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A size of 1 au for the radio source Sgr A* at the centre of the Milky Way


Although it is widely accepted that most galaxies have supermassive black holes at their centres1,2,3, 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 proof5,6,7, 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 (sizewavelength1.09) explicitly rules out explanations other than those emission models with stratified structure, which predict a smaller emitting region observed at a shorter radio wavelength.

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Figure 1: High-resolution VLBI image of Sgr A* at 3.5 mm obtained with the VLBA on 20 November 2002.
Figure 2: Intrinsic major axis size versus observing wavelength.


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The Very Large Array and the Very Long Baseline Array are operated by the National Radio Astronomy Observatory, which is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities Inc. Z.-Q.S. acknowledges support from the One-Hundred-Talent programme of the Chinese Academy of Sciences

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Correspondence to Zhi-Qiang Shen.

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Supplementary information

Supplementary Notes

A description of the model fitting procedure used to determine apparent structure of Sgr A* quantatively, and the revision of the wavelength-dependent scattering law. (DOC 138 kb)

Supplementary Figure

Measured (FWHM) angular size of Sgr A* vs. observing wavelength. (PDF 27 kb)

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Shen, ZQ., Lo, K., Liang, MC. et al. A size of 1 au for the radio source Sgr A* at the centre of the Milky Way. Nature 438, 62–64 (2005).

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