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The radio galaxy 3C 273 hosts one of the nearest and best-studied powerfulquasar jets. Having been imaged repeatedly by the Hubble Space Telescope (HST)over the past twenty years, it was chosen for an HST program to measure propermotions in the kiloparsec-scale resolved jets of nearby radio-loud activegalaxies. The jet in 3C 273 is highly relativistic on sub-parsec scales, withapparent proper motions up to 15$c$ observed by VLBI (Lister et al., 2013). Incontrast, we find that the kpc-scale knots are compatible with beingstationary, with a mean speed of $-$0.2$\pm$0.5$c$ over the whole jet. Assumingthe knots are packets of moving plasma, an upper limit of 1c implies a bulkLorentz factor $\Gamma<$2.9. This suggests that the jet has either deceleratedsignificantly by the time it reaches the kpc scale, or that the knots in thejet are standing shock features. The second scenario is incompatible with theinverse Compton off the Cosmic Microwave Background (IC/CMB) model for theX-ray emission of these knots, which requires the knots to be in motion, butIC/CMB is also disfavored in the first scenario due to energeticconsiderations, in agreement with the recent finding of Meyer & Georganopoulos(2014) which ruled out the IC/CMB model for the X-ray emission of 3C 273 viagamma-ray upper limits.

An HST Proper-Motion Study of the Large-scale Jet of 3C273