  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.
