The radio jet in 3C 120 at very long baseline interferometry and VeryLarge Array scales.

The radio jet and lobes of 3C 120 have been mapped on scales from 1 pc to over 400 kpc. The observations show that the jet is continuous from the parsec scales, where superluminal motions are seen, to over 100 kpc. The wide range of scales observed allows an underlying power-law function of central-jet brightness vs distance from the core to be distinguished from order-of-magnitude local-brightness fluctuations. The power law has an index of about -2.2. Following the usual minimum-energy assumptions, it is found that this index gives an energy per unit length of the jet that is nearly constant and a magnetic field that scales with the inverse of the core distance. It appears that the basic physical parameters of the jet are established on subparsec scales, where there are relativistic motions, and evolve in a simple manner for over five orders of magnitude in distance from the core.