The radio nucleus of NGC 4151 at 5 and 8 GHz

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Abstract
We present 5- and 8-GHz observations of the Seyfert nucleus of NGC 4151. The observations have angular resolutions raging from 400 to 50 mas and sensitivities as high as 7 µJy beam–1. We confirm the picture of a two-sided radio jet along PA 77°, and also detect a weak component 6 arcsec (400 pc) to the east which shows that collimated ejecta associated with the jet extend at least twice as far as revealed by earlier measurements. The central knots in the jet have significantly higher pressure than components in the outer parts. Most of the components in the source have steep spectra, although that of the eastern component in the central double is relatively flat (α = – 0.3), consistent with it being associated with the optical continuum nucleus. The high- resolution MERLIN 5-GHz image of this component has revealed a small (150 mas) jet, which is further evidence that this component is the nucleus. We have also discovered extended weak, steep-spectrum, low-brightness emission with an ‘S’-shaped symmetry, and suggest the structure to be a consequence of relativistic particles from the jet drifting along density gradients away from the galactic plane. The high sensitivity of the 8-GHz VLA measurements allows us to set an upper limit of 105 pc cm–6 on the emission measure of thermal gas outside the radio source. There is, however, possible thermal emission straddling the optical nucleus, which we tentatively identify with narrow line region (NLR) gas. We discuss the geometry of the nucleus and consider models to account for the misalignment between the NLR and extended narrow-line region (ENLR) gas and the radio jet. We favour a model in which the misalignment is caused by density bounding of the UV cone, which implies that the collimation axis of the nucleus is at an angle of 25° to the rotation axis of NGC 4151 and 40° to the line of sight. We discuss difficulties of fitting NGC 4151 into unified schemes in which Seyfert 1 and 2 nuclei differ only in orientation.

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