Paper: astro-ph/0512515
Date: Wed, 21 Dec 2005 00:18:33 GMT (183kb)

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

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

Comments: 18 pages, 4 figures

Journal-ref: Nature, 438(2005)62
\\
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* (\sgras)^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 \sgras 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 \sgras at a wavelength of 3.5mm, demonstrating that its size is
$\sim$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 \sgras is an SMBH. The power-law relationship
between wavelength and intrinsic size (size $\propto$ 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.

\\ ( http://arXiv.org/abs/astro-ph/0512515 , 184kb)