Massive galaxy cluster to shed light on cosmic lumpiness


By Rachel Courtland (Image: ESA XMM-Newton/EPIC/LBT/LBC/AIP/J Kohnert) Astronomers have found the most massive distant cluster of galaxies yet seen. A search of the rest of the sky for such objects could help measure the lumpiness of the universe and the effects of dark energy, the mysterious entity that is causing space to expand ever faster. The cluster, called 2XMM J083026+524133, is as massive as 1000 Milky Way galaxies. A large part of this bulk is contained in 95 million-degree gas that lies between the cluster’s galaxies. Massive clusters are fairly common in the nearby universe. But models predict such galaxies are some 200 times rarer where the newly discovered cluster resides, some 7.7 billion light years away, says astronomer Georg Lamer of the Astrophysikalisches Institut Potsdam in Germany. Since 2001, another cluster, called CL J1226.9+3332, has held the title as the most massive distant cluster, says Lamer. But the new cluster is more than 500 million light years farther away and slightly heavier. Lamer and colleagues discovered the cluster while combing through archived X-ray data from Europe’s XMM-Newton observatory. The catalogue contained some 20,000 large X-ray sources and covered 1% of the sky. But the team found only one massive cluster candidate in the distant universe. Follow-up observations with the Large Binocular Telescope on Mount Graham in Arizona confirmed the find, and showed what appeared to be about 10 individual galaxies inside the cluster. But Lamer says the cluster likely contains far more galaxies that remain unseen. Lamer estimates that 100 or so clusters might turn up in future X-ray surveys of the distant universe over the entire sky. This rarity might be a powerful cosmological probe, because small variations in the properties of the universe can alter the number of massive clusters astronomers expect to see billions of years after the big bang. Counting such massive clusters could yield a more precise measurement of a cosmological measure of the lumpiness of the early universe, called Ó8. A measure of the strength of fluctuations in the density of matter in the early universe, this factor helps reveal how quickly matter gravitationally collapsed to form stars, galaxies, and ultimately galaxy clusters. “The number of clusters is extremely sensitive to that [figure],” says Richard Mushotzky, a galaxy cluster expert at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Currently, Ó8 is only known to an accuracy of about 15%, Mushotzky says. Massive clusters are also affected by dark energy, which pushes space apart and slows down the formation of clusters. Future surveys of clusters could thus put limits on dark energy’s strength at different cosmic times. One all-sky survey that could be used to search for more massive, early clusters is a German X-ray telescope called eROSITA. The instrument is set to launch in 2011 on a Russian satellite. Journal reference:
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