Kiloparsec-scale motions in 3C 120 - Revisited

Walker, Walker, & Benson reported the detection of superluminal motion on kiloparsec scales in 3C 120, a source with well-studied superluminal motion on parsec scales. This result was based on two-epoch observations of a knot in the jet 4'' from the core using the VLA at 5 GHz. Muxlow & Wilkinson attempted to confirm this motion using 1.67 GHz observations with MERLIN, but instead got a result consistent with no motion and inconsistent with the earlier VLA result at about the 2 sigma level. Third-epoch VLA observations by Walker & Benson also produced the result of no motion between epochs 2 and 3, for which the images were superior to that of epoch 1. Fourth-epoch VLA observations were made in 1996 November. The results, along with a reanalysis of the earlier epochs, are presented here. It is now clear that the 4'' knot is moving much more slowly than the parsec scale features, if at all. The observation of superluminal motion is good evidence for relativistic motions near the line of sight in a jet. This is important information for the understanding of jet physics. The observation of lack of motion, on the other hand, does not say much about the jet velocity. It might suggest that the jet is not relativistic. However, a number of effects in a relativistic jet, including standing shocks and fortuitous bends, could also lead to stationary features. As a by-product of these observations, an extremely high dynamic range image of the 3C 120 jet on scales to 25'' was produced and is discussed briefly.