  Jets powered by active galactic nuclei appear impressively stable compared
with their terrestrial and laboratory counterparts-they can be traced from
their origin to distances exceeding their injection radius by up to a billion
times. However, some less energetic jets get disrupted and lose their coherence
on the scale of their host galaxy. Quite remarkably, on the same scale, these
jets are expected to become confined by the thermal pressure of the
intra-galactic gas. Motivated by these observations, we have started a
systematic study of active galactic nuclei jets undergoing reconfinement via
computer simulations. Here, we show that in the case of unmagnetized
relativistic jets, the reconfinement is accompanied by the development of an
instability and transition to a turbulent state. During their initial growth,
the perturbations have a highly organized streamwise-oriented structure,
indicating that it is not the Kelvin-Helmholtz instability, the instability
which has been the main focus of the jet stability studies so far. Instead, it
is closely related to the centrifugal instability. This instability is likely
to be behind the division of active galactic nuclei jets into two morphological
types in the Fanaroff-Riley classification.
