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Young Universe with an Old Galaxy Cluster

ESA’s XMM-Newton Satellite Discovers Most distant, Mature Galaxy Cluster

This image shows the galaxies in the galaxy cluster CL J1449+0856 as observed by the NASA/ESA Hubble Space Telescope. Most of the galaxies exhibit 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 a sign that this structure is a mature cluster, as evidenced also by the diffuse X-ray emission detected by ESA's XMM-Newton. The image has been taken with NICMOS-3 on board Hubble using one filter only (the F160W band, or H-band); the colours in the image have been added using information from ground-based data. The region depicted above is about 45 arc minutes on a side. Courtesy NASA/STScI.

The universe, as seen by those who study its earliest ages, shows much more interesting structure in

its early history than scientists used to think.  According to the most widely accepted theory about how the universe grew, the infant cosmos began with small objects and grew steadily larger by a process called “accretion.” This simply means that smaller objects coalesced together under the influence of their mutual gravitational pull. The cosmos was populated with ever-growing objects.  This is why

galaxy clusters, the largest cosmic structures that are bound together by gravity, are the last things to form. They take a lot of material and a strong gravitational field to hold together.

It’s not clear exactly when the first gravitationally bound galaxy clusters appeared, and so astronomers are focusing their attention, and the instruments of such satellites as the XMM-Newton observatory, to study them. iven 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 à l’Énergie Atomique (CEA), in France, who led an extensive study that revealed what appears to be the most distant galaxy cluster detected yet.

The cluster, called CL J1449+0856, is seen as it was when the universe was only about a quarter of its current age –around 3 billion years old. 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.

This image shows the X-ray emission (in purple) coming from the diffuse intra-cluster medium of the galaxy cluster CL J1449+0856 as detected by XMM-Newton. The X-ray signal is superimposed onto a composite image of very long exposures taken at near-infrared wavelengths with ESO's Very Large Telescope in Chile and the Subaru telescope on Hawaii. Most of the objects visible in the field are very faint and distant galaxies: the galaxy cluster is visible as a clump of faint, red objects near the center. Credit: ESA/ESO/Subaru/R. Gobat, et al.

This multi-wavelength study is based on observations performed with a number of ground- and space-based observatories: infrared data from NASA’s Spitzer Space Telescope; 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.

The x-ray data from both observatories exhibit detections that correspond to the cluster, however, only the XMM-Newton detection is associated with the diffuse intra-cluster gas, which is key to the findings presented in this paper, whereas the Chandra detection is associated with one of the galaxies belonging to the cluster.

The diffuse x-ray emission is a key signature of the cluster’s advanced evolutionary state. “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.

This fully mature galaxy cluster is located just over 1o billion light-years away, which is a record-breaking distance for such an object.

For more details about this remarkable distant galaxy cluster, visit XMM-Newton’s web page.

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October 2014
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