Astronomers observe biggest galactic collision in universe

The European Space Agencys (ESAs) XMM Newton and NASAs Chandra X ray observatories have caught a pair of galaxy clusters merging into a giant cluster.

Paris, July 19 : The European Space Agency's (ESA's) XMM Newton and NASA's Chandra X ray observatories have caught a pair of galaxy clusters merging into a giant cluster.

Scientists say the discovery adds to existing evidence that galaxy clusters can collide faster than previously thought.

When individual galaxies collide and spiral into one another, they discard trails of hot gas that stretch across space, providing signposts to the mayhem. Still, recognising the signs of collisions between whole clusters of galaxies is not as easy.

Now, Renato Dupke and colleagues from the University of Michigan have used the ESA's XMM-Newton and NASA's Chandra orbiting X-ray observatories to disentangle the puzzling galaxy cluster, Abell 576.

Previous X-ray observations had hinted that the gas was not moving uniformly across the cluster.

The team took readings from two locations in the cluster and saw that there was a distinct difference in the velocity of the gas. One part of the cluster seemed to be moving away from us faster than the other.

However, the moving gas was found cold by astronomical standards. At such high speeds, it ought to have had a temperature of more than double the measured 50 million degrees Celsius.

"The only explanation was to take the Bullet Cluster and turn it in the line of sight, such that one galaxy cluster is directly behind the other" said Dupke.

Dupke said Abell 576 was also bullet collision, in which one galaxy passes through the other, like a bullet travelling through an apple, but since this was happening across our line of sight, so it could be seen.

"Nevertheless, the Bullet Cluster is estimated to have a collision speed similar to the Abell 576 system. There is now a growing body of evidence that these high collision velocities are possible," said Dupke.

"The job of explaining these high speeds now rests with the cosmologists. Major cluster-cluster collisions are expected to be rare, with estimates of their frequency ranging from less than one in a thousand clusters to one in a hundred. On collision, their internal gas is thrown out of equilibrium and if unrecognised, causes underestimation of its mass by between 5 and 20 percent," he said.

Dupke further said the 'cold' clouds of gas - the cores of each cluster, which have survived the initial collision, will eventually fall back together to become one.

The study, 'The merger in Abell 576: a line of sight bullet cluster?' appears in the Astrophysical Journal.

ANI