Mar. 16, 2005 Combining observations with ESO's Very Large Telescope and ESA's XMM-Newton X-ray observatory, astronomers have discovered the most distant, very massive structure in the Universe known so far.
It is a remote cluster of galaxies that is found to weigh as much as several thousand galaxies like our own Milky Way and is located no less than 9,000 million light-years away.
The VLT images reveal that it contains reddish and elliptical, i.e. old, galaxies. Interestingly, the cluster itself appears to be in a very advanced state of development. It must therefore have formed when the Universe was less than one third of its present age.
The discovery of such a complex and mature structure so early in the history of the Universe is highly surprising. Indeed, until recently it would even have been deemed impossible.
Clusters of galaxies are gigantic structures containing hundreds to thousands of galaxies. They are the fundamental building blocks of the Universe and their study thus provides unique information about the underlying architecture of the Universe as a whole.
About one-fifth of the optically invisible mass of a cluster is in the form of a diffuse, very hot gas with a temperature of several tens of millions of degrees. This gas emits powerful X-ray radiation and clusters of galaxies are therefore best discovered by means of X-ray satellites (cf. ESO PR 18/03 and 15/04).
It is for this reason that a team of astronomers  has initiated a search for distant, X-ray luminous clusters "lying dormant" in archive data from ESA's XMM-Newton satellite observatory.
Studying XMM-Newton observations targeted at the nearby active galaxy NGC 7314, the astronomers found evidence of a galaxy cluster in the background, far out in space. This source, now named XMMU J2235.3-2557, appeared extended and very faint: no more than 280 X-ray photons were detected over the entire 12 hour-long observations.
A Mature Cluster at Redshift 1.4
Knowing where to look, the astronomers then used the European Southern Observatory's Very Large Telescope (VLT) at Paranal (Chile) to obtain images in the visible wavelength region. They confirmed the nature of this cluster and it was possible to identify 12 comparatively bright member galaxies on the images (see ESO PR Photo 05b/05).
The galaxies appear reddish and are of the elliptical type. They are full of old, red stars. All of this indicates that these galaxies are already several thousand million years old. Moreover, the cluster itself has a largely spherical shape, also a sign that it is already a very mature structure.
In order to determine the distance of the cluster - and hence its age - Christopher Mullis, former European Southern Observatory post-doctoral fellow and now at the University of Michigan in the USA, and his colleagues used again the VLT, now in the spectroscopic mode. By means of one of the FORS multi-mode instruments, the astronomers zoomed-in on the individual galaxies in the field, taking spectral measurements that reveal their overall characteristics, in particular their redshift and hence, distance .
The FORS instruments are among the most efficient and versatile available anywhere for this delicate work, obtaining on the average quite detailed spectra of 30 or more galaxies at a time.
The VLT data measured the redshift of this cluster as 1.4, indicating a distance of 9,000 million light-years, 500 million light years farther out than the previous record holding cluster.
This means that the present cluster must have formed when the Universe was less than one third of its present age. The Universe is now believed to be 13,700 million years old.
"We are quite surprised to see that a fully-fledged structure like this could exist at such an early epoch," says Christopher Mullis. "We see an entire network of stars and galaxies in place, just a few thousand million years after the Big Bang".
"We seem to have underestimated how quickly the early Universe matured into its present-day state," adds Piero Rosati of ESO, another member of the team. "The Universe did grow up fast!"
Towards a Larger Sample
This discovery was relative easy to make, once the space-based XMM and the ground-based VLT observations were combined. As an impressive result of the present pilot programme that is specifically focused on the identification of very distant galaxy clusters, it makes the astronomers very optimistic about their future searches. The team is now carrying out detailed follow-up observations both from ground- and space-based observatories. They hope to find many more exceedingly distant clusters, which would then allow them to test competing theories of the formation and evolution of such large structures.
"This discovery encourages us to search for additional distant clusters by means of this very efficient technique," says Axel Schwope, team leader at the Astrophysical Institute Potsdam (Germany) and responsible for the source detection from the XMM-Newton archival data. Hans Boehringer of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, another member of the team, adds: "Our result also confirms the great promise inherent in other facilities to come, such as APEX (Atacama Pathfinder Experiment) at Chajnantor, the site of the future Atacama Large Millimeter Array. These intense searches will ultimately place strong constraints on some of the most fundamental properties of the Universe."
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