Accurate optical-to-radio registration of Seyfert galaxies with HST images Absolute astrometry of HST images?

We propose a method to obtain accurate absolute astrometry of Hubble Space Telescope images. Ground based images are used to linkHST images to standard astrometric stars defining the optical frame of reference. The method has been used to register optical images of the Seyfert 2 galaxy NGC 5929 to the corresponding radio maps, allowing the exploration of the relationship between radio and optical emission within the galaxy. The total uncertainty in the absolute astrometry of one HST image is∼ 62mas, an order ofmagnitude better than it is usually achieved with the Guide Star Catalog 1.1 positions, and the final registration with the radio images is derived to better than 0.′′1. The radio core of NGC 5929 falls within a nuclear dust lane. No optical counterpart to the radio nucleus is seen, in agreement with the Unified Model for Seyfert galaxies. The strong association between radio and line emission in this object is confirmed by this work. There are several different contributions to the final accuracy of the optical-to-radio registration:measuring errors, errors in the absolute radio and optical positions, local systematic deviations in the optical-to-radio alignment of the fundamental frame, etc. A significant decrease of the astrometric error budget has to wait for substantial improvements as, for example, those expected from the radio-optical link in the forthcoming Hipparcos Catalog.