VLBA and Global VLBI observations of M87 at 43 GHz , some new and some previously published , are used to study the structural evolution of the jet with a spatial resolution of under 100 Schwarzschild radii .
The images , taken between 1999 and 2004 , have an angular resolution of 0 \farcs 00043 \times 0 \farcs 00021 .
An edge-brightened jet structure and an indication of a large opening angle at the jet base are seen in all five epochs .
In addition , a probable counter-jet is seen in the latter three epochs .
A 22 GHz VLBA image also confirms many of the structures seen at the higher frequency , including the counter-jet .
A comparison of the counter-jet flux density at 22 and 43 GHz reveals that it is not free-free absorbed at these frequencies . Attempts to obtain speeds from the proper motions of jet and counter-jet components indicate that these observations are undersampled .
The closest pair of images gives apparent speeds of 0.25 to 0.40c for the jet and 0.17c for the counter-jet .
These speeds should be treated as lower limits because of possible errors in associating components between epochs .
If they are real , they indicate that the jet is oriented 30-45° from the line-of-sight and that the component speeds along the jet are 0.3-0.5c .
Using the jet orientation derived from proper motions , the spectral index of the the counter-jet , and a jet-to-counter-jet brightness ratio of 14.4 , the inferred bulk flow is 0.6-0.7c , which , given the considerable uncertainties in how to measure the brightness ratio , is not significantly larger than the component speed .