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A new cosmic crowd has captured the distance and heavyweight titles for galaxy clusters discovered deep in the universe. The record-breaker sits billions of light-years from Earth and weighs about a thousand times the mass of the Milky Way, astronomers report in an upcoming issue of Astronomy & Astrophysics.
“To discover a cluster that is so distant yet so big was quite lucky,” says study coauthor Georg Lamer of the Astrophysical Institute of Potsdam in Germany.
Lamer and his Potsdam colleagues first spotted the massive cluster, dubbed 2XMM J083026+524133, when scrutinizing data from the European Space Agency’s XMM-Newton space telescope. In 2001,the X-ray satellite captured the cluster’s signature while imaging a distant, active galaxy. Surveying the satellite’s catalogue earlier this year for nearby galaxies and distant clusters, the team was startled that the new cluster’s X-ray signal had been overlooked.
“It was so bright,” Lamer says.
Optical images from the Sloan Digital Sky Survey established that the light could not be coming from a nearby galaxy in that particular cosmic region. So the team took a deep field exposure with the Large Binocular Telescope at the Mt. Graham International Observatory near Safford, Ariz. The cluster appeared and was calculated to be 7.7 billion light-years from Earth. The previous record-holding cluster sits only 3.5 billion light-years away and weighs slightly less thana thousand Milky Ways .
“The new cluster, at its great distance and with its mass,” Lamer says, “can only be explained by the existence of dark energy.”
Dark energy is an unexplained force that accelerates the expansion of the universe. Without this force, Lamer says, nearby clusters should be much more massive than those that are billions of light-years away. Distant clusters, he says, should be less massive because they had less time to conglomerate.
“It is notoriously hard to compare cluster masses,” he notes. “But, the ‘neighboring’ Coma Cluster and this new, distant cluster actually seem to have comparable masses.”
Still, it is an overstatement to claim that dark energy exists based on observations of this one cluster, comments astrophysicist Stephen Murray of the Harvard-SmithsonianCenter for Astrophysics in Cambridge, Mass. Although, he says, the discovery does add an important data point in the study of galaxy clusters, which help astronomers test cosmological models that include dark energy.
The strength of dark energy at various cosmic times can be determined if astronomers compare the number of massive clusters found at different distances, Lamer says. But far-off, massive clusters are rare, and XMM-Newton scans too little of the sky to find them. So astronomers must wait until 2011 for the launch of eROSITA, a German X-ray telescope, to scan the entire sky for the predicted 100 or so remaining deep-space, cluster heavyweights, he adds.