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Thursday, 19 September, 2002, 12:59 GMT 13:59 UK
'Missing link' black holes found
Artist's view
Artist's view of a black hole in a globular cluster

A search for black holes in globular star clusters by the Hubble Space Telescope has been successful in allowing new insights into how black holes form.

Globular star clusters contain the oldest stars in the Universe, and if they contain black holes now they most likely had black holes when they originally formed.

"These findings may be telling us something very deep about the formation of star clusters and black holes in the early Universe," says Roeland Van Der Marel of the Space Telescope Science Institute.

"Black holes are even more common in the Universe than previously thought," he adds.

Cosmic building blocks

The black holes found in globular clusters may provide a link between stellar-mass and the supermassive black holes found inside galaxies.

"Not only will we learn about the formation of black holes, but these new data from Hubble help us connect globular clusters to galaxies, providing information on one of the most important unsolved problems in astronomy today: how galaxies form in the Universe," says Michael Rich of the University of California.

G1
Globular cluster G1
Understanding such a link is important because it may provide clues about how supermassive black holes form in galaxies.

An unexplained fact is that a black hole's mass is related to the mass of the stellar environment it inhabits. That is, supermassive black holes are found in the centres of galaxies.

The newfound black holes in globular clusters, which are 10,000 times less massive than a galaxy, also obey the trend. Astronomers speculate that some unknown process ties a black hole to its host in a fundamental way.

"The intermediate-mass black holes that have now been found in globular clusters may be the building blocks of the supermassive black holes that dwell in the centres of most galaxies," says Karl Gebhardt of the University of Texas at Austin.

Two main theories

Dr Van Der Marel led a team that uncovered a black hole in the centre of the globular star cluster M15 which is 32,000 light-years away in the constellation Pegasus. It has a mass some 4,000 times that of our Sun.

A separate series of observations found a 20,000-solar-mass black hole in the giant globular cluster G1, located 70 times farther away.

By contrast, stellar-mass black holes are only a few times the mass of our Sun, and galactic-centre black holes can be millions or billions of times more massive than our Sun.

"There are two main theories of black hole formation," says Dr Gebhardt.

"You could either make the black hole all at once, when the galaxy is forming, by dumping a lot of material in the middle, or you could start with a seed black hole that subsequently grows over time.

"The observational evidence now points to the idea that you start out with a small seed black hole."

See also:

11 Sep 02 | Science/Nature
22 Mar 99 | Science/Nature
14 May 02 | Science/Nature
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