Astronomers of SRON have discovered a new cluster of galaxies, hidden behind a previously identified cluster of galaxies. The recently exposed cosmic giant is apparently just as bright as the first group, but is six times further away. The astronomers made the discovery as part of an international team using the space telescope XMM-Newton.

Being fooled by a cosmic giant is no laughing matter for an astronomer. For years astronomers racked their brains over the relation between two in X-rays equally bright and large regions in the cluster of galaxies known as Abell 3128. ‘That is the charm of science’, says Norbert Werner, PhD student at SRON. ’You are always finding things that you did not expect.’

Clusters of galaxies are the largest structures in the universe. They consist of tens to hundreds of massive galaxies, of which each in turn consists of hundreds of billions of stars. Gravity is the binding factor. The hot gas of tens of millions degrees Celsius, present in the clusters, emits X-rays, which renders the cluster visible for space telescopes such as XMM-Newton. Detailed analyses of these X-rays tell astronomers more about the composition of the gas and accordingly, its origin.

What was so intriguing about the two X-ray spots in cluster Abell 3128 was the fact that although they had the same size and brightness, the gas clouds seemed to have completely different compositions. Werner: ‘While one spot was clearly caused by a hot gas cloud rich in metals released by supernova explosions in the galaxies, the other spot seemed to contain a much lower amount of metals than any other cluster previously observed. What we observed completely contradicted the current theories about how large structures in the universe arise.’

The observations with the XMM-Newton made the surprise complete. The gas cloud behind the puzzling X-ray spot was found to be 4.6 billion light years away, at least six times further than cluster Abell 3128. ‘We were therefore looking at two completely different objects, which from our perspective were in exactly the same line of sight’, says Norbert Werner.

‘The research into this large cluster of galaxies mainly centres on the question as to how the large structures of the universe have been formed’, explains project leader Jelle Kaastra. According to current insights, material is spread throughout the universe as a web of thread-like structures of rarefied hot gas: the cosmic web. Between these threads are cavities that are becoming increasingly larger as the universe expands. ‘Compare it to bubbles in a bubble bath’, says the astronomer. The density of the material is highest at the intersections in the web. Therefore that is where clusters of galaxies develop.

Due to their enormous mass and attractive force, the clusters have their own dynamics. Kaastra: ‘They attract each other, collide and fly through each other; a whole host of things happen that we can study with X-ray telescopes such as the XMM-Newton.’

XMM-Newton is the X-ray telescope of the European Space Agency (ESA) for which SRON built an instrument capable of analysing the X-rays in detail. XMM-Newton was launched in 1999 from French Guyana and still functions superbly. ESA recently extended the operation of the satellite for a further 5 years.

The results from the research of Norbert Werner and Jelle Kaastra were recently published in the scientific journal Astronomy & Astrophysics. Reference: Werner, N., Churazov, E., Finoguenov, A., Markevitch, M., Burenin, R., Kaastra, J. S., and Böhringer, H., “Complex X-ray morphology of Abell 3128: a distant cluster behind a disturbed cluster” A&A 474, 707–716 (2007).