  Although it is widely accepted that most galaxies have supermassive black
holes (SMBHs) at their centers^{1-3}, concrete proof has proved elusive.
Sagittarius A* (Sgr A*)^4, an extremely compact radio source at the center of
our Galaxy, is the best candidate for proof^{5-7}, because it is the closest.
Previous Very Long Baseline Interferometry (VLBI) observations (at 7mm) have
detected that Sgr A* is ~2 astronomical unit (AU) in size^8, but this is still
larger than the "shadow" (a remarkably dim inner region encircled by a bright
ring) arising from general relativistic effects near the event horizon^9.
Moreover, the measured size is wavelength dependent^{10}. Here we report a
radio image of Sgr A* at a wavelength of 3.5mm, demonstrating that its size is
\~1 AU. When combined with the lower limit on its mass^{11}, the lower limit on
the mass density is 6.5x10^{21} Msun pc^{-3}, which provides the most stringent
evidence to date that Sgr A* is an SMBH. The power-law relationship between
wavelength and intrinsic size (The size is proportional to wavelength^{1.09}),
explicitly rules out explanations other than those emission models with
stratified structure, which predict a smaller emitting region observed at a
shorter radio wavelength.
