A flow of cold gas unexpectedly feeds the distant Anthill Galaxy


A long cold stream of gas feeds a very distant galaxy like a huge flexible straw. The discovery suggests a new way for galaxies to grow in the early universe, researchers report in Science on March 31.

Computer simulations predicted that gas flows should connect galaxies to the cosmic web. But astronomers expected this gas to be warm, making it unsuitable for fueling star formation and galaxy growth.

So astronomer Björn Emonts and his colleagues were surprised to see a stream of cold, star-forming gas leading to the Anthill Galaxy, a massive galaxy whose light takes 12 billion years to reach Earth.

The team spotted the flow while mapping cold gas in the galaxy’s outskirts with the Atacama Large Millimeter/Submillimeter Array, or ALMA, in Chile. Emmonts was particularly interested in the radio wavelengths of light emitted by carbon atoms when the temperature is about -260° to -160° Celsius.

“People didn’t think these streams could get this cold,” says Emmonts of the National Radio Astronomy Observatory in Charlottesville, Virginia.

But there, according to the data, the cold stream extended at least 325,000 light-years from the galaxy. The stream carries the mass of 70 billion suns and deposits the equivalent of about 450 suns of cold gas into the galaxy each year, the team calculated. This is enough to double the mass of the galaxy within a billion years.

Emmonts believes that no one has seen such a stream before because his team used ALMA in an unusual configuration, with its telescopes as close together as possible. This gave the observatory a lower resolution but a wider field of view.

“People don’t usually do that,” Emmonts says. “We actually defocused ALMA to the worst possible degree.”

If other galaxies are fed by similar structures, it could mean that early galaxies grew mostly by feeding directly from cosmic streams, rather than through the leading hypothesis of violent galaxy mergers.

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