title: A size of approximately 1 au for the radio source Sgr A* at the centre of the Milky Way.
abstract: Although it is widely accepted that most galaxies have supermassive black holes at their centres, concrete proof has proved elusive. Sagittarius A* (Sgr A*), an extremely compact radio source at the centre of our Galaxy, is the best candidate for proof, because it is the closest. Previous very-long-baseline interferometry observations (at 7 mm wavelength) reported that Sgr A* is approximately 2 astronomical units (au) in size, but this is still larger than the 'shadow' (a remarkably dim inner region encircled by a bright ring) that should arise from general relativistic effects near the event horizon of the black hole. Moreover, the measured size is wavelength dependent. Here we report a radio image of Sgr A* at a wavelength of 3.5 mm, demonstrating that its size is approximately 1 au. When combined with the lower limit on its mass, the lower limit on the mass density is 6.5 x 10(21)M(o) pc(-3) (where M(o) is the solar mass), which provides strong evidence that Sgr A* is a supermassive black hole. The power-law relationship between wavelength and intrinsic size (size proportional, variantwavelength(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.
authors.affiliation: Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai 200030, China. zshen@shao.ac.cn
authors.forename: Zhi-Qiang
authors.initials: ZQ
authors.lastname: Shen
authors.forename: K Y
authors.initials: KY
authors.lastname: Lo
authors.forename: M-C
authors.initials: MC
authors.lastname: Liang
authors.forename: Paul T P
authors.initials: PT
authors.lastname: Ho
authors.forename: J-H
authors.initials: JH
authors.lastname: Zhao