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Colliding Galaxies Left a Stream of Gas 2.6 Million Light Years Long

It's by far the longest gas trail ever discovered, and it weighs more than the Milky Way and Andromeda combined.
A galactic collision between NGC 2207 & IC 2163. Image: Hubble Heritage/Flickr

Astronomers have discovered the mother of all gas streams, stretching a full 2.6 million light years across. Yes, that's right: This stream is well over 20 times the diameter of our own galaxy, and so massive that it outweighs both the Milky Way and Andromeda (and that's saying something, because our sister galaxy is a real beefcake).

The gas bridge is composed of atomic hydrogen, and is suspended between a large galaxy on one end, and a tangle of smaller ones on the other. The whole scene is located about 500 million light years away in the NGC 7448 galactic group. It was discovered by a team led by Rhys Taylor of the Czech Academy of Sciences (who also happens to make fantastic space art).


"This was totally unexpected," Taylor said in a statement from the RAS. "We frequently see gas streams in galaxy clusters, where there are lots of galaxies close together, but to find something this long and not in a cluster is unprecedented."

Taylor's team used the William E. Gordon telescope to capture the stream, which is part of the Arecibo Observatory in Puerto Rico (or as it's more popularly known, the observatory that ends up with a splatted Sean Bean on it in GoldenEye). The data was collected between 2008 and 2011, and the team published their results and methods today in The Monthly Notices of the Royal Astronomical Society.

This astonishing structure was probably formed in some kind of epic galaxy collision, or at least that's what the team is speculating at this point. But there are also other possible origin stories for the stream that are even more exotic (I know:moreexotic than a galaxy fight).

"It is possible that this is an example of so-cold 'cold accretion,' where primordial gas, left over from the formation of the universe, condenses onto the cosmic web of dark matter filaments thought to exist between galaxies," Taylor told me. "This has been proposed as a—very controversial—explanation for some other similar features."

"However, in this case, I don't think it's very likely," he continued. "There's nothing particularly unusual about these galaxies other than the gas stream, so there's nothing to suggest that they would be more susceptible to accretion than any other galaxies. Long streams like this are especially unusual, whereas if accretion were the cause we'd expect to see them everywhere."


So, the stream was probably formed by the large galaxy clipping the group of smaller ones as they hurtled passed each other, or perhaps by the big guy just steamrolling right on through the smaller group, leaving behind a trail of gassy carnage.

"Another reason that a galaxy collision, or near miss, seems like a good explanation here is that the disc galaxy is roughly parallel to the stream," Taylor explained. "It's a lot easier to remove gas by dragging it out in the same plane as the disc, since the gas is already orbiting the center of the galaxy in that way."

"The bottom line is that if the galaxy were oriented perpendicular to the stream rather than parallel to it, that would be much better evidence for an accretion origin," he said. "We can't rule out that it formed by accretion, but it doesn't seem a very likely explanation to me."

The discovery could shed light on a number of astronomical questions.

Beyond the wow factor of the stream itself (and the tumultuous way in which it formed), the discovery could shed light on a number of astronomical questions.

"One element that is definitely of interest is just how much gas we see outside of the galaxies," Robert Minchin, the principal investigator of the project, told me. "This will make a reservoir of gas that will eventually fall back onto the galaxies, where it can be used to form stars. We don't see streams like this everywhere, so we know this isn't the main source of gas falling onto galaxies, but discoveries like this will help quantify how much gas is contained in streams and other such features."

The discovery also has the added bonus of participation from undergraduate students. Roberto Rodriguez and Clarissa Vazquez from the Universidad de Puerto Rico en Humacao contributed to the research, as well as Hannah Herbst of the University of Florida.

"Student involvement is very important to us," said Minchin in the RAS statement. "We are proud to be inspiring the next generation of astronomers, and particularly proud of the involvement of Puerto Rican students."

Indeed, as far as college stories go, discovering a gas trail 2.6 million light years long left by an ancient galactic punch-up is a hard one to top. Undergraduate astronomy students of the world: the gauntlet has been thrown down.