# A transition from parabolic to conical shape as a common effect in nearby AGN jets

## TL;DR

This study identifies a common transition from parabolic to conical shapes in nearby AGN jets, suggesting a universal effect linked to jet energetics and ambient medium conditions, based on VLBA observations of 367 AGN.

## Contribution

It provides the first systematic search for shape transitions in a large AGN sample, revealing their prevalence and proposing a physical model for the transition mechanism.

## Key findings

- 10 out of 29 nearby AGN show shape transition
- Break occurs at 10^5-10^6 gravitational radii
- Jet shape transition linked to energy flux balance

## Abstract

Observational studies of collimation in jets in active galactic nuclei (AGN) are a key to understanding their formation and acceleration processes. We have performed an automated search for jet shape transitions in a sample of 367 AGN using VLBA data at 15 GHz and 1.4 GHz. This search has found ten out of 29 nearby jets at redshifts z<0.07 with a transition from a parabolic to conical shape, while the full analyzed sample is dominated by distant AGN with a typical z about 1. The ten AGN are UGC00773, NGC1052, 3C111, 3C120, TXS0815-094, Mrk180, PKS1514+00, NGC6251, 3C371, and BL Lac. We conclude that the geometry transition may be a common effect in AGN jets. It can be observed only when sufficient linear resolution is obtained. Supplementing these results with previously reported shape breaks in the nearby AGN 1H0323+342 and M87, we estimate that the break occurs at 10^5-10^6 gravitational radii from the nucleus. We suggest that the jet shape transition happens when the bulk plasma kinetic energy flux becomes equal to the Poynting energy flux, while the ambient medium pressure is assumed to be governed by Bondi accretion. In general, the break point may not coincide with the Bondi radius. The observational data supports our model predictions on the jet acceleration and properties of the break point.