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 ( ) .
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 1 c 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 ( ) which ruled out the IC/CMB model for the X-ray emission of 3C 273 via gamma-ray upper limits .