The Milky Way ate a galaxy called Loki, and scientists think they found its bones

Astronomers have identified some strange stars in the Milky Way that may have once belonged to a different galaxy.

By studying the chemistry of these stars and their motion close to the galactic disk, the researchers found that the stars' home galaxy, nicknamed "Loki," might have merged with our galaxy about 10 billion years ago.

Studying these stars could be "very important" in understanding the history of the Milky Way and the universe itself, lead author of the study Federico Sestito, an astrophysicist at the University of Hertfordshire in the U.K, told Live Science in an email. Sestito added that Loki may have been among "the very first small galaxies formed in the young universe."

Massive galaxies are not born whole. They are assembled over billions of years through mergers with smaller galaxies, which are sometimes absorbed. In the early universe, shortly after the Big Bang, matter clumped into clouds of gas that collapsed into the first primitive galaxies. These small systems then fell into one another, merged and gradually built up into the large structures we see today.

In the new study, published March 23 in the Monthly Notices of the Royal Astronomical Society, astronomers identified 20 old, very-metal-poor stars orbiting unusually close to the galactic disk — the flat, rotating region of the Milky Way where most stars, including the sun, reside — and examined whether a past merger might explain what they were seeing.

A chemical timestamp

The very first stars that formed in the universe were made of hydrogen and helium. It was only inside those early stars that hydrogen and helium fused into heavier elements, which astronomers call metals. These stars, when they eventually exploded, enriched the surrounding gas with those metals. Each successive generation of stars was therefore born from material slightly more enriched than the last.

As these small galaxies collided and merged, their stars, gas and dark matter became part of the growing young Milky Way. Because of this, computer simulations suggest that stars from the earliest mergers are expected to be found deeper inside the Milky Way today, while stars from galaxies that merged later are more likely to be scattered farther out in the galactic halo — a vast, spherical region that extends beyond the bright disk.

However, very few metal-poor stars have been found in the inner regions of the Milky Way to test this idea. So, when the team identified 20 metal-poor stars orbiting close to the galactic disk, they wondered whether the stars could be remnants of an ancient merger.

Hide and seek

The team identified these stars from an existing catalog of metal-poor stars. They observed each one using a powerful spectrograph at the Canada-France-Hawaii Telescope, which revealed their chemical abundances. Using precise positional data from the Gaia space telescope, they calculated the stars' distances and how they orbit in our galaxy.

Sestito said that "a mixture of information from the chemistry and the orbits of these stars" nudged them to examine the stars' origin. Rather than drifting through the halo of the galaxy where ancient, metal-poor stars have been mostly observed, these stars were tracing paths close to the Milky Way's disk within just 6,500 light-years from the sun.

"Usually, stars in the disk are metal-rich and younger, like the sun," he said, "while our stars [in the study] are old and very metal-poor (like in dwarf galaxies)."

Additionally, some of these stars were found moving in the same direction as the Milky Way's rotation, while others traveled in the opposite direction. But these two groups did not show any difference in their chemical abundances. Explaining how a single infalling galaxy could leave stars moving in opposite directions was also challenging.

The answer came from computer simulations of galaxy formation. If the merger happened early enough, when the young Milky Way was still lightweight and had not yet settled into a spinning disk, the infalling galaxy would have had enough freedom to scatter its stars in all directions.

"The early merging history of a large galaxy might be very chaotic, with various smaller systems merging together and dispersing their stars with many different orbits," Sestito explained. This scenario could produce both prograde and retrograde orbits, placing the merger event around 3 billion years after the Big Bang.

As a result, the simulations showed that a single dwarf galaxy swallowed by the young Milky Way more than 10 billion years ago, could have scattered its stars into exactly the orbital pattern observed today. The models also helped estimate the total mass of this galaxy to be around 1.4 billion solar masses.

The team nicknamed this infalling galaxy Loki.

"Loki, in the Norse mythology, is the God of mischief, and, as a trickster, his intents are hard to decipher," Sestito said. "Similarly, our accreted stars gave us some hard time in understanding their origin."

The search continues

Anirudh Chiti, an astrophysicist at Stanford University who was not part of the study, told Live Science that the new discovery shows promise.

"The chemical abundance analysis is intriguing, and part of the argument rests on the fact that the chemistry of the stars seems more clustered than those in the Milky Way halo," Chiti wrote in an email. "This is a nice example of the kind of discovery that those samples could turn up or verify."

Still, the new findings fall short of certainty. Sestito acknowledged that more observations are needed to confirm them.

"Our work is surely limited in terms of the number of observed stars," Sestito said. Observing stars with high-resolution spectroscopy is time-intensive — each star requires around four hours of telescope time, which is why the current sample is small.

Because researchers are still in the early stages of exploring the chemical signatures of the lowest-metallicity stars in the Milky Way disk, it remains plausible that these stars belong to a subgroup of stars or substructure within the Milky Way, Chiti noted. "I'm looking forward to what future work mapping the chemistry of large samples of very metal-poor stars in the Milky Way disk may show," he said.

To confirm the nature of Loki, the team would need to observe its stars and other non-Loki targets with the same telescope setup to better understand the differences between this system and other parts of the Milky Way halo.

With upcoming advanced spectroscopic facilities, astronomers will be able to observe hundreds of stars with available high-quality data on their trajectories and chemical abundances. Sestito thinks the search should not be limited to the halo. The hidden systems in the inner regions of the galaxy could hold clues to the primitive galaxies of the young universe, though detecting them in the crowded disk would be challenging.