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

Zhi-Qiang Shen, K. Y. Lo, M.-C. Liang, Paul T. P. Ho, J.-H. Zhao,

Paper: Nature, 438(2005)62

EPrint Server: astro-ph/0512515


Although it is widely accepted that most galaxies have supermassive black holes (SMBHs) at their centers1-3, concrete proof has proved elusive. Sagittarius A* (Sgr A*)4, an extremely compact radio source at the center of our Galaxy, is the best candidate for proof5-7, because it is the closest. Previous Very Long Baseline Interferometry (VLBI) observations (at 7mm) have detected that Sgr A* is 2 astronomical unit (AU) in size8, but this is still larger than the "shadow" (a remarkably dim inner region encircled by a bright ring) arising from general relativistic effects near the event horizon9. Moreover, the measured size is wavelength dependent10. Here we report a radio image of Sgr A* at a wavelength of 3.5mm, 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*1021 Msun pc-3, which provides the most stringent evidence to date that Sgr A* is an SMBH. The power-law relationship between wavelength and intrinsic size (size \propto 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.

Preprints available from the authors at llshang@shao.ac.cn , or the raw TeX (no figures) if you click here.

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