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Last Updated: Thursday, 7 April, 2005, 12:07 GMT 13:07 UK
Deepest X-rays tell merger story
By Jonathan Amos
BBC News science reporter, in Birmingham

An artist's impression of galaxy merger (CXC/M.Weiss)
An artist's impression of what the galaxy collisions might look like
Scientists have seen giant black holes growing rapidly in the cores of massive star-forming galaxies.

The observations from the Chandra space telescope are the deepest X-ray images ever obtained, viewing events that are 10 billion light-years away.

It is also clear most of these galaxies are merging with close neighbours.

David Alexander told the UK Astronomy Meeting the collisions were probably providing the material to feed the holes and drive the birth of new stars.

"We compared what you see in these galaxies with what you see in the field galaxy population, other galaxies at the same epoch and distance, and there is a stark difference - far more star formation and black hole growth," the Cambridge University scientist and Royal Society research fellow explained to the BBC News website.

"There is something specific going on in these sources and when you look at them very closely, there appear to be major mergers going on with galaxies coming together."

The work, which is described in the journal Nature, actually draws on observations made by several astronomical facilities, including the Keck and James Clerk Maxwell telescopes in Hawaii and the Hubble Space Telescope in orbit around the Earth.

The different instruments were needed to track down the target galaxies, measure star formation and identify mergers, and, crucially, with Chandra, see the X-ray emissions that come from heated material as it is sucked into a black hole.

Our own galaxy, the Milky Way, has a colossal black hole at its core, too, which is some three million times the mass of our Sun. But the black holes seen in Chandra's far-off galaxies are up to 100 times bigger.

"The X-ray observations also show these black holes appear to be surrounded by dense shrouds of gas and dust, which is the material that is going to be consumed by the holes.

Black holes separated by 70,000 light-years (NASA/CXC/IoA/Alexander et al)
What Chandra sees: The X-rays from two black holes in merging galaxies
"It also explains why they have been difficult to see," said Dr Alexander.

Eventually, though, it is likely the prodigious growth of the holes will lead to the emergence of what astronomers call a quasar.

This describes a black hole region that has become so luminous its emissions not only break out from any shroud but also outshine the entire galaxy in which the active core is sited.

A recent computer simulation, performed by Dr Tiziana Di Matteo of Carnegie Mellon University and collaborators, has shown how big galaxy mergers can drive material towards the central regions of galaxies, producing stars and fuelling black hole growth.

"These recent observations [by Dr Alexander and colleagues] are in good agreement with our simulation," said Dr Di Matteo.

"It is exciting that we seem to be converging on a consistent picture of galaxy formation with both observations and theory."

The Royal Astronomical Society's National Astronomy Meeting is being held at the University of Birmingham.

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15 Apr 04 |  Science/Nature

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