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

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

2005-12-21

arxiv.org/abs/astro-ph/0512515v2

Nature 438:62,2005

Astronomy and Astrophysics

Astrophysics

18 pages, 4 figures

Scientific paper

10.1038/nature04205

Although it is widely accepted that most galaxies have supermassive black holes (SMBHs) at their centers^{1-3}, 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 proof^{5-7}, 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 size^8, 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 horizon^9. Moreover, the measured size is wavelength dependent^{10}. 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 mass^{11}, the lower limit on the mass density is 6.5x10^{21} 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 (The size is proportional to 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.

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