Superluminal motion in the quasar 3C 345

Observations of the compact radio structure of the quasar 3C 345 have been made at eight epochs between 1979 and 1981, using a very long baseline interferometer comprising five telescopes. The sequence of hybrid maps, alternating between 5.0 and 10.7 GHz, shows a bright unresolved "core" and a curved jet at each epoch. Two "knots" in the jet separate from the core at apparent transverse speeds ≡15c (H0 = 55 km s-1 Mpc-1); their proper motion is the best direct evidence to date for superluminal expansion in 3C 345. The one-sided morphology, proper motion, radio spectrum, and weak X-ray flux of 3C 345 can be explained in the relativistic-jet model with the jet aligned close to the line of sight. The jet is constrained to lie at θ ⪉ 7° to the line of sight, and to move with a bulk Lorentz factor γ ⪆ 6.