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Last Updated: Friday, 8 April, 2005, 21:30 GMT 22:30 UK
Cosmic 'traffic pile-ups' probed
By Paul Rincon
BBC News science reporter, in Birmingham

XMM Newton (Esa)
XMM-Newton has been in orbit since 1999
Astronomers have shed light on the violent effects of collisions between galaxy clusters, using the space-based XMM-Newton X-ray telescope.

Violent shock waves rack the colliding clusters, squeezing the gas between them and raising temperatures to many millions of degrees Celsius.

In several billion years this will happen to the Milky Way, when the cluster it is in merges with another.

The results were shown at the National Astronomy Meeting in Birmingham.

Galaxy clusters, which can measure up to six million light-years across, are the largest objects whose mass can be measured by astronomers.

They grow by continuously pulling in smaller galaxies, but occasionally collisions occur between clusters of similar sizes - with catastrophic consequences.

Temperature increase

The energy released irreversibly changes the physical conditions inside the cluster by generating compression waves and shocks that heat the gas to temperatures 10,000 times those on the surface of the Sun.

Using data from XMM-Newton, Elena Belsole, from the University of Bristol, and colleagues measured the origins and energy of X-rays coming from these clusters.

We were surprised to find that some clusters that look quite established and serene are in their temperature very perturbed
Elena Belsole
University of Bristol
"We often study clusters through optical light. But in fact, this is just a minority of what we can see," Dr Belsole told the BBC News website.

"Previous satellites have been able to see in the X-ray spectrum but could not see very high energies. So we knew about the morphology of clusters but not about the temperatures there."

The X-ray energy allowed the researchers to calculate the gas temperature and, knowing where the X-rays were coming from, allowed them to map how temperatures changed throughout the clusters.

"[XMM Newton] allows us to look at the temperature structure.

"And what we were surprised to find was that some clusters that look quite established and serene are in their temperature very perturbed," said Dr Belsole.

Milky Way's fate

One of the clusters the team has been studying is A3921, which lies 1.2 billion light-years from Earth.

Data shows it was formed through the collision of two clusters that were unequally matched.

One of the clusters was three times smaller than the other, and was almost totally destroyed by the encounter. "The process is still taking place today.

"In several billion years, the group of which our galaxy, the Milky Way, is a member will be torn apart as it merges with the nearby Virgo cluster," said Dr Belsole.

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


SEE ALSO:
Europe has lander goal for Mars
08 Apr 05 |  Science/Nature
Fossil galaxies 'eat neighbours'
08 Apr 05 |  Science/Nature
Cosmic particle accelerator seen
08 Apr 05 |  Science/Nature


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