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 2.5×10⁶ 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(⋆) by less than 20 R_sc(⋆), with the major axis oriented essentially north-south, where R_sc(⋆) (≡ 7.5×10¹¹ cm) is the Schwarzschild radius for a 2.5×10⁶ M_☉ black hole. Contrary to previous expectation that the intrinsic structure of Sgr A* is observable only at λ≤1 mm, we can discern the intrinsic source size at λ=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 λ=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*.