CfA Colloquium Schedule Spring 2004
 12 February 2004

12 February 2004

Speaker: Mark Reid (Center for Astrophysics)

Title: Must Sgr A* be a super-massive black hole?

Abstract: The case for a super-massive black hole at the center of the Galaxy is extremely strong. The proper motions, accelerations, and orbits of stars about a common gravitational center are now being determined to high accuracy at infrared wavelengths. A total mass of about 4 x 106 solar masses is required within a radius of about 100 AU. These dramatic results are fully consistent with the theory that Sgr A*, the compact radio source at the Galactic center, is a super-massive black hole. But must this matter be contained in a super-massive black hole and must Sgr A* be the super-massive black hole? We have been using the VLBA for 8 years to measure the position of Sgr A* with respect to extragalactic radio sources. The apparent proper motion of Sgr A* is almost entirely in the plane of the Galaxy, and the effects of the orbit of the Sun around the Galactic center can account for this motion. The residual proper motion of Sgr A* perpendicular to the plane of the Galaxy is incredibly small: 0.4 plus or minus 0.8 km/s. This exceedingly low speed indicates that Sgr A* contains more than 50% of the mass deduced from stellar orbits. This is the first direct evidence that a compact radio source at the center of a galaxy contains of order 106 solar masses and provides overwhelming evidence that it is in the form of a super-massive black hole. No other form of dark matter can explain both the extreme mass concentration in the Galactic center and the direct association with a stationary, compact, radiative source.

References for students:

  1. Infrared observations of stellar orbits: Schoedel et al 2002, Nature, 419, 694 Ghez et al 2003, ApJ, 586, L127
  2. Position of SgrA* with respect to infrared orbits: Reid et al 2003, ApJ, 587, 208
  3. Early papers on the proper motion of SgrA*: Reid et al 1999, ApJ, 524, 816 Backer and Sramek 1999, ApJ, 524, 805


Section Photo