Apparent 'superrelativistic' expansion of the extragalactic radio source 3C 345

The compact extragalactic radio source 3C 345 was observed by very-long-baseline interferometry (wavelength about 3.8 cm) at 12 epochs distributed over the nearly four-year period from February 1971 to October 1974. For one of these epochs, the multibaseline data were sufficient to allow the brightness distribution to be estimated in a model-independent manner; the resultant distribution was clearly dominated by two components. The remaining sets of data were also represented adequately by two-component models. The angular separation of the two components increased during this period from about 1.00 to 1.30 milliarcsec, corresponding to an apparent average speed of expansion of approximately 2.5 c at a fixed position angle of 105 (plus or minus 5) deg. These results, coupled with the fact that contraction has never been observed, seem difficult to reconcile with the so-called Christmas-tree model of the 'superrelativistic' expansion of extragalactic radio sources.