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The velocity field of the M87 jet from milli-arcsecond (mas) to arcsecondscales is extensively investigated together with new radio images taken by EVNobservations. We detected proper motions of components located at between 160mas from the core and the HST-1 complex for the first time. Newly derivedvelocity fields exhibits a systematic increase from sub-to-superluminal speedin the upstream of HST-1. If we assume that the observed velocities reflect thebulk flow, we here suggest that the M87 jet may be gradually acceleratedthrough a distance of 10^6 times of the Schwarzschild radius of thesupermassive black hole. The acceleration zone is co-spatial with the jetparabolic region, which is interpreted as the collimation zone of the jet(Asada & Nakamura 2012). The acceleration and collimation take placesimultaneously, which we suggest a characteristic of magnetohydrodynamic flows.Distribution of the velocity field has a peak at HST-1, which is considered asthe site of over-collimation, and shows a deceleration downstream of HST-1where the jet is conical. Our interpretation of the velocity map in the M87 jetgives a hypothesis in AGNs that the acceleration and collimation zone ofrelativistic jets extends over the whole scale within the sphere of influenceof the supermassive black hole.

Discovery of Sub- to Superluminal Motions in the M87 Jet: An Implication of the Acceleration from Sub-relativistic to Relativistic Speeds