Violent galactic collisions have been invoked to explain surprisingly intense bouts of star formation occurring in ancient galaxies, but new evidence suggests a much gentler explanation.
Astronomers using the European Southern Observatory's Very Large Telescope in Chile have observed three primeval galaxies with patches of star formation near their centers, away from the heavy elements that signal the remains of previous stars.
The team found that these galaxies were sucking in cool hydrogen and helium from the space between galaxies as fuel. [ Illustration of Galaxy Gas Guzzling ]
"It solves the problem of providing to the galaxies fuel to form their stars in a continuous way, without having to invoke violent mergers and galaxy interactions," said study researcher Giovanni Cresci of Italy's OsservatorioAstrofisico di Arcetri. "Those certainly exist, but these new findings show that they are not the main driver of star formation in the early universe."
Theoretical models suggest that the first galaxies formed within a billion years after the Big Bang, but these galaxies were much smaller than the Milky Way, which contains some 100 billion stars. Somehow the galaxies had to accumulate new stars to reach their current size, and galaxy mergers were an obvious explanation.
Recently, however, evidence has accumulated against the merger idea. A collision between galaxies would result in a burst of star formation that would fade within a few million years, Cresci told SPACE.com. But when astronomers peered at ancient star-forming galaxies that existed 10 billion years ago, they found evidence of star formation lasting up to a billion years or more far longer than the merger model could explain.
In addition, some ancient star-forming galaxies showed no evidence of mergers.
Based on these hints, researchers proposed that early galaxies could have sucked in narrow streams of cold gas from the space around them, driving star formation for long periods of time.
In their new study, Cresci and his colleagues provide the first direct evidence for the cold gas hypothesis. They observed three pristine, disk-shaped galaxies undisturbed by mergers each containing approximately one to 10 Milky Way 's worth of stars. The distance to the galaxies corresponded to an age of two billion years after the Big Bang.
Cresci said that although the number of galaxies they observed is small, the result proves that inflows of cold gas can drive star formation, which could go a long way to resolving how the universe came to appear the way it does today.
"This is the link between the large-scale structures dominated by dark matter and the local Hubble-type galaxies such as our own," he said. "We are trying to understand how our home in the universe, the Milky Way, was built."
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