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

Nature volume 438, pages 62–64 (2005)Cite this article

Abstract

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 (size∝wavelength1.09) 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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Acknowledgements

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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Authors and Affiliations

  1. Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai, 200030, China

    Zhi-Qiang Shen

  2. National Radio Astronomy Observatory, 520 Edgemont Road, Virginia, 22903, Charlottesville, USA

    K. Y. Lo

  3. Division of Geological and Planetary Sciences, California Institute of Technology, California, 91125, Pasadena, USA

    M.-C. Liang

  4. Harvard-Smithsonian CfA, 60 Garden Street, Massachusetts, 02138, Cambridge, USA

    Paul T. P. Ho & J.-H. Zhao

  5. Institute of Astronomy & Astrophysics, Academia Sinica, PO Box 23-141, 106, Taiwan, Taipei, China

    Paul T. P. Ho

  6. Astrophysics, Academia Sinica, PO Box 23-141

    Paul T. P. Ho

Authors
  1. Zhi-Qiang Shen
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  2. K. Y. Lo
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  3. M.-C. Liang
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  4. Paul T. P. Ho
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  5. J.-H. Zhao
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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). https://doi.org/10.1038/nature04205

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