# Detection of helicoidal motion in the optical jet of PKS 0521-365

## TL;DR

This study reports the discovery of helicoidal motions in the optical jet of the AGN PKS 0521-365, using spectroscopic observations that reveal sinusoidal velocity patterns indicative of complex jet dynamics at kiloparsec scales.

## Contribution

The paper presents the first detection of helicoidal motion in the optical jet of PKS 0521-365, expanding understanding of jet behavior at large scales in active galaxies.

## Key findings

- Helicoidal velocity pattern fitted by sinusoidal function.
- Detection of extended emission line regions with S-shaped morphology.
- Estimated ionized gas outflow mass of ~10^4 solar masses.

## Abstract

The jet activity of Active Galactic Nuclei (AGN), and its interaction with the interstellar medium (ISM), may play a pivotal role in the processes which regulate the growth and star formation of its host galaxy. Observational evidence which pinpoints the conditions of such interaction is paramount to unveil the physical processes involved. We report on the discovery of extended emission line regions exhibiting an S-shaped morphology along the optical jet of the radio-loud AGN $\text{PKS}\,0521-365$ ($z=0.055$), by using long-slit spectroscopic observations obtained with FORS2 on VLT. The velocity pattern derived from the $\text{[O II]}\,\lambda 3727\,\AA,\, \text{H}\beta\, \lambda 4861\,\AA\,$ and [O III] $\lambda\lambda4959,5007\,\AA\,$ emission lines is well-fitted by a sinusoidal function of the form: $v(r)=\alpha r^{1/2}sin(\beta r^{1/2}+\gamma)$, suggesting helicoidal motions along the jet up to distances of $20\,\text{kpc}$. We estimate a lower limit for the mass of the outflowing ionized gas along the jet of $\sim$10$^4\,M_\odot$. Helical magnetic fields and jet precession have been proposed to explain helicoidal paths along the jet at pc scales; nevertheless, it is not clear yet whether these hypotheses may hold at kpc scales.