March 9, 2011
Old Galaxy Cluster Found In Young Universe
Astronomers working with data from several observatories, including ESA's 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.
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 Ã l'Ã“°nergie 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 fr 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 characterization of its properties. The researchers estimate that the mass of the cluster is about 5-8 Ã– 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 analyzing 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.
The galaxy cluster discovered in this study, named CL J1449+0856, was originally identified in archival images from the Spitzer Space Telescope as an excess density of galaxies with emission peaking at red and infrared wavelengths. The data cover 342 square arc minutes of the 'Daddi Field'.
Further observations revealed a wealth of distant galaxies in this field, with emission that peaks at different wavelengths "“ galaxies with younger stellar populations peaking at bluer wavelengths, those with older stellar populations peaking at redder wavelengths. The redshifts of 109 'blue' galaxies have been measured spectroscopically and their redshift distribution peaks at z=2.07. For the 'red' galaxies, only photometric redshifts were estimated and the distribution of their redshifts peaks at z=2.05. The agreement of these two estimates indicates that the cluster redshift lies in the range z=2"“2.1.
The cluster luminosity (L), measured from the X-ray data, is approximately 7 Ã– 1043 erg/s in the soft X-ray energy range (0.1-2.4 keV), and corresponds to a temperature of 2 keV (about 23 million Kelvin) and an estimated mass of 5-8 Ã– 1013 solar masses. Relying on the results of numerical simulations, the cluster is predicted to grow and reach a mass of 0.9-1.5 Ã– 1014 solar masses at z=1.5 and of 4.9-8.2 Ã– 1014 solar masses at z=0; these estimates fall well within the range of measured mass values for clusters at these redshifts.
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.
Related publication: Gobat, R., et al., "A mature cluster with X-ray emission at z=2.07", A&A, 2011, 526, A133. DOI: 10.1051/0004-6361/201016084
Image 1: Composite near-infrared and X-ray image of the galaxy cluster CL J1449+0856, from XMM-Newton data and ground based observations. Credit: ESA/ESO/Subaru/R. Gobat et al.
Image 2: Hubble's near-infrared view of galaxy cluster CL J1449+0856 Credit: NASA/ESA/R. Gobat et al.
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