A distant galaxy is being strangled by the cosmic web

The cosmic web kills a galaxy. Galaxies can only continue to form stars when they are full of gas, and a dwarf galaxy nearly 100 million light years away is starved of its stellar fuel by the enormous web of matter that stretches across the universe.

One side of this galaxy, called AGC 727130, appears completely normal. On the other side, however, the gas extends far beyond the boundaries of the galaxy, driven by an invisible force. Nicholas Luber of Columbia University in New York and his colleagues spotted this disintegrating galaxy using the Very Large Array, a radio observatory located in New Mexico.

While AGC 727130 is near two other dwarf galaxies, the researchers found that it is not close enough for interactions with them to cause the gas disruption. Instead, their calculations suggest that its gas is expelled through a process called dynamic pressure removal, in which a galaxy moving through an intragalactic cloud – in this case, a bit of the cosmic web – leaves its gas behind. Without it, the galaxy is “extinct,” meaning it can no longer form stars.

The filaments of the cosmic web are extremely tenuous, so just one probably wouldn’t be enough to strip a galaxy of its gas, but AGC 727130 is located at the intersection of several filaments. “The idea that the cosmic web is capable of removing gas pressure from galaxies is not shocking in itself and probably happens quite often, but it is very difficult to see,” says Luber. “Catching this one was just pure luck.”

Finding galaxies like this is difficult because it is a gradual process, and those that have already been stripped of their gas tend to be too dark to spot. “What is striking about this result is the fact that low-mass extinct dwarf galaxies are extremely rare, with only a few – less than 0.06% of galaxies – known to exist outside of the presence of a massive host galaxy,” says Julia Blue Bird, a radio astronomer in New Mexico.

Of this tiny number of extinct dwarf galaxies, even fewer have been deprived of their gas by the cosmic web, rather than by interactions with another galaxy. “This is…perhaps the first clear example of such an event,” says Jacqueline van Gorkom of Columbia University. Several large radio telescopes are preparing to publish new gas maps across large swaths of the universe, which should tell us a lot more about these types of galaxies, she says.

This is important because of a question in cosmology known as the missing satellite problem: according to our best model of cosmology, there should be many more dwarf galaxies orbiting larger galaxies than we have actually found. “We don’t find many extinct dwarfs, but is that because they are hard to find or are they not? This tells us that this extinction is happening even far from the largest galaxies,” says team member Sabrina Stierwalt from Occidental College in California. If we can spot more galaxies extinguished by the cosmic web, then this could help bridge the gap between models and observations.