Intrinsic size of Sagittarius A*: 72 Schwarzschild radii

Recent proper motion studies of stars at the very center of the Galaxy strongly suggest that Sagittarius A*, the compact nonthermal radio source at the Galactic center, is a 2.5 x 10(6) M. black hole. By means of near-simultaneous multiwavelength Very Long Baseline Array measurements, we determine for the first time the intrinsic size and shape of Sgr A* to be 72 R-sc(star) by less than 20 R-sc(star), with the major axis oriented essentially north-south, where R-sc(star)(= 7.5 x 10(11) cm) is the Schwarzschild radius for a 2.5 x 10(6) M. black hole. Contrary to previous expectation that the intrinsic structure of Sgr A* is observable only at lambda less than or equal to 1 mm, we can discern the intrinsic source size at lambda = 7 mm because (1) the scattering size along the minor axis is half that along the major axis and (2) the near-simultaneous multiwavelength mapping of Sgr A* with the same interferometer makes it possible to extrapolate precisely the minor axis scattering angle at lambda = 7 mm. The intrinsic size and shape place direct constraints on the various emission models for Sgr A*. In particular, the advection-dominated accretion flow model may have to incorporate a radio jet in order to account for the structure of Sgr A*.