Magnetic collimation of the relativistic jet in M87
  We apply a two-zone MHD model to the jet of M87. The model consists of an inner relativistic outflow, which is surrounded by a non-relativistic outer disk-wind. The outer disk-wind collimates very well through magnetic self-collimation and confines the inner relativistic jet into a narrow region around the rotation axis. Further, we show by example, that such models reproduce very accurately the observed opening angle of the M87 jet over a large range from the kiloparsec scale down to the sub-parsec scale. 
J. Gracia, K. Tsinganos, S. V. Bogovalov