Morphological transition of the compact radio lobe in 3C84 via the strong jet-cloud collision

TL;DR

This study uses multi-epoch VLBI observations to reveal how a dense cloud collision causes morphological changes and flux variations in the radio lobe of 3C84, demonstrating jet-cloud interaction effects in an active galactic nucleus.

Contribution

First direct detection of jet-cloud collision effects causing morphological transition in a radio galaxy's lobe at parsec scales.

Findings

Hotspot experienced a one-year frustration due to jet-cloud collision.

Radio lobe transitioned from FR II to FR I morphology.

Radio flux of the lobe decreased after the collision.

Abstract

We report multi-epoch Very Long Baseline Interferometric (VLBI) observations of the compact radio lobe in the radio galaxy 3C84 (NGC1275) during 2016 - 2020. The image sequence of 3C84 reveals that the hotspot in the radio lobe showed the one-year long frustration in 2017 within a compact region of about 0.07 parsec, suggesting a strong collision between the jet and a compact dense cloud with the estimated average density about $(4\-- 6)\times 10^{5}~{\rm cm^{-3}}$. Although the hotspot and the radio lobe began to move south again after its breakout, the radio lobe showed a morphological transition from FR II- to FR I-class radio lobe and its radio flux became fainter. This is the first detection of the dynamical feedback from the cloud to the jet where the cloud located on the jet axis significantly interferes with the jet propagation and evolution at the central one-parsec region in active galactic nucleus.