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Why a galaxy far, far away has shattered records for birthing stars

Astronomers identify a giant cluster of galaxies 5.7 billion light-years from Earth. At its core new stars are being formed at a rate that could explain how supermassive black holes govern a galaxy's growth.

By Staff writer / August 15, 2012

Astronomers peering deep into the heart of a cluster of galaxies some 5.7 billion light-years away have found that the cluster’s central galaxy is producing stars at an extraordinary rate – up to hundreds of times more stars per year than any of its siblings in a handful of other well-studied clusters.

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The largest, most productive galaxies in the universe are found at the centers of galaxy clusters, researchers say. And astronomers thought they had a pretty good handle on why these galaxies were at the top of the star-formation heap – acquisitions and mergers involving other galaxies.

The new find, however, in what the astronomers are calling the Phoenix Cluster, suggests an unexpected source of gas available to make new stars in these central galaxies – the gas between galaxies in the cluster.

Even more intriguing, it also could shed light on the interplay between supermassive black holes at the centers of galaxies and the galaxies themselves. That interplay is thought to limit the size each actor in this cosmic tug of war can reach.

As clusters go, the Phoenix Cluster is among the largest yet detected. The entire cluster spans some 8.5 million light-years. It contains hundreds of thousands of Milky Way-size galaxies, and sports an overall mass of 2,500 trillion suns. X-ray emissions from all of the hot gas it contains shine brighter than the X-ray emissions from any other known cluster.

The galaxy at the center of the Phoenix Cluster is "crushing the record" for star birth in galaxy clusters, says Michael McDonald, a researcher at the Kvali Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology in Cambridge. Dr. McDonald is the lead author of the discovery's formal report, which appears in Thursday's issue of the journal Nature.

The cluster's formal title is SPT-CLJ2344-4243. It lies in the direction of the constellation Phoenix, a minor grouping of stars in the Milky Way and visible in the Southern Hemisphere.

Prior to 2010, no one knew this enormous collection of gas and galaxies existed.

That year, a microwave telescope conducting a sky survey from its frigid spot at the South Pole detected a "shadow" in the universe's microwave background – emissions that were discovered in 1964 as a ubiquitous radio hiss all across the sky. Theorists identified it as the echo of the Big Bang – the sudden release of energy that gave rise to the universe some 13.8 billion years ago.

That shadow was cast by the Phoenix Cluster, explained Bradford Benson, an astrophysicist at the University of Chicago, in a briefing for reporters on Wednesday. The cluster was blocking the microwave radiation as it traveled from its more distant source – the edge of the observable universe. The cluster's traits only became clear after researchers trained eight additional ground- and space-based telescopes on the object to view it at different wavelengths.

Using NASA's Chandra X-Ray Observatory, currently in orbit around Earth, the team saw that the cluster's X-ray emissions not only were the brightest of any known cluster, but that the brightest region appears toward the cluster's center. That increase in X-ray brightness is due to emissions the gas gives off as it cools during its migration from the cluster's periphery toward its center.


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