A study of the small galaxies swallowed by our nearest large galactic neighbor has revealed something of how galaxies grow. In the process, it has hinted at something that, if true, would change the way we see the universe in profound ways.

All galaxies, including our own Milky Way, grow by consuming smaller ones. Inevitably, the nearest large galaxy to our own, M31, also known as Andromeda, has eaten many small neighbors. Our first insight into the nature of these events is useful because Andromeda and the Milky Way are on course to meet. That's 4 billion years away, but it's good to be prepared.

Dr Dougal Mackey of the Australian National University and Professor Geraint Lewis from the University of Sydney analyzed streams of stars within Andromeda's halo and identified some as the remnants of galaxies, even gaining approximations of when this occurred. Rather than a steady process, they identified two bursts separated by billions of years. The galaxies in the two rounds arrived at right angles to each other and maintained that motion even after being pulled apart.

“We are cosmic archaeologists, except we are digging through the fossils of long-dead galaxies rather than human history,” Lewis said in a statement. 

Mackey told IFLScience the consumed galaxies contained globular clusters, groups of stars bound so closely together that only the most powerful gravitational disruptions pull them apart.

Individual stars in Andromeda’s halo are usually too distant and faint for us to track their movements, but the clusters are a different matter, allowing the pair to identify in Nature the streams whose shared movements pin them as past galaxies.

“Once you get below a certain mass, galaxies don’t contain globular clusters,” Mackey said, so the smallest are currently untraceable. However, he added that the contribution of these to Andromeda's growth was minor.

One day, Andromeda will swallow the Milky Way as well. However, Mackey acknowledged to IFLScience this will be a very different process. The galaxies Andromeda has eaten up until now have masses a tenth of its total at most, usually much smaller. The Milky Way, on the other hand, is at least half Andromeda’s size.

The merger of the two will destroy the spiral structure of each, creating an egg-like elliptical galaxy. By then, the Sun will have burned the Earth beyond recognition, but if humanity somehow survives elsewhere, consequences will be limited. Our view of the night sky will change utterly, but the space between the stars is so great that even when stars from Andromeda become mixed among our nearest neighbors, it is unlikely any will pass close enough to disrupt orbits within the inner solar system.

One feature of the pair's work is still tentative but could turn out to be awe-inspiring. Around a decade ago astronomers noticed a curious pattern in the dwarf galaxies that surround Andromeda. Around half these galaxies orbit in a specific plane, something astronomers had not expected and struggled to explain. We now know this pattern is unstable, and can neither have been in place for a long time (astronomically speaking) or have lasted that far into the future.

This study found that some of the swallowed galaxy remnants conform to the same pattern, but surprisingly, it is the much older set that do this, rather than the more recent arrival. This suggests these long-lost galaxies also once lay in a similar plane, indicating an abiding underlying structure to the local area of the universe. The most likely explanation is the universe is made up of a web of dark matter filaments and Andromeda lies on one of these, with gravity so powerful it aligns the movement of clusters of small galaxies.