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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.

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Based on that brightness, the team estimates that gas in this cluster is cooling faster than gas in any other cluster. It's this gas that ultimately chills enough to form stars – hot gas is too diffuse and energetic to collapse into the small, dense clumps that continue to contract under gravity until their cores ignite.

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Meanwhile, observations at other wavelengths showed the central galaxy as a bright blue object – signaling the formation of large numbers of stars. Indeed, the team estimates that the central galaxy was adding as many as 740 newborns a year when the light the team recorded left the cluster 5.7 billion years ago. The previous record-holder, a cluster 3.3 billion light-years away and known as Zw 33146, pumped out a leisurely 79 new stars a year. The Milky Way, by contrast, is a sluggard, at 1 new star per year.

The difference in star-formation rates among clusters' central galaxies, and the unusually rapid cooling that gas in the Phoenix Cluster displays, suggests that intergalactic gas is a far-more important source of raw material for stars in these central galaxies than previously thought.

More broadly, the discovery could shed light on a vexing question: Why do nearby clusters show high levels of X-ray emissions, but very small flow of cooler gas toward the cluster's center? Based on the X-ray emissions, these clusters should be in star-forming overdrive. But analysis of their central galaxies show puny star-formation rates compared with the Phoenix Cluster.

Researchers have invoked supermassive black holes as the likely regulator.

In galaxies where star formation is prodigious, the black holes snag some of that in-falling gas and slowly ramp up the amount of radiation emitted as that material begins to fall into them. This radiation is emitted in jets of highly accelerated plasma that reach far above and below the galaxies, sending shock waves into the intergalactic gas and heating it.

At some point, a supermassive black hole's emissions grow so large and the intergalactic gas becomes so hot that the cooling process virtually halts and continued star formation occurs largely from supplies of gas already in the galaxy – or from gas stolen from other galaxies during close passes or collisions.

With the galaxy in the Phoenix Cluster, the rate at which gas is falling toward the cluster's center "is so huge that, vast as the black hole itself probably is, it's not able to hold this in-fall of gas at bay," says Martin Rees, an astrophysicist at the University of Cambridge in Britain, who is not a member of the research team but offered his perspective during the briefing.

One explanation for what the team sees is that such high star-formation rates occur only briefly, before the black hole is activated and exerts its influence on the inflow of cold gas.

That explanation "is the most likely one," says Avi Loeb, an astrophysicist who heads the astronomy department at Harvard University in Cambridge, Mass. If the feeding period is short, occurring early in a galaxy's history, "when you take snapshots of clusters, only a small fraction of those snapshots would show this phase."

Or the high rate of star formation that the team detected could be part of a cycle, since once the black hole shuts off the supply of gas, it's ultimately starving itself. Once its activity falls and the radiation jets disappear, the intergalactic gas has a chance to cool, and the process begins anew.

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