Distant galaxy may contain primeval stars

UV light hints at cache of members of earliest stellar population

STAR LIGHT  Ultraviolet light from clouds of hydrogen and helium in a distant galaxy, illustrated, suggest that primordial stars lurk inside.

M. Kornmesser/ESO

A stash of one of the earliest generations of stars might be lurking in a galaxy whose light has taken nearly 13 billion years to reach Earth. The finding possibly provides a rare look at how, when and where stars arose out of the pristine gas that was left behind in the wake of the Big Bang.

While other galaxies house clusters that could be typical of first-generation stars, the new observations provide the most direct evidence yet of such a population, astrophysicist David Sobral of the Institute of Astrophysics and Space Sciences in Lisbon, Portugal, and colleagues report. Their findings, described online June 4 at arXiv.org, will appear in the Astrophysical Journal.

A galaxy designated CR7 is loaded with hydrogen that is blasting out ultraviolet radiation — about three times as much as any other galaxy from that time. The galaxy is also blazing with light from helium atoms that have been stripped of one electron.

“We see indications of very, very hot sources,” says Sobral, “hotter than any star we know of in our galaxy.” To ionize helium, the surfaces of such stars must sizzle at around 100,000° Celsius. The sun, by comparison, is a relatively cool 5,500°.

Stars typical of the first stellar generation, known as Population III stars, are prime candidates as the source of all that energy. Researchers suspect that Population III stars are incredibly large, possibly up to a thousand times as massive as the sun. Such stars burn hot and die young, lasting at most a few million years.

Certain types of dying stars as well as gassy disks swirling around supermassive black holes can also provide that much energy. But what’s special about CR7, says Sobral, is the apparent lack of heavier elements such as carbon and oxygen. All atoms heavier than helium are forged in the centers of stars. The absence of such elements indicates that the gas contains only hydrogen and helium, and is therefore typical of the gas out of which the first stars formed.

“It’s definitely an unusual object,” says George Becker, an astrophysicist at the Space Telescope Science Institute in Baltimore. But Population III stars aren’t the only, or even the most likely, possibility, he says. The first stars are thought to have arisen a few hundred million years after the Big Bang. As they die and explode, they quickly pollute the surrounding gas with heavier elements. To have a large burst of pristine star formation roughly 1 billion years after the Big Bang seems unusual, Becker says. By that time, ordinary star formation should have overwhelmed the Population III nurseries.

The light could be coming from a collection of stars that have trace amounts of carbon and oxygen, undetectable with current instruments. Or it could be coming from pristine gas that isn’t itself collapsing to form stars, Becker says, but is cooling off from earlier bursts of star formation.

“The observations they’re making are very challenging,” he says, “which is part of why they’re exciting.”

This galaxy offers a preview of what the James Webb Space Telescope, scheduled to launch in 2018, could see, Sobral says. Hunting for Population III stars is one its primary goals. A large mirror in space combined with instruments sensitive to infrared light will be able to tell which stars are members of the first generation and which are not.

Editor’s Note: This story was updated July 2, 2015, to correct the temperature of the surface of the sun. It’s 5,500° Celsius. The story had said it was 5.800°, but that’s how hot it is as measured in kelvins.

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