Compact radio structure in the Seyfert nucleus of NGC 5929.

Abstract

The authors present MERLIN and VLA radio measurements of the Seyfert galaxy NGC 5929 at 15, 8, 5, 1.6 and 0.4 GHz. All measurements have subarcsecond angular resolution ranging from 0.6 arcsec at 0.4 GHz to 50 mas at 5 GHz. The authors confirm the triple structure reported in earlier studies. The weak central source of the triplet is almost certainly associated with the active nucleus. It has a flat spectral index and is unresolved even at 50-mas resolution, consistent with a synchrotron self absorbed core. The two components which straddle the core have sizes of order 10 pc and have steep spectral indices, and hence can be considered as small radio "lobes". Both of these components show evidence for low-frequency turnovers at 0.4 GHz, which the authors interpret as evidence for free-free absorption by ionized gas with an emission measure of ≡10⁵pc cm^-6. As the north-eastern component shows greater free-free absorption than the south-western component, the authors propose that it is behind most of the narrow-line region (NLR) ionized gas. This implies that the nuclear collimated ejection is towards the observer in the south-west and away from the observer in the north-east. There is evidence of weak extended emission, particularly at 0.4 GHz, which may represent flows approximately perpendicular to the central triplet. The authors compare the radio structure of NGC 5929 to recent HST images and discuss the possible models to relate the radio and optical structures.