure of the mini-cocoon, which was inflated by the jet and contains hot gas that cannot confine the jet efficiently as it propagates further away from the core. Finally, we also constrained the magnetic field strength in the core region and the hotspot.