A pair of orbiting X-ray telescopes have captured images of two galaxies merging into a giant cluster.

Renato Dupke and colleagues from the University of Michigan used the European Space Agency's XMM-Newton and Nasa's Chandra orbiting X-ray observatories to investigate a "puzzling" galaxy cluster known as Abell 576.

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

Using the superior sensitivity and spectral resolution of the XMM-Newton and Chandra X-ray telescopes, Dupke 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 Earth faster than the other.

According to the researchers it was puzzling because the moving gas was cold by astronomical standards. If this gas moved at such high speeds, it should have had a temperature of more than double the measured 50 million degrees Celsius.

Dupke realised that Abell 576 is undergoing a collision, but seen head on, so that one cluster is now almost directly behind the other.

The 'cold' clouds of gas are the cores of each cluster, which have survived the initial collision but will eventually fall back together to become one.

The data reveals that the clusters have collided at a speed of over 3,300km/s. This is "interesting" because there are some computer models of colliding galaxy clusters which suggest that such a high speed is impossible to reach.