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Thursday, October 28, 1999 Published at 14:05 GMT


Sci/Tech

Hubble homes in on black hole

A Hubble close-up of the jet emerging from the black hole

By BBC News Online Science Editor Dr David Whitehouse

Astronomers have obtained their first glimpse of the mysterious region near a black hole at the centre of a distant galaxy.

From the black hole a powerful stream of subatomic particles travel outward at nearly the speed of light. They form a jet that is nearly straight for thousands of light-years.

The astronomers used also radio telescopes in Europe and the United States to make the most detailed images ever of the centre of the galaxy M87, some 50 million light-years away.


[ image: Radio observations show how narrow the jet is]
Radio observations show how narrow the jet is
"This is the first time anyone has seen the region in which a cosmic jet is formed into a narrow beam," said Bill Junor of the University of New Mexico, US.

"We had always speculated that the jet had to be made by some mechanism relatively near the black hole, but as we looked closer and closer to the centre, we kept seeing an already-formed beam. That was becoming embarrassing, because we were running out of places to put the formation mechanism that we knew had to be there."

Professor Junor, along with John Biretta and Mario Livio of the Space Telescope Science Institute, in Baltimore, have now shown that M87's jet is formed within a few tenths of a light-year of the galaxy's core, presumed to be a black hole three billion times more massive than the sun.

In the formation region, the jet is seen opening widely, at an angle of about 60 degrees, nearest the black hole, but is squeezed down to only six degrees a few light-years away.


[ image: Radio observations of the start of the jet]
Radio observations of the start of the jet
"The 60-degree angle of the inner part of M87's jet is the widest such angle yet seen in any jet in the universe," said Professor Junor. "We found this by being able to see the jet to within a few hundredths of a light-year of the galaxy's core -- an unprecedented level of detail."

At the centre of M87, material being drawn inward by the strong gravitation of the black hole is formed into a rapidly-spinning flat disk, called an accretion disk.

The subatomic particles are thought to be pushed outward from the poles of this disk. The scientists believe that magnetic fields in the disk are twisted tightly as the disk spins and then channel the electrically-charged particles into a pair of narrow jets.

"Our new image of M87 supports this idea of magnetic fields doing the work of forming the stream of particles into a narrow jet," said Professor Biretta.

Jets such as the one in M87 are seen emerging from numerous galaxies throughout the Universe. "What we learn about how M87's jet is formed and shaped can be applied to others," said Professor Livio.

"These jets coming from radio galaxies and quasars are among the greatest 'particle accelerators' in the universe, but we don't fully understand how they work. This new information will help scientists decipher the physics of these powerful 'engines,'" he added.

Professor Junor summed up, saying: "The jets are fascinating to us because they show how nature is somehow using the accretion disk and the jet to tap into the enormous gravitational energy of a black hole and use that energy to 'light up' the outer regions of the galaxy."





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