By Leah Crane
NRAO/AUI/NSF, S. Dagnello
A galaxy from the early universe has been found to be far smoother than expected, which is evidence for a speedy sort of galaxy formation never spotted before. And it could mean that galaxies like the Milky Way may have started taking shape far earlier in the universe’s history than we thought they could.
Our current models of the universe suggest that in the first few billion years after the big bang, galaxies formed as a result of clumps of dark matter attracting hot gas, which eventually formed stars. When young, we expect galaxies formed like this to be lumpy and misshapen.
But that isn’t always the case. Marcel Neeleman at the Max Planck Institute for Astronomy in Germany and his colleagues used the Atacama Large Millimeter/submillimeter Array in Chile to spot a galaxy in the early universe that isn’t lumpy. This one dates to just 1.5 billion years after the big bang.
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Instead, it appears to be a smooth, rotating disc that looks more like our own spiral galaxy than the typical galaxies of the early cosmos. Officially called DLA0817g and dubbed the Wolfe Disk by the researchers, it formed 2.5 billion years earlier than the next oldest disc galaxy we have seen.
“The other extremely early galaxies we’ve seen just look like train wrecks, with clumps of gas everywhere, but this one is not like that,” says Neeleman. But if it formed from hot gas, it wouldn’t have had enough time since the big bang to smooth out its clumps and form a defined disc.
Instead, the researchers think it formed via a process called cold accretion, where gas flows smoothly into the galaxy along dense filaments instead of falling in clumps. This process is expected to be gentler, allowing the gas to settle into a disc more quickly in the early universe.
This is the first galaxy that we have seen in the early universe that seems to have formed this way, and we will need to find more to determine if it is a common process. “We don’t think that this is an extraordinary galaxy,” says Neeleman. “We think it’s quite normal and we should be able to find a whole bunch more.”
Journal reference: Nature, DOI: 10.1038/s41586-020-2276-y
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