Context : Very long baseline interferometry ( VLBI ) imaging of radio emission from extragalactic jets provides a unique probe of physical mechanisms governing the launching , acceleration , and collimation of relativistic outflows .

Aims : VLBI imaging of the jet in the nearby active galaxy M 87 enables morphological and kinematic studies to be done on linear scales down to \sim 100 Schwarzschild radii ( R _ { \mathrm { s } } ) .

Methods : The two-dimensional structure and kinematics of the jet in M 87 ( NGC 4486 ) have been studied by applying the Wavelet-based Image Segmentation and Evaluation ( WISE ) method to 11 images obtained from multi-epoch Very Long Baseline Array ( VLBA ) observations made in January-August 2007 at 43 GHz ( \lambda = 7 mm ) .

Results : The WISE analysis recovers a detailed two-dimensional velocity field in the jet in M 87 at sub-parsec scales .
The observed evolution of the flow velocity with distance from the jet base can be explained in the framework of MHD jet acceleration and Poynting flux conversion .
A linear acceleration regime is observed up to z _ { obs } \sim 2 mas .
The acceleration is reduced at larger scales , which is consistent with saturation of Poynting flux conversion .
Stacked cross correlation analysis of the images reveals a pronounced stratification of the flow .
The flow consists of a slow , mildly relativistic layer ( moving at \beta \sim 0.5 c ) , associated either with instability pattern speed or an outer wind , and a fast , accelerating stream line ( with \beta \sim 0.92 , corresponding to a bulk Lorentz factor \gamma \sim 2.5 ) .
A systematic difference of the apparent speeds in the northern and southern limbs of the jet is detected , providing evidence for jet rotation .
The angular velocity of the magnetic field line associated with this rotation suggests that the jet in M87 is launched in the inner part of the disk , at a distance r _ { 0 } \sim 5 R _ { \mathrm { s } } from the central engine .

Conclusions : The combined results of the analysis imply that MHD acceleration and conversion of Poynting flux to kinetic energy play the dominant roles in collimation and acceleration of the flow in M 87 .