1. Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai 200030, China
2. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22903, USA
3. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
4. Harvard-Smithsonian CfA, 60 Garden Street, Cambridge, Massachusetts 02138, USA
5. Institute of Astronomy & Astrophysics, Academia Sinica, PO Box 23-141, Taipei 106, Taiwan, China

Correspondence to: Zhi-Qiang Shen¹ Correspondence and requests for materials should be addressed to Z.-Q.S. (Email: zshen@shao.ac.cn).

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 (sizewavelength^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.

1. Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai 200030, China
2. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22903, USA
3. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
4. Harvard-Smithsonian CfA, 60 Garden Street, Cambridge, Massachusetts 02138, USA
5. Institute of Astronomy & Astrophysics, Academia Sinica, PO Box 23-141, Taipei 106, Taiwan, China

Correspondence to: Zhi-Qiang Shen¹ Correspondence and requests for materials should be addressed to Z.-Q.S. (Email: zshen@shao.ac.cn).

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