Illustration courtesy M. Weiss, CXC/NASA
The Phoenix Cluster, as seen in x-ray, optical, and ultraviolet wavelengths.
Published August 15, 2012
But as many as ten telescopes have confirmed the strange case of the Phoenix Cluster. And despite the cluster's rarity, scientists say, the findings may help explain the evolution of all such clusters.
(Related: "Glowing, Green Space Blob Forming New Stars, Hubble Shows.") "The discovery of this cluster was a bit of a roller coaster, since, with every new observation, we found something even more exciting," said MIT astrophysicist Michael McDonald, lead author of the new study detailing the Phoenix findings, released by the journal Nature Wednesday.
"We were all very skeptical at first, since it doesn't fit our current picture of how galaxy clusters"—congregations of hundreds of galaxies bound by gravity—"should look," McDonald said.
"But over the span of a couple months, we convinced ourselves that this was truly an exceptional, unique system."
Named after the southern constellation it resides in, 5.7 billion light-years from Earth, the Phoenix Cluster is an island of thousands of galaxies.
It's so massive, McDonald said, that our own galaxy group—not quite a cluster, at about 30 galaxies, including the Milky Way—has only about 0.1 percent the mass of Phoenix. Among its other claims to fame: the brightest output of x-ray light ever seen from a cluster.
(See a picture of another galaxy cluster.)
Star Factory at Full Steam
What's puzzling astronomers most is that the galaxy at the center of the Phoenix Cluster appears to be in an active "starburst" period—birthing more than 740 stars a year, based on data from NASA's Chandra X-ray Observatory, the National Science Foundation's South Pole Telescope, and eight other telescopes on Earth and in orbit.
Among central cluster galaxies, "the previous record-holder forms stars at a rate of around 150 stars per year, so this is absolutely crushing that record," McDonald said.
It's especially confounding given that central galaxies are generally the oldest in their clusters, as evidenced by a telltale red glow—far past star-bearing age.
The core of the Phoenix cluster shines bright blue, though—the color of baby starlight, McDonald said.
Case of the Cooling Cluster
The newfound galaxy cluster may shed light on a long standing astrophysical puzzle known as the cooling-flow problem.
According to theoretical models, gases thrown out by star explosions—especially plentiful in aging central galaxies—should naturally cool over time, forming a gas flow cold enough to condense and form new stars quickly (star pictures).
Problem was, no one had seen this happen until now.
In most cases, jets of material from supermassive black holes at the centers of galaxies are heating surrounding gases, preventing them from reaching temperatures at which star birth can kick into high gear, according to McDonald and to astrophysicist Brian McNamara, who wasn't part of the new study.
"For some reason this [heating] cycle isn't working in the Phoenix Cluster, and we don't know why—that's the mystery," said McNamara, of Canada's University of Waterloo.
Study leader McDonald may have the answer.
We may have simply happened upon a central galaxy at a previously unknown, pivotal stage in the normal course of galactic evolution, McDonald said—before its black hole has grown powerful enough to fully turn up the heat.
"We suspect that this phase should only last for about a hundred million years, which is less than one percent of the age of the universe," McDonald said. "So it is not surprising that these systems are rare, since the odds of catching one during this starburst phase should be around one in a hundred."
Waterloo's McNamara isn't so sure.
The Phoenix Cluster, he said, is "not behaving like most galaxies in cooling flows, and while we have lots of ideas for why this would be, we are not sure which, if any, are correct."
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