Abstract

On the one hand, the jets of Active Galactic Nuclei appear to be remarkably stable – if jets produced by aircraft engines were that stable they would be able to reach to the Moon. On the other hand, many of them eventually get disrupted, most likely due to some kind of instabilities. The reasons behind this behaviour remain unclear. In my talk I will describe one possible explanation, based on the causality principle and the fact that the jet surrounding is highly inhomogeneous. I will also present the results of our recent 3D simulations of relativistic hydro-jets supporting the idea. In the simulations, the key role is played by the centrifugal instability associated with the curved streamlines of the simulated jets and not by the Kelvin-Helmholtz instability normally analysed in stability studies of AGN jets. Motivated by this discovery, we explored the onset and evolution of the Relativistic Centrifugal Instability (RCFI) in the simpler case of axisymmetric rotating flows as well. Following the original physical argument of Lord Rayleigh, we derived the relativistic version of the Rayleigh criterion for this problem and used axially symmetric computer simulations to verify its predictions.