The nature of extragalactic radio-jets from high-resolution radio-interferometric observations

TL;DR

This paper reviews high-resolution VLBI observations of extragalactic radio jets, providing evidence that observed wave-like structures are real features of the jets, not artifacts, and discusses their implications for understanding jet physics.

Contribution

It demonstrates that ridge-lines in VLBI images are genuine features of jets and explores their wave-like motions, advancing understanding of jet dynamics and instabilities.

Findings

Ridge-lines of radio jets are consistent across frequencies and epochs.

Observed wave-like motions suggest physical perturbations or instabilities.

High-resolution observations are essential for studying jet wave propagation.

Abstract

Extragalactic jets are a common feature of radio-loud active galaxies. The nature of the observed jets in relation to the bulk flow is still unclear. In particular it is not clear whether the observations of parsec-scale jets using the very long baseline interferometric technique (VLBI) reveal wave-like structures that develop and propagate along the jet, or trace the jet flow itself. In this contribution I review the evidence collected during the last years showing that the ridge-lines of helical radio-jets do not correspond to observational artifacts. This conclusion was reached by studying a number of VLBI observations of the radio jet in the quasar S5~0836+710 at different frequencies and epochs. The ridge-line of the emission in the jet coincides at all frequencies within the errors. Moreover, small differences between the ridge-lines as observed at different epochs reveal wave-like motion transversal to the jet propagation axis. I also discuss similar results, albeit with different interpretations, obtained by other authors. The current challenge is to measure the propagation velocities of these waves and to try to characterise them in terms of simple perturbations or Kelvin-Helmholtz instability, which would help understanding the physical conditions of the flow where the waves develop. This problem can only be tackled by high-resolution observations such as those that can be achieved by the space radio-antenna Radioastron.