In contrast to other structures observed in the young Universe, this object is already in its prime, as is evident from its diffuse X-ray emission and evolved population of galaxies. This shows that fully-grown galaxy clusters were already in place this early in cosmic history.

According to the most widely accepted theory, structure in the Universe grows in a hierarchical fashion: under the effect of gravity, smaller objects build up first, and then accrete into progressively larger ones.

This is why galaxy clusters, the largest cosmic structures that are bound by gravity, are the last to form. When these enormous assemblies of galaxies, hot gas and dark matter appeared in the cosmos is still an open question, and the urge to resolve this question drives astronomers to observe galaxy clusters that are as distant as possible.

Given that galaxy clusters are scarce, and even more so at high redshifts where they are still taking shape, these observations are challenging. "This is why discovering a galaxy cluster at a redshift of 2 feels like unearthing a rare and valuable gem," says Raphael Gobat from the Commissariat a l'Energie Atomique (CEA), in France, who led the extensive study that revealed what appears to be the most distant galaxy cluster detected yet.

CL J1449+0856, the newly-found galaxy cluster, is seen at an era when the Universe was only 3 billion years old - less than a quarter of its present age. "As well as being at a record-breaking distance, what makes this object rather unique is that it's not a proto-cluster undergoing formation, as are many that have been detected at high redshifts, but it is already mature, a proper galaxy cluster," adds Gobat.

A signature of its advanced evolutionary state is the diffuse X-ray emission associated with the cluster, observed with ESA's XMM-Newton. "Only galaxy clusters that have had time to fully develop, collapsing under the influence of their own gravity, are visible in X-rays," explains Alexis Finoguenov from the Max-Planck-Institut fur extraterrestrische Physik (MPE), in Germany, co-author of the paper in which the result is presented.

The X-ray emission originates from the hot intra-cluster gas: subject to the cluster's gravitational potential, the gas is compressed and heated to temperatures of over 10 million Kelvin, and shines at X-ray wavelengths.

The X-ray data not only show evidence that this is a fully-formed cluster, but also allow characterisation of its properties. The researchers estimate that the mass of the cluster is about 5-8 x 1013 solar masses. "The cluster has a moderate mass, as expected at such an early epoch, and corresponds to the progenitor of massive structures observed in the local Universe, such as the Coma cluster," adds Finoguenov.

With masses that have not yet reached extreme values, high-redshift clusters are relatively faint X-ray sources and are thus particularly hard to observe. Furthermore, their emission peaks in the soft X-ray band.

"As it is more sensitive to soft X-rays than other instruments, XMM-Newton is the ideal tool to detect the signal from mature clusters in the young Universe," remarks Norbert Schartel, ESA'S XMM-Newton Project Scientist.

This remarkable finding is the result of a multi-wavelength study which lasted roughly five years and involved a number of observation facilities on the ground as well as space-based telescopes. Originally, the cluster was identified while analysing infrared data from NASA's Spitzer Space Telescope.

"We stumbled upon this galaxy cluster by chance: the structure caught our eyes as a clump of galaxies which appeared much redder and more concentrated than those in the surroundings," notes co-author Emanuele Daddi, also from CEA. The team then ran a follow-up campaign to assess the redshifts of the galaxies, and their measurements confirmed that the galaxies are indeed close to each other, forming a cluster at redshift z~2.

They also examined the characteristics of the galaxies in greater detail using data that included observations by the NASA/ESA Hubble Space Telescope, and found that most of them have quite compact shapes, consistent with what is expected for galaxies with a bulge dominated by old stars. "The fact that the stellar populations in the galaxies had time to evolve is an additional sign of the cluster's maturity, along with its X-ray signal," Daddi adds.

The evidence obtained all points in the same direction, but the team's efforts are not over and they are investigating this structure in even greater detail to study how early galaxy formation and the assembly process of a galaxy cluster are affected by the environment.

A serendipitous discovery corroborated by a body of deep and high-quality data, the record set by this finding is unlikely to be broken in the near future, as theory predicts that chances of finding similar clusters at redshifts of 2 or more using state-of-the-art instruments are low - only about a few per cent.

"A single detection like ours is compatible with this small probability and can still be accommodated within current cosmological models," explains Gobat. However, if deeper and larger surveys were to detect more objects like this, the most widely accepted theory for the formation of cosmic structure would require revision.

"This discovery has shown, for the first time, that old clusters were already present in the young Universe and that we are able to observe them. Only time will tell us what 'old' and 'young' really mean and what this implies in the broader context of cosmology," he adds.

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