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

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1. Introduction

With the launch of HST and the use of VLA and MERLIN radio observations, both optical and radio images with a resolution better than [FORMULA] are routinely available. However, the poor accuracy in the registration of these images prevents taking advantage of such increased resolution to a better understanding of the relationship between radio and optical emission, which is of great interest in several fields of extragalactic astrophysics, and in particular for Seyfert and radio galaxies. The absolute astrometry of HST images is limited by the uncertainties of the Guide Star Catalog positions to, typically, [FORMULA] 5 in the Northern hemisphere (Taff et al. 1990).

In this paper, we propose a method to improve the absolute astrometry of HST images using ground based images to link to astrometric standard stars. The resulting astrometry is of high accuracy and it can be used to define a common reference frame to which both HST and the radio images are referred. This method is applied here for the first time to the Seyfert 2 galaxy NGC 5929, for which both HST and MERLIN images are available.

The observations used for this test are described in Sect. 2. In Sect. 3 we derive the absolute astrometry of the ground based images. Section 4 outlines the method to register ground based and HST images, while Sect. 5 addresses the issue of systematic deviations between radio and optical frames. In Sect. 6, we register the HST and radio images and briefly discuss the results. A summary and our conclusions are given in the last section.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998