An old galaxy cluster discovered in the young Universe
Astronomers working with data from several observatories, including the VLT and XMM-Newton, have discovered the most distant, mature galaxy cluster yet. The cluster is seen as it was when the Universe was only about a quarter of its current age. 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.
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," says Raphael Gobat from the Commissariat à l'Énergie Atomique (CEA), in France, who led the extensive study that revealed what appears to be the most distant galaxy cluster detected yet.
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 für 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 × 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.
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. Follow-up observations used optical and near-infrared data from the Subaru Telescope of the National Astronomical Observatory of Japan, the Very Large Telescope of the European Southern Observatory, the William Herschel Telescope, the NASA/ESA Hubble Space Telescope and the W.M. Keck Observatory; X-ray data from ESA's XMM-Newton and NASA's Chandra X-ray observatories.