The XXL extragalactic survey, made possible notably by ESA’s XMM-Newton satellite and the telescopes at the European Southern Observatory (ESO) in Chile, has located and identified 450 galaxy clusters and 22,000 active galaxies. The Astronomy & Astrophysics Review is publishing a first series of results of this international collaboration led by CEA, the French atomic energy and alternative energies commission, and backed by CNES, mobilizing research scientists and resources from the French scientific research centre CNRS, Aix-Marseille University and the Côte-d’Azur Observatory.
The largest structures in the Universe, galaxy clusters can attain masses more than 100,000 billion times that of the Sun. These giant objects where galaxies merge amidst clouds of gas in temperatures of millions of degrees are the hot topic in modern astronomy. How did something this large form? How old are they, what is their history and what will become of them in the future? What role does the enigmatic dark matter play inside them? What can their evolution, deduced by comparing the oldest and newest clusters, tell us about the evolution of the Universe itself and in particular about the effects of the elusive dark energy?
The XXL X-ray survey conducted from 2011 to 2013 by the XMM-Newton satellite aims to catalogue hundreds of galaxy clusters out to distances where the Universe is roughly only half its current age. The data collected will enable research scientists to retrace the evolution and spatial distribution of these structures and test different cosmological scenarios.
The results presented in the first series of articles concern the 100 brightest clusters detected in the XXL survey and will enable a first reconstruction of the structure of the Universe out to distances of more than 11 billion light-years.
These results have revealed a significantly lower density of clusters than predicted by cosmological models, and lower amounts of gas than expected. A similar deficit was observed for the most massive clusters detected directly in the all-sky survey conducted by the Planck satellite. This suggests one of two things: either we need to rethink certain parameters describing the physics of clusters, or the cosmological model is more complex than—or simply different to—the one currently believed. The full study of the sample of 450 clusters surveyed by the XXL project, which will be published in two years’ time, is expected to give us a clearer answer.
Initial analyses of the XXL survey have revealed five new "super clusters" or "clusters of galaxy clusters". One of them, XLSSC-e, in the Cetus (Whale) constellation, approximately 4.5 light-years away, comprises six different clusters covering a region of 0.3 by 0.2 degrees, which at this distance makes it 7 x 4.5 million light-years big.
The XXL project is a key stepping stone to the future new-generation surveys that aim to cover all or a significant portion of the sky, such as DES (Dark Energy Survey), eROSITA (extended ROentgen Survey with an Imaging Telescope Array), LSST (Large Synoptic Survey Telescope) and Euclid.
XMM-Newton is an ESA satellite. CNES is helping to fund the satellite and contributing to data processing through support for French research laboratories involved.
The XXL survey I : Scientific motivations, XMM observing plan - Follow-up observations and simulation programme, Pierre M., Pacaud F., Adami C. et al. 2015 A&A in press
The XXL survey II : The bright cluster sample, Pacaud F., Clerc N., Giles, P. et al. 2015 A&A in press
The XXL survey III : Luminosity-Temperature Relation of the Bright Cluster Sample, Giles P., Maughan B., Pacaud F. et al. 2015 A&A in press
The XXL survey IV : Weak lensing mass – X-ray temperature scaling relation for the bright cluster sample, Lieu M., Smith G., Giles P. et al. 2015 A&A in press
The XXL survey V: Detection of the Sunyaev-Zel'dovich effect of the Redshift 1.9 Galaxy Cluster XLSSU J021744.1-034536 with CARMA, Mantz A>, Abdulla Z., Carlstrom J.et al. 2014, ApJ 794, 157
The XXL survey VI: The 1000 brightest X-ray point-sources, Fotopoulou S., Pacaud F., Paltani S. et al 2015 A&A in press
The XXL survey VII: A supercluster of galaxies at z=0.43, Pompei E., Adami C., Eckert D. et al 2015 A&A in press
The XXL survey VIII : Spectroscopic MUSE and imaging CFHT view of Intra Cluster Light in a z=0.54 cluster of galaxies, Adami C., Pompei E., Sadibekova T. et al. 2015 A&A in press
The XXL survey IX : 3 GHz VLA radio observations towards a supercluster at z=0.43, Baran N., Smolcic V., Milakovic D. et al. 2015 A&A in press
The XXL survey X : Weak-lensing mass - K-band luminosity relation and implication on cluster galaxies, Ziparo F., Smith G., Mulroy S. et al. 2015 A&A in press
The XXL survey XI : ATCA 2 GHz continuum observations, Smolcic V., Delhaize J., Huyn M. et al. 2015 A&A in press
The XXL survey XII : Optical spectroscopy of X-ray-selected clusters and evidence of AGN suppression in superclusters, Koulouridis E., Poggianti B., Altieri B. et al. 2015 A&A in press
The XXL survey XIII: The baryon content of the bright cluster sample, Eckert D., Ettori S., Coupon J. et al. 2015 A&A in press
The XXL survey XIV: AAOmega redshifts for the southern XXL field, Lidman C., Ardila F., Owers M. et al. 2015 PASA in press
 Active galaxies are those with a black hole at their centre.
 543 observations that required more than six million seconds of exposure time, making this the largest survey conducted since the launch of XMM-Newton in 1999.
 To retrace the history of the Universe as far back in time as this, cosmologists need to be able to observe very distant zones. They therefore point their instruments at "empty" darker regions where they can "see" the faintest sources.