3 November 2005
Your nearest black hole?
Most galaxies are thought to have a supermassive black hole at their
centres, but proving it is very difficult. The centre of our own Galaxy,
occupied by the compact radio source Sagittarius A* (Sgr A*) is a good place
to start looking as it is so close to us, just 26,000 light-years from the
Sun. A new short-wavelength radio image of Sgr A* has made it possible
establish the intrinsic size of Sgr A* for the first time. It is 1 atomic
unit (about 1.6 light years) across. This suggests that its mass density is
more than 10 orders of magnitude greater than any other known cosmic object,
well into supermassive black hole territory.
News and Views
: Astronomy: Light on a dark place
The sharpest images ever taken of matter around the probable black hole at
the centre of our Galaxy bring us within grasp of a crucial test of general
relativity - a picture of the black hole's 'point of no return'.
Christopher Reynolds
Paper:
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 and J.-H. Zhao
Abstract:
Although it is widely accepted that most galaxies have supermassive black
holes at their centres, concrete proof has proved elusive. Sagittarius A*
(Sgr A*), an extremely compact radio source at the centre of our Galaxy, is
the best candidate for proof, 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 size, 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 hole. Moreover, the measured size is wavelength dependent. 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 mass, the
lower limit on the mass density is 6.5 10^21M 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.
Links to the Articles and Papers at Nature (only the summaries and abstracts
are free)
http://www.nature.com/nature/journal/v438/n7064/edsumm/e051103-09.html
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Posted by
Robert Karl Stonjek
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