  So far, all relativistically boosted jets with superluminal motion have only
been detected in typical radio galaxies with early type host galaxies. We have
now discovered superluminal motion in the Seyfert I galaxy III Zw 2, classified
as a spiral. The lower limit for the apparent expansion speed is 1.25 c.
Spectral and spatial evolution are closely linked. Before and after this rapid
expansion we have seen a period of virtually no expansion with an expansion
speed less than 0.04 c. However, at 15 GHz the picture is completely different.
III Zw 2 shows slow expansion ($\sim 0.6 c$) during the time of no expansion at
43 GHz and no expansion during the rapid expansion at 43 GHz. The difference
between the two frequencies is qualitatively explained by optical-depth effects
in an 'inflating- balloon model', describing the evolution of radio lobes on an
ultra-compact scale. The stop-and-go behavior could be explained by a jet
interacting with a molecular cloud or the molecular torus. Since III Zw 2 is
also part of a sample of so- called radio-intermediate quasars (RIQ), it
confirms earlier predictions of superluminal motion for this source, based on
the argument that RIQs could be relativistically boosted jets in radio-weak
quasars and Seyfert galaxies.

Astrophysics