THE combined might of 11 radio telescopes has finally pierced the hazy veil
of dust obscuring the massive black hole at the centre of our Galaxy. An
international team of astronomers says the unexpected nature of the radiation
emanating from it may force astrophysicists to rethink the way matter falls into
the jaws of hungry black holes.
Smack in the middle of our Galaxy, in the direction of Sagittarius, is a
mysterious object called Sgr A* which emits bright radio waves. Astronomers
think that it contains a black hole as heavy as 2.6 million Suns because stars
whirl about it very rapidly. Matter 鈥渁ccreting鈥� or falling into the hole would
be heated to huge temperatures and emit the bright radiation.
But because the object is at the centre of the dense disc of the Galaxy,
obscured by clouds of dust and gas, getting a clear view of Sgr A* is difficult.
Another problem is that because the region is swarming with stars, it is
difficult to distinguish the stellar light from the black hole鈥檚 radiation.
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Now Paul Ho and Jun-Hui Zhao, astronomers at the Harvard-Smithsonian Center
for Astrophysics in Cambridge, Massachusetts, and their colleagues in Taiwan and
China, have trained 11 radio telescopes on the centre of the Galaxy to get an
unprecedented look at its centre. Much to their surprise, they discovered that
the radiation from Sgr A* has a highly elliptical shape. It measures about 30 by
8 light minutes, making it more than three times as long as it is wide.
This doesn鈥檛 fit in with theoretical models of how matter falls into black
holes, according to Ho. 鈥淭he most popular models are spherical accretion
models鈥攖hey look like a round ball,鈥� he says. 鈥淭here鈥檚 no structure, no
elliptical shape.鈥�
Ho and his colleagues then thought the elliptical shape of the radiation
might be caused by jets of gas, which often erupt from black holes. But they say
that models predict jets that are either too big or too small for Sgr
A*鈥攏one explains its shape precisely. The team鈥檚 results will appear in a
forthcoming issue of Astrophysical Journal Letters.
鈥淚t is a very interesting result, and it does look pretty convincing,鈥�
comments Wei Cui, an astronomer at the Massachusetts Institute of Technology in
Cambridge. However, Cui believes that existing jet models might be flexible
enough to explain the elliptical shape of the radio source. 鈥淵ou can鈥檛 take the
theoretical calculations too literally,鈥� he says.
Donald Backer, an astronomer from the University of California at Berkeley,
agrees. 鈥淎s a class of models, I think there is room to wiggle,鈥� he says. But he
thinks that the observations could play an important role in weeding out models
of how a black hole behaves. 鈥淭here鈥檚 a good sense we鈥檙e closing in on it in a
rather dramatic fashion here,鈥� he says. 鈥淲e鈥檙e putting a face on the black
丑辞濒别.鈥�