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How The Universe's Brightest Galaxies Became So Bright

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Jonathan O'Callaghan

Senior Staff Writer

2543 How The Universe's Brightest Galaxies Became So Bright
These galaxies formed stars faster than any other. NASA/Space Telescope Science Institute.

In the universe 10 to 12 billion years ago, there existed a rare breed of super-active galaxies. Known as submillimeter galaxies (SMGs), they formed stars at an extraordinary rate, 1,000 times faster than the Milky Way, but how they came to be was somewhat of a mystery. Now, scientists think they may have an answer.

Two theories exist for their formation. The first is that they formed from the merging of two smaller galaxies, causing a short and intense period of star formation. The second theory says that the process is much longer, over a billion years, and involves a continual cycle of gas from a reservoir replenishing stellar fuel.

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The new study in the journal Nature suggests that the second theory is the dominant cause of SMGs, based on computer simulations. Lead author Desika Narayanan from Haverford College in Pennsylvania, speaking to IFLScience, said their results indicated that more than 80% of SMGs formed this way.

It is thought that SMGs eventually evolved into the most massive galaxies in the universe, known as type-Cd galaxies. "These monstrous hulks live in the center of clusters today, and are the fossil relics of what were once active and lively places," said Narayanan.

The video above shows a rotating view of SMGs in the early universe. Robert Thompson (NCSA).

The basic idea for how the majority of SMGs formed is that they began to accumulate gas as soon as they were born. They started to form stars rapidly, at a rate of 50 to 100 per year, which resulted in a process called stellar feedback. This is where exploding and brightly shining stars dumped energy back into the galaxy, preventing the formation of new stars from the gas.

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The gas was forced into the outer reaches of the galaxy, where it sat like a reservoir until it eventually rained back into the galaxy, forming stars extremely rapidly, up to 1,000 per year. For comparison, the Milky Way produces one or two per year.

"You can tell there’s a lot of gas in these galaxies," said Narayanan. "Something like 40% of non-dark matter mass is in the gas, which is an extraordinary amount compared to the Milky Way, which has about 15% of its stellar mass in the form of gas."

One other major implication of the research is that it’s possible all of the universe’s biggest galaxies went through such a period of rapid star formation. And according to Narayanan, that would be "revolutionary" in how these extreme systems formed.

In cosmic terms, SMGs are fairly rare, with only a few hundred known of in the early universe. Coupled with the fact they are only visible in the infrared, they are fairly hard to study. But if this theory is confirmed, it could be another step in understanding how our modern universe evolved.


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