The Structure of the M87 Jet: A Transition from Parabolic to Conical Streamlines

The structure of the M87 jet, from milli-arcsec to arcsecond scales, is extensively investigated, utilizing the images taken with the EVN, MERLIN and VLBA. We discover that the jet maintains a parabolic streamline over a range in sizescale equal to $10^{5}$ times the Schwarzschild radius. The jet then transitions into a conical shape further downstream. This suggests that the magnetohydrodynamic jet is initially subjected to the confinement by the external gas which is dominated by the gravitational influence of the supermassive black hole. Afterwards the jet is then freely expanding with a conical shape. This geometrical transition indicates that the origin of the HST-1 complex may be a consequence of the over-collimation of the jet. Our result suggests that when even higher angular resolution is provided by a future submm VLBI experiment, we will be able to explore the origin of active galactic nuclei jets.