Restarted activity in the 3C 328 radio galaxy

As a rule, both lobes of Fanaroff-Riley (FR) type-II radio sources are terminated with hotspots, but the 3C 328 radio galaxy is a specimen of an FR II-like object with a hotspot in only one lobe. A conceivable reason for such asymmetry is that the nucleus of 3C 328 was temporarily inactive. There was no energy transfer from it to the lobes during the period of quiescence, and so they began to fade out. However, under the assumption that the axis connecting the two lobes makes an appreciable angle with the sky plane, and hence one is considerably farther from the observer than the other, the lobes are observed at two distinct stages of evolution due to the light-travel lag. While the far-side lobe is still perceived as being of the FR II type with a hotspot, decay of the near-side lobe is already apparent. No jets are visible in the VLA images, but the VLBA observations of the inverted-spectrum core component of 3C 328 have revealed that it has a jet of a sub-arcsecond length pointing towards the lobe that shows evidence of decay. Since the jet always points to the near side, its observed orientation is in line with the scenario proposed here. The presence of the jet supports the inference that the nucleus of 3C 328 is currently active; however, given the fact that the jet is short (similar to 200 pc in projection), the activity must have restarted very recently. The lower and upper limits of the quiescent period length have been calculated.

Automated summary

The radio galaxy 3C 328 is an example of an FR II-like object with asymmetric distribution of hotspots. The lobes show different evolutionary stages, suggesting a recent restart of activity. The presence of a jet supports the inference of current activity in the nucleus.