THE MILLIARCSECOND STRUCTURE OF 3C273 AT 22GHZ

We present the first VLBI images at 22 GHz of the jet in the quasar 3C 273. In addition to the compact core region, two emission regions can be identified with features seen at lower frequencies; they separate from the core with constant speeds of 0.65 ± 0.09 and 0.92 ± 0.11 mas yr-1, corresponding to apparent superluminal motion of 4.3 ± 0.3c and 6.1 ± 0.3c (for H0 = 100 km s-1 Mpc-1, q0 = 0.5). The core region brightened at about the estimated epoch of zero separation for the latest superluminal component, suggesting a causal relationship The curved ridge line of the jet smoothly extends inward towards the core, although no pronounced bends in the range of core distance 0.5-2.5 mas are seen. We find no significant evidence against a common path of subsequent superluminal features. An apparent frequency dependence in the position of one superluminal feature tentatively suggests that opacity effects across the jet direction are present. Our results are consistent with an interpretation of the superluminal features as shocks in an underlying relativistic flow, although alternative explanations cannot be ruled out.