Scanning the sky for X-ray sources, ESA’s XMM-Newton X-ray observatory has been busy with the XXL Survey, its largest observational programme to date. The second batch of data from the survey has just been released, including information on 365 galaxy clusters, which trace the large-scale structure of the Universe and its evolution through time, and on 26,000 active galactic nuclei (AGN). The first results using this data are published in a special issue of Astronomy & Astrophysics.

X-rays are produced in some of the most energetic processes in the Universe, but because they are blocked by Earth’s atmosphere, they can only be observed from space. When X-ray telescopes observe the extragalactic Universe, they basically see two sources: the hot gas pervading clusters of galaxies, and Active Galactic Nuclei (AGN) – bright, compact regions at the centres of some galaxies where a supermassive black hole is accreting the surrounding matter.

ESA’s XMM-Newton is one of the most powerful X-ray telescopes ever placed in orbit. Over the last eight years, it has spent 2,000 hours measuring X-ray radiation as part of the XXL Survey, which searched for galaxy clusters and AGN by scanning two areas of seemingly-empty sky each measuring 25 square degrees (as a reference, the full moon measures about half a degree across).

The survey mapped X-ray clusters so distant that the light left them when the Universe was less than half of its present age, and AGN that are even further away. Some of the observed sources are so far-flung that XMM-Newton received no more than 50 X-ray photons from them, making it challenging to tell whether they are clusters or AGN.

"This is of critical importance, since one of the ways we can test our models of the Universe is to count the number of galaxy clusters at different distances from us. If we mis-classify clusters as AGN, or vice-versa then we would get completely wrong answers.", explains Ben Maughan of the astrophysics research group at the University of Bristol, one of the main institutions working on the XXL survey.

Maughan's PhD student Crispin Logan led a project to scrutinise the most distant galaxy clusters found in XXL, using NASA's Chandra X-ray observatory.

"While Chandra is not as sensitive as XMM, it has far sharper resolution, so we can easily test whether an object that we thought was a galaxy cluster was really an AGN.", said Logan. "These very distant galaxy clusters are the most difficult to classify, but the sophisticated computer algorithms used by XXL passed this test, giving us confidence in the final results of the survey."

Galaxy clusters form from the densest parts of the early Universe and they are so large that the gravitational forces that drive their formation must overcome the expansion of the Universe which acts to pull them apart. "If we were transported to a parallel universe where the rate of cosmic expansion, or amounts of dark matter or dark energy were a bit different from our own, we would find the numbers of galaxy clusters that were able to form would be very different," explains Ben Maughan. "This makes galaxy clusters very powerful tools for studying our Universe and measuring the parameters of the models we use to describe it."

ESA’s Planck satellite determined values for these cosmological parameters by studying the cosmic microwave background, which is information from the very early Universe. After estimating these parameters using the latest data from the XXL Survey – which is based on information from the more recent Universe – scientists compared their findings against the Planck values.

“Although we didn’t find as many galaxy clusters as predicted by the Planck cosmological model, we obtained a distribution of clusters and AGN that is compatible with the currently favoured cosmological model, which resorts to Einstein's cosmological constant as an explanation for the accelerated expansion of the Universe, rather than invoking even more exotic possibilities,” explains project leader Marguerite Pierre, from CEA Saclay, France. “We can already improve on the Planck estimate for the cosmological constant, even though our analysis has only been carried out on half of the XXL cluster sample; we will spend the next couple of years analysing the rest of the data with the aim of refining the cosmological constraints.”

The ultimate goal of the XXL Survey is to provide an extensive, well-characterised catalogue of clusters that can be used to place even better constraints on the cosmological parameters. The final XXL data release containing even more X-ray sources, as well as the complete cosmological analysis, is foreseen for 2021.

“It is very exciting that data from this space telescope is contributing to our understanding of the evolution of the Universe,” concludes Norbert Schartel, XMM-Newton Project Scientist at ESA. “This was made possible thanks to the collaboration between a huge number of institutions across many different countries.”