The Milky Way Ate Another Galaxy 10 Billion Years Ago—And We Now Know More About This Cosmic Cannibalism Event

Ten billion years ago, the Milky Way gobbled up another galaxy, creating the cosmic structure our solar system sits in today. By analyzing the age distribution of stars in the Milky Way's inner halo, scientists have now been able to create a better picture of our galaxy's formation.

They found that many of the stars in the inner halo are up to 10 billion years old, providing more evidence of the timing of the merger—and helping identify the Milky Way's original stars that were there at its onset, 13.5 billion years ago.

Galaxies are not in fixed positions in space. They move around over time, occasionally smashing into one another.

Evidence of a massive merger between the Milky Way and the Gaia Enceladus galaxy came in 2018 when scientists used data from the European Space Agency's Gaia satellite to show a vast number of stars appeared to be out of place. In a letter published by Nature, the team said the inner halo of the Milky Way is "dominated by debris" from another galaxy. This galaxy was found to be about a quarter of the size of the Milky Way.

However, when this huge collision took place has been debated.

In a study published in Nature Astronomy on Monday, scientists led by Carme Gallart, from Spain's Institute of Astrophysics of the Canary Islands, looked at the stars in the Milky Way's inner halo and created cosmological simulations to find the age limit for when the merger took place.

Their findings showed that most of the stars in the inner halo are up to 10 billion years old—suggesting this is the point when the merger took place. By being able to put a constraint on the timing of the merger, the team was also able to identify the Milky Way's original stars.

Concluding, they say the simulations provide a "clear picture of the formation" of the Milky Way: "In this picture, a primitive Milky Way had been forming stars over a period of some [three billion years] when a smaller galaxy, which had been forming stars on a similar timescale but was less chemically enriched owing to its lower mass, was accreted into it."

Milky way
The Milky Way and Andromeda galaxies will collide in 4.5 billion years NASA Goddard

They said heat from the merger helped create the halo-like configuration we see today, and that a large supply of gas "ensured the maintenance of a disk-like configuration, with the thick disk continuing to form stars at a substantial rate."

Gallart told Newsweek: "Finding the date of the merger helps to understand what are its effects in the Milky Way. For example, in our study we see that, after the merger, the rate at which stars form in the Milky Way disk go up, and thus we can infer that this merger contributed to star formation in the disk of the Milky Way."

This is not the only time the Milky Way has merged with other galaxies. It is thought that throughout its history it has consumed many other, smaller galaxies.

Effects of mergers are not really noticeable on a small scale, Gallart said. "The distance between stars in a galaxy are so huge...that the two galaxies intermix, change their global shape, more star formation may happen in one and maybe the other—the small one—stops forming stars.

"But the individual stars in each galaxy don't collide, don't really notice the force of the event in a way that affect their individual evolution, or the evolution of the planetary systems that may be attached to them."

At the moment, the Milky Way is colliding with the Canis Major Dwarf galaxy. Our solar system is believed to have entered the Milky Way during a merger with the Sagittarius dwarf galaxy.

Eventually, the Milky Way will merge with our largest, closest neighbor, Andromeda. When this happens, around 4.5 billion years from now, the Milky Way as we know it will cease to exist.