A stream of cold gas is unexpectedly feeding the far-off Anthill Galaxy

The stream could keep the galaxy supplied with star-forming fuel for a billion years

A telescope image of a long stream of cold cosmic gas seen as a blue line coming off a big purple circle on a dark background.

A long stream of cold cosmic gas, highlighted in light blue, flows into the Anthill Galaxy in this image from the ALMA telescope array. A large reservoir of gas inside the galaxy is shown in purple and dark blue.


A long, cold stream of gas is feeding a very distant galaxy like a vast bendy straw. The finding suggests a new way for galaxies to grow in the early universe, researchers report in the March 31 Science.

Computer simulations predicted that streams of gas should connect galaxies to the cosmic web (SN: 3/6/23). But astronomers expected that gas to be warm, making it unsuitable for star-forming fuel and galaxy growth.

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

The team spotted the stream while mapping cold gas in the galaxy’s neighborhood using the Atacama Large Millimeter/submillimeter Array, or ALMA, in Chile. Emonts was particularly interested in radio wavelengths of light that carbon atoms emit when the temperature is between about -260° and -160° Celsius.

“People didn’t think that these streams could get so cold,” says Emonts, of the National Radio Astronomy Observatory in Charlottesville, Va.

But there, in the data, a frigid stream stretched at least 325,000 light-years away from the galaxy. The stream carries the mass of 70 billion suns and deposits the equivalent of about 450 suns in cold gas onto the galaxy every year, the team calculated. That’s enough to double the galaxy’s mass within a billion years.

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

“People don’t normally do that,” Emonts says. “We basically defocused ALMA to the worst possible extent.”

If other galaxies are fed by similar structures, it could mean that early galaxies grew mostly by drinking directly from the cosmic streams, rather than by the leading hypothesis — violent galaxy mergers (SN: 6/28/19).

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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