The jet in M 87.

Observations of the M87 jet are used to investigate the nature of the jet. The jet appears to be expanding and twisted into a helix. Equipartition conditions are calculated at the position of the knots along the expanding jet, and the implications of jet helicity are explored. Jet helicity is interpreted as being produced by helical instability of the flow, and the polarization properties of a helical expanding jet are investigated. Jet helicity produced by instability is shown to imply a supersonic Mach 15 jet more dense than the surrounding medium. When this result is combined with the calculated jet conditions it is found that the jet flow velocity is approximately 30,000 km/s and energy flow is about 200 times the jet luminosity. This provides an energy reservoir for particle acceleration and adiabatic expansion of the radio lobe. While the radio lobes would require 5 x 10 to the 6th yr to develop, the jet could achieve its present configuration in less than 2 x 10 to the 5th yr.