An HST Proper-Motion Study of the Large-scale Jet of 3C273
  The radio galaxy 3C 273 hosts one of the nearest and best-studied powerful
quasar 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 proper
motions in the kiloparsec-scale resolved jets of nearby radio-loud active
galaxies. The jet in 3C 273 is highly relativistic on sub-parsec scales, with
apparent proper motions up to 15$c$ observed by VLBI (Lister et al., 2013). In
contrast, we find that the kpc-scale knots are compatible with being
stationary, with a mean speed of $-$0.2$\pm$0.5$c$ over the whole jet. Assuming
the knots are packets of moving plasma, an upper limit of 1c implies a bulk
Lorentz factor $\Gamma<$2.9. This suggests that the jet has either decelerated
significantly by the time it reaches the kpc scale, or that the knots in the
jet are standing shock features. The second scenario is incompatible with the
inverse Compton off the Cosmic Microwave Background (IC/CMB) model for the
X-ray emission of these knots, which requires the knots to be in motion, but
IC/CMB is also disfavored in the first scenario due to energetic
considerations, 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 via
gamma-ray upper limits.
