VLBA Observations of Astrometric Reference Sources in the Galactic Center

We report here on multi-frequency VLBA observations of three extragalacticsources within 1 degree of the Galactic Center. These sources have been used asastrometric reference sources for VLA and VLBA determinations of the propermotion of Sagittarius A*, the compact nonthermal radio source in the GalacticCenter. Each source has a main component with a brightness temperature inexcess of 10^{7.5} K, confirming that the sources are active galactic nuclei.The sources have simple structure that can be characterized by one or twoGaussian components. The frequency dependence of the structure indicates thatthe positions of Sgr A* determined by the VLA astrometry of Backer & Sramek(1999) at 4.8 and 8.4 GHz should have an offset of \sim 2 mas. This offset isin the same direction as the 5 mas shift measured by Backer & Sramek (1999).The structure is unlikely to bias the 43 GHz VLBA results of Reid et al (1999).Motions of components in the calibrator sources could lead to errors in theproper motion of Sgr A* on the order of a few km s^{-1}. All three sources show frequency dependent structure consistent withscattering which is significantly stronger than that of the Galactic scatteringmodel of Taylor & Cordes (1993) but significantly weaker than that of thehyperstrong Galactic Center scattering. Combined with other observations, thissuggests the existence of a new component of Galactic scattering locatedseveral kpc from the Galactic Center.Comment: 12 pages, 4 figures, accepted for publication in Ap