VLBA observations of astrometric reference sources in the Galactic center

We report here on multifrequency VLBA observations of three extragalactic s ources within 1 degrees of the Galactic center. These sources have been use d as astrometric reference sources for VLA and VLBA determinations of the p roper motion of Sagittarius A*, the compact nonthermal radio source in the Galactic center. Each source has a main component with a brightness tempera ture in excess of 10(7.5) K, confirming that the sources are active galacti c nuclei. The sources have simple structure that can be characterized by on e or two Gaussian components. The frequency dependence of the structure ind icates that the positions of Sgr A* determined by the VLA astrometry of Bac ker & Sramek at 4.8 and 8.4 GHz should have an offset of similar to 2 mas. This offset is in the same direction as the 5 mas shift measured by Backer & Sramek. The structure is unlikely to bias the published 43 GHz VLBA resul ts of Reid et al. Motions of components in the calibrator sources could lea d to errors in the proper motion of Sgr A* on the order of a few km s(-1). All three sources show frequency-dependent structure consistent with scatte ring significantly stronger than that of the Galactic scattering model of T aylor & Cordes but significantly weaker than that of the hyperstrong Galact ic center scattering. Combined with other observations, this suggests the e xistence of a new component of Galactic scattering located several kpc from the Galactic center.