Discovery of Sub- to Superluminal Motions in the M87 Jet: An Implication
  of the Acceleration from Sub-relativistic to Relativistic Speeds
  The velocity field of the M87 jet from milli-arcsecond (mas) to arcsecond
scales is extensively investigated together with new radio images taken by EVN
observations. We detected proper motions of components located at between 160
mas from the core and the HST-1 complex for the first time. Newly derived
velocity fields exhibits a systematic increase from sub-to-superluminal speed
in the upstream of HST-1. If we assume that the observed velocities reflect the
bulk flow, we here suggest that the M87 jet may be gradually accelerated
through a distance of 10^6 times of the Schwarzschild radius of the
supermassive black hole. The acceleration zone is co-spatial with the jet
parabolic region, which is interpreted as the collimation zone of the jet
(Asada & Nakamura 2012). The acceleration and collimation take place
simultaneously, which we suggest a characteristic of magnetohydrodynamic flows.
Distribution of the velocity field has a peak at HST-1, which is considered as
the site of over-collimation, and shows a deceleration downstream of HST-1
where the jet is conical. Our interpretation of the velocity map in the M87 jet
gives a hypothesis in AGNs that the acceleration and collimation zone of
relativistic jets extends over the whole scale within the sphere of influence
of the supermassive black hole.
