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Astron. Astrophys. 318, 997-1002 (1997)

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6. Results and discussion

By combining the uncertainty in the absolute astrometry of the ground based image with the accuracy of the registration of the HST image, we find that the absolute astrometry of the HST image has been obtained to [FORMULA] 62 mas.

Finally, to complete the error budget analysis of the optical-to-radio registration, we have to include the local systematic difference between the two systems (20 mas, see above) and the intrinsic precision of MERLIN observations, which is estimated at 50 mas. Therefore, the total error of our registration of NGC 5929 is estimated at [FORMULA] mas.

In Fig. 2 the contours from the MERLIN radio image by Su et al. (1996) are overlaid onto the HST line and continuum images. The cross on the lower right corner represents the uncertainty in the registration. The radio core falls within the nuclear dust lane. No optical counterpart to the radio nucleus is seen. In the framework of the Unified model for Seyfert galaxies (e.g. Antonucci, 1993), the active nucleus of NGC 5929 is obscured at optical wavelength by a circumnuclear torus. As observed in several Seyfert 2 galaxies (e.g. Capetti et al., 1996), nuclear dust lanes are often associated with the obscuring torus. The nucleus itself is expected to be offset from the peak of emission in a direction perpendicular to the dust lane orientation, in agreement with our results.

Fig. 2b confirms the association of the line ([FORMULA] and [N II]) emitting component with the radio lobes already noted by Whittle et al. (1986) and Bower et al. (1994), and which is commonly observed in Seyfert galaxies (Capetti et al., 1996). In particular the SW radio and optical components are essentially coincident. The lower brightness line emission linking the two main knots is closely aligned with the radio axis.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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