• Proper motions of Local Group galaxies.
• The structure of the Milky Way: The BeSSeL Survey
• The Galactic Center: Sgr A*
• Supernova 2008iz in M82
• Extragalactic H2O Masers: Local Group, 3C 403, MG J0414+0534
• Circular Polarization in AGN: M81
• Jets in AGN: The case of III Zw 2
• VLBI Astrometry and Phase-referencing.

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The Galactic Center: Sgr A*

We report measurements with the VLBA of the position of Sgr A* with respect to two extragalactic radio sources over a period of eight years. The apparent proper motion of Sgr A* relative to J1745-283 is 6.379 +/- 0.024 mas/y along a position angle of 209.60 +/- 0.18 degrees, almost entirely in the plane of the Galaxy. The effects of the orbit of the Sun around the Galactic center can account for this motion, and the residual proper motion of Sgr A* perpendicular to the plane of the Galaxy is -0.4 +/- 0.9 km/s. A maximum-likelihood analysis of the motion expected for a massive object within the observed Galactic center stellar cluster indicates that Sgr A* contains more than about 10% of the approximately 4 x 10^6 Msun deduced from stellar orbits. The intrinsic size of Sgr A*, as measured by several investigators, is less than 1 AU, and the implied mass density of ~10^22 Msun/pc^3 is within about three orders of magnitude of a comparable super-massive black hole within its Schwarzschild radius. Our observations provide the first direct evidence that a compact radiative source at the center of a galaxy contains of order 10^6 Msun and provides overwhelming evidence that it is in the form of a super-massive black hole. Finally, the existence of "intermediate mass" black holes more massive than ~10^4 Msun between roughly 10^3 and 10^5 AU from Sgr A* are excluded.

Published in:

Reid M.J., Brunthaler A., 2004, ApJ, 616, 872

Position residuals of Sgr A* relative to J1745--283 on the plane of the sky. Each measurement is indicated with an ellipse, approximating the apparent scatter-broadened size of Sgr A* at 43 GHz and 1 sigma error bars, which include estimates of systematic uncertainties. The dashed line is the variance-weighted best-fit proper motion, and the solid line gives the orientation of the Galactic plane, which is tilted by 31.40 deg east of north in J2000 coordinates

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