A distant galaxy is being strangled by the cosmic web

The cosmic web is killing the galaxy. Galaxies can only continue to form stars when they are full of gas, and one dwarf galaxy nearly 100 million light-years away is being stripped of its stellar fuel by a vast web of matter stretching across the universe.

One side of this galaxy, called AGC 727130, looks completely normal. However, on the other hand, the gas extends far beyond the galaxy, pulled away by some invisible force. Nicholas Luber from Columbia University in New York and his colleagues discovered this disintegrating galaxy using the Very Large Array radio observatory in New Mexico.

Although AGC 727130 is close to two other dwarf galaxies, the researchers found that it is not close enough for any interaction with them to disturb the gas. Instead, their calculations suggest that its gas is expelled through a process called “pressure clearing,” in which a galaxy moving through an intragalactic cloud—in this case, part of the cosmic web—leaves its gas behind. Without this, the galaxy is “hardened”, i.e. can no longer form stars.

Threads cosmic web are extremely sparse, so one would probably not be enough to starve the galaxy of gas, but AGC 727130 is located at the intersection of several filaments. “The idea that the cosmic web is able to squeeze gas out of galaxies is not in itself shocking and probably happens quite often, but it is very difficult to see,” says Luber. “Catching this one was just dumb luck.”

Finding galaxies like this is difficult because it is a gradual process, and those that are already stripped of gas are usually too dim to be detected. “What's striking about this result is the fact that low-mass, quenched dwarf galaxies are extremely rare, and few of them—less than 0.06 percent of galaxies—are known to exist outside the presence of a massive host galaxy,” says Julia Blue Birdradio astronomer from New Mexico.

Of this tiny number of quenched dwarf galaxies, even less of the gas was taken away by the cosmic web rather than by interaction with another galaxy. “This… may be the first clear example of such an event,” says Jacqueline van Gorkom at Columbia University. Several large radio telescopes are preparing to release new maps of gas across vast swaths of the Universe, which should tell us much more about these types of galaxies, she said.

This is important because of a question in cosmology known as problem with lack of satellite: According to our best model of cosmology, there should be many more dwarf galaxies orbiting larger galaxies than we have actually discovered. “We don't find many extinct dwarfs there, but is it because they are hard to find, or are they not there at all? This tells us that this extinction actually occurs even far away from larger galaxies,” says a team member. Sabrina Stirwalt at Occidental College in California. If we can detect more galaxies engulfed in the cosmic web, it will help close the gap between models and observations.