If small galaxies approach large ones, like Andromeda, "the tidal forces of the larger system destroy them and separate them until its mass is assimilated"says Dougal Mackey, an astronomer at the National University of Australia.
Scientists knew for 10 or 15 years that Andromeda had a vigorous history of accumulation and destruction of its neighbors, however "it seems to have a much more intense history than the Milky Way," said Mackey.
How astronomers understood Andromeda's past
The assimilation process leaves obvious traces. The remains of the destroyed galaxies accumulate in a halo around the main galaxy. Some of these remnants are globular clusters, compact balls of thousands of stars that are very bright.
The researchers were able to measure their movements, using them as headlights, while orbiting the Andromeda galaxy.
The team detected that many of the globular clusters got into stellar currents that were clearly remnants of recently destroyed galaxies.
"They had to have been destroyed recently because we can still see the currents stretched," Mackey said.
In addition, the team found other clusters that orbited differently and appear to have originated in galaxies that were destroyed long ago, perhaps even 12,000 million years ago, he said.
"That tells us that there were two main events that formed the Andromeda halo," Mackey said. "One happened a long time ago. The other must have happened recently."
On the other hand, the scientist stressed that there could have been a countless amount of smaller galaxies devoured by Andromeda that left no trace.
"We are cosmic archaeologists. We excavated fossils of dead galaxies a long time ago instead of fossils in human history, "said Geraint Lewis, one of the main members of the study.
Andromeda vs. Milky Way
The Andromeda galaxy and the Milky Way are at a distance of 2.5 million light years and approach at a speed of between 100 and 140 kilometers per second. In this way, they will collide in about 4 billion years.
"Knowing what kind of monster our galaxy faces is useful in figuring out the final fate of the Milky Way," said Mackey. "Andromeda has a stellar halo much larger and more complex than the Milky Way, which indicates that it has assimilated many more galaxies, possibly larger," he explained.
In the future, Andromeda will merge not only with several additional small galaxies, but also with the Milky Way, a process in which our Local Group of galaxies will evolve into a smaller and smaller number of galaxies until "finally, become a single megagalaxy"said Paul Mason, an astrophysicist at the State University of New Mexico in Las Cruces.
"After approximately 100,000 million years, we will fall into the heart of the Virgo Supercluster" which includes dozens of galaxy clusters similar to our own Local Group, he added.
The article was published in the journal Nature.