46 8 v 1 2 8 Se p 20 00 III Zw 2 : superluminal motion and compact lobe expansion in a Seyfert galaxy

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 (∼ 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 ’inflatingballoon 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 socalled 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.