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Giant cluster phenomenally fertile

Stars form fast thanks to snoozing black hole

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10:57am, August 16, 2012
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Nearly 6 billion light-years away, one of the most massive galaxy clusters ever seen is birthing stars at an incredible rate, with its central member producing more than 700 suns per year in its cold, cold heart.

“It should be producing less than one, and it’s producing 740,” says Michael McDonald of MIT, coauthor of a study describing the cluster in the Aug. 16 Nature.

The cluster’s cold core and excessive fertility suggest the presence of a slacker black hole at the galaxy’s center.  Normally, radiation from a supermassive black hole — in this case, something around 10 billion times the mass of the sun — warms the surrounding environment, making it hard for gas to coalesce into stars. Whether the black hole in this cluster is simply overworked,  permanently off-duty or just taking a break is unclear, but scientists guess they’ve captured the cluster at an exceptionally frisky moment with respect to star formation.

“Stars are forming at a much higher rate than we’ve seen before in any of these galaxies,” says astrophysicist Martin Rees of the University of Cambridge in England.  “It’s a fascinating step toward putting this picture together of the tussle between the black holes and the star formation.

But the rate of star formation in the cluster may be somewhat overestimated, says astronomer Andrew Fabian, also at Cambridge. “There’s no doubt that they’ve found a really exciting and fascinating object,” he says. “Just how extreme it is, we don’t know yet.”

Astronomers discovered the riot of starbirth using the South Pole Telescope, a radio antenna that searches for the shadows cast by clusters on the cosmic microwave background, the thermal radiation left over from the Big Bang. Called “Phoenix” by the team — its official name is SPT-CLJ2344-4243 — the cluster comprises roughly 1,000 galaxies, is more than 4 million light-years across, and contains more mass than 2,000 Milky Ways.

In other words, it’s big.

It’s bright, too: The galactic conglomerate is the most luminous cluster ever observed in the X-ray spectrum. At its center lies a galaxy 50 times larger than the Milky Way. “It’s kind of the boss galaxy,” McDonald says.

It’s here in the cluster’s epicenter that scientists are observing stars forming much faster than expected for a galaxy of this type. Calculating the stellar birth rate involves measuring the amount of X-rays emitted by molecular gas as it cools off and falls toward the central galaxy. Though the swirling gas starts off at temperatures exceeding 10 million degrees Celsius, stars can’t begin coalescing until it cools to temperatures nearer the ambient temperature of space — several hundred degrees below zero.

Fabian and his colleagues suggested decades ago that star formation proceeds through such “cooling flows,” but that process hadn’t been observed in this type of central galaxy until now.

McDonald and his colleagues calculated that approximately 740 stars are bursting out of this central galaxy each year. But the sprinter’s pace is unlikely to be a permanent feature of the cluster, because over a marathon distance it would produce a monster galaxy 1,000 times larger than anything ever observed.

It’s more likely that scientists have captured the cluster in a fit of fertility, though it’s unknown just how long that fit will last. “I think something around 100 million years is reasonable,” McDonald says. Determining exactly how much molecular gas — fuel for the fertile fire — is present will help resolve the timescale question. Scientists hope to make that measurement using the ALMA telescopes in northern Chile.

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