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Accurate optical-to-radio registration of Seyfert galaxies with HST images
Absolute astrometry of HST images *
A. Capetti 2 and
Received 8 March 1996 / Accepted 21 May 1996
We propose a method to obtain accurate absolute astrometry of Hubble Space Telescope images. Ground based images are used to link HST 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 62 mas, an order of magnitude 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 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.
Key words: astrometry galaxies: individual: NGC 5929 galaxies: Seyfert methods: data analysis
* Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555 and by STScI grant GO-3594.01-91A.
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Online publication: July 3, 1998