Record-breaking galaxy cluster discovered

(Press-News.org) A massive galaxy cluster nearly six billion light years from Earth has been discovered with an astounding and unexpected burst of star formation – more prodigious than any galaxy cluster yet observed, an international team of astronomers and NASA announced today.

In a wide-ranging discussion on the eve of the announcement, two of the leading astronomers on the project talked about the record-breaking galaxy cluster, called Phoenix, and how its surprising properties are prompting astronomers to re-think how galaxy clusters – among the largest structures in the universe – form and evolve over cosmic time.

"This discovery in the Phoenix Cluster suggests a whole new twist to … how massive galaxies at the center of galaxy clusters grow," said Hubble Fellow Michael McDonald, a researcher at the Kavli Institute for Astrophysics and Space Research at MIT and also the lead author of a paper appearing Aug. 16 in the journal Nature. "It allows for another mechanism for the growth of these galaxies. It opens up a whole new area of research."

Bradford Benson, a researcher at the Kavli Institute for Cosmological Physics at the University of Chicago and a co-author of the paper, noted that the massive Phoenix Cluster, which is named for the constellation that marks its direction on the sky, was discovered during a survey by the South Pole Telescope. "The Phoenix Cluster stood out as having very high X-ray emission from its center, so much that it made the entire cluster the most luminous X-ray cluster ever observed," Benson said. "That immediately peaked our interest, because it suggested that cool gas was condensing in the center of the cluster."

The prevailing view of astronomers who study clusters has been that their central galaxies are largely dormant, full of old stars and lacking the kind of vigorous star formation that marked their youth. But the central galaxy in Phoenix signaled new star formation.

The research team followed up the initial discovery of Phoenix with observations by numerous telescopes – compiling enough data to convincingly conclude that Phoenix was unlike any galaxy cluster ever seen.

"I am a very glass half-full guy," McDonald said. "I immediately thought that it was extremely exciting, and I needed people to reign me in." Still, McDonald noted that to go from the initial excitement of finding evidence for rapid star formation to actual confirmation "required several months of checking results and getting additional data."

McDonald and Benson shared why they think the central galaxy is so active with starbursts, and how they may have caught the galaxy cluster during a brief period that all clusters go through in their evolution – although to be sure about that, more data is needed. As for next steps, according to McDonald, "We're going to try to find more clusters, either like this or unlike this. The reason is that it's really hard to draw any conclusion based on one galaxy cluster. So while this is exciting, it doesn't necessarily tell us about the overall evolution of galaxies and galaxy clusters."

Benson said the exciting discovery promises to be a gift that keeps on giving. "Cosmology tries to answer some of the biggest questions about the universe, such as 'How old is it?' and 'How did it evolve?'" Benson noted that studying galaxy clusters helps astronomers better understand those mysteries, such as the nature of dark energy, the unknown force that's causing the universe to expand at accelerating rates. "We want to understand (galaxy clusters) better so we can use them as cosmological tools," he said. "But they are also interesting by themselves."

The complete discussion can be found at: http://www.kavlifoundation.org/science-spotlights/phoenix-cluster-mcdonald-benson

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