THE STRUCTURE OF THE M87 JET: A TRANSITION FROM PARABOLIC TO CONICAL STREAMLINES

The structure of the M87 jet, from milliarcsecond to arcsecond scales, is extensively investigated, utilizing the images taken with the European VLBI Network, MERLIN, and Very Long Baseline Array. We discover that the jet maintains a parabolic streamline over a range in size scale equal to 10 super(5) times the Schwarzschild radius. The jet then transitions into a conical shape farther 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. Afterward the jet then freely expands with a conical shape. This geometrical transition indicates that the origin of the HST-1 complex may be a consequence of the overcollimation of the jet. Our result suggests that when even higher angular resolution is provided by a future submillimeter very long baseline interferometry experiments, we will be able to explore the origin of active galactic nucleus jets.