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Tuesday, 17 September, 2002, 09:41 GMT 10:41 UK
Old galaxies have youthful shine
Galactic cluster Abell 2104
There are more black holes inside it than we realised

Astronomers have discovered more black holes in a cluster of galaxies than they expected.

It is a finding that has profound implications for theories about how these strange objects grow in galaxies by absorbing the gas that drifts between the stars.

It suggests that black holes might be as energetic in old galaxies as they are in younger ones.

This is a surprise for astronomers who thought that black holes in older galaxies were faint because they had run out of the gas they use as fuel.

The discovery was made by the orbiting Chandra X-ray Observatory, and the Walter Baade Telescope at the Las Campanas Observatory in Chile.

Hidden activity

"This changes our view of galaxy clusters as the retirement homes for old and quiet black holes," says Paul Martini of the Carnegie Institution in Pasadena, US.

He adds: "We now want to know how these black holes produce bright X-ray sources, similar to what we see from much younger galaxies?"

Chandra x-ray observatory
Chandra eyes the Universe
"With Chandra we can see that even ancient galaxies with 10-billion-year-old stars can have central black holes still actively pulling in large amounts of interstellar gas," says Dr Martini.

"This activity has simply been hidden from us all this time. It means these galaxies aren't over the hill after all and our theories need to be revised."

Supermassive black holes - having the mass of millions to billions of Suns squeezed into a region about the size of our Solar System - are believed to be the powerhouses in the cores of bright active galaxies.

Many astronomers think that all galaxies have central, supermassive black holes, yet only a small percentage show activity. What is needed to power them is fuel in the form of gas and dust.

Out of retirement

The theory now in question is that older galaxies have little interstellar gas available to activate a central black hole.

Previous research suggested that only about one percent of the galaxies in a cluster have an active black hole at their core. This latest Chandra observation increases that count to about 5%.

"It seems that the supermassive black holes have somehow retained a fuel source, and are now coming out of retirement," says Dr Martini.

This could imply that galaxies are better at holding on to a supply of gas and dust than previously thought, particularly at their cores near the supermassive black hole.

The reawakening of galaxies in their old age may have other implications.

What was once thought to be radiation from new stars may be from black holes instead, implying that scientists may be overestimating the amount of star formation taking place in clusters.

See also:

27 Aug 99 | Science/Nature
26 May 00 | Science/Nature
08 Jun 00 | Science/Nature
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