ASTRONOMY

Leaky galaxy found that could shed light on early universe

PUBLISHED : Sunday, 26 October, 2014, 5:19am
UPDATED : Monday, 27 October, 2014, 10:42am
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Astronomers searching for hints of leaky galaxies have picked up radiation from a star-forming galaxy in the nearby universe that behaves rather like some of the earliest stellar factories that gave a dark universe its first rays of light.

The galaxy known as J0921 + 4509, as described in the journal Science, is pushing out 50 solar masses' worth of new stars per year - about an order of magnitude greater than the Milky Way's rate. This galaxy could help scientists shed light on one of the earliest epochs after the universe's birth - a formative period known as reionisation, which took place a few hundred million years after the birth of the universe.

During reionisation, the first stars that were coalescing out of the soup of neutral gas filling the universe began to send radiation out into their surroundings, separating the electrons from protons in the neutral hydrogen around them.

This process gave the first starlight to the dark universe and helped to determine its cosmic structure, said Sanchayeeta Borthakur, an astronomer at Johns Hopkins University in Baltimore in the United States.

"It has consequences for how (large-scale) structures formed, how galaxies got bigger and bigger, and what kind of dark matter makes up the universe," she said.

Scientists aren't sure exactly what caused reionisation, but they knew that the process needed starlight that was much more energetic than visible light: extreme ultraviolet rays or higher, known as Lyman continuum radiation.

But even though these young stars in the universe were presumably putting out a whole lot of Lyman continuum radiation, it's unclear how extreme ultraviolet radiation could have escaped and affected the surrounding universe. That's because the same clouds of neutral gas that give birth to the stars also imprison their light. If that high-energy ultraviolet could not escape, then how did reionisation happen?

Researchers have found what appeared to be "leaky" galaxies, which appeared to be letting light escape the cloudy prisons. But these galaxies seem few and far between - high-energy ultraviolet is not easy to look for, and it often quickly gets used up as it ionises the atoms it hits. The handful of other Lyman continuum-emitting galaxies in the nearby universe are allowing something like 2 per cent or 3 per cent of that radiation to escape. That's about a tenth of what would have been needed to reionise the universe.

To trigger reionisation, it's estimated that a galaxy would need to be leaking some 20 per cent of its Lyman continuum radiation. Lo and behold, J0921 4509 was leaking 21 per cent.

The scientists say these galaxies offer a valuable analogue they can use to study aspects of reionisation.

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