A size of ∼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^1,2,3, concrete proof has proved elusive. Sagittarius A* (Sgr A*)⁴, an extremely compact radio source at the centre of our Galaxy, is the best candidate for proof^5,6,7, because it is the closest. Previous very-long-baseline interferometry observations (at 7 mm wavelength) reported that Sgr A* is ∼2 astronomical units (au) in size⁸, 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 hole⁹. Moreover, the measured size is wavelength dependent¹⁰. Here we report a radio image of Sgr A* at a wavelength of 3.5 mm, demonstrating that its size is ∼1 au. When combined with the lower limit on its mass¹¹, the lower limit on the mass density is 6.5 × 10²¹M_⊙ pc^-3 (where M_⊙ 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∝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.