Jet Collimation in Action: Realignment on Kiloparsec Scales in 3C 279

We report a change in the trajectory of a well-studied jet component of the quasar 3C 279. The component changes in apparent projected speed and direction, and we find it to be moving with a Lorentz factor γ ≳ 15 at an initial angle of ≲1° to the line of sight. The new trajectory of the component has approximately the same speed and direction as an earlier superluminal feature, originally seen in the early 1970s. The new direction for the component is also much better aligned with larger scale Very Large Baseline Array and Very Large Array structure out to 0.″1. We suggest that the trajectory change is a collimation event occurring at ≳1 kpc (deprojected) along the jet. While the change in trajectory on the sky appears to be 26°, the intrinsic change is ≲1°. We estimate the Doppler factor prior to the change in direction to be δ ≳ 28 and after the change to be δ ≳ 23. A comparison with independent constraints on the Doppler factor suggests that the energy in the radiating particles cannot greatly exceed the energy in the magnetic field unless the volume filling factor is very much less than 1.