Scientists have discovered extremely old galaxies - formed less than a billion years after the Big Bang - which create stars more than a hundred times faster than our own Milky Way.
The discovery could help solve a cosmic puzzle - a mysterious population of surprisingly massive galaxies from when the universe was only about 10 per cent of its current age.
After first observing these galaxies a few years ago, astronomers proposed that they must have been created from hyper-productive precursor galaxies, which is the only way so many stars could have formed so quickly.
However, astronomers had never seen anything that fit the bill for these precursors until now.
This newly discovered population could solve the mystery of how these extremely large galaxies came to have hundreds of billions of stars in them when they formed only 1.5 billion years after the Big Bang, requiring very rapid star formation.
Researchers led by Roberto Decarli of Max Planck Institute for Astronomy made this discovery by accident when investigating quasars, which are supermassive black holes that sit at the centre of enormous galaxies, accreting matter.
They were trying to study star formation in the galaxies that host these quasars.
"But what we found, in four separate cases, were neighbouring galaxies that were forming stars at a furious pace, producing a hundred solar masses' worth of new stars per year," Decarli said.
"Very likely it is not a coincidence to find these productive galaxies close to bright quasars. Quasars are thought to form in regions of the universe where the large- scale density of matter is much higher than average," said Fabian Walter, from Max Planck Institute for Astronomy.
"Those same conditions should also be conducive to galaxies forming new stars at a greatly increased rate," Walter said.
"Whether or not the fast-growing galaxies we discovered are indeed precursors of the massive galaxies first seen a few years back will require more work to see how common they actually are," said Eduardo Banados from Carnegie Institution for Science in the US.
The team also found what appears to be the earliest known example of two galaxies undergoing a merger, which is another major mechanism of galaxy growth.
The new observations provide the first direct evidence that such mergers have been taking place even at the earliest stages of galaxy evolution, less than a billion years after the Big Bang.
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