Gaia data revealed that a dwarf galaxy that merged with the Milky Way about 8–11 billion years ago changed its structure and left “migrant” stars in its halo. Today, the Large Magellanic Cloud is already pulling on our galaxy and shaping its future.
Credit: Vasily Belokov, based on an image by Juan Carlos Muñoz/ESO, licensed under CC BY-NC-SA.
When you look at the Milky Way on a dark night, it's easy to think of it as a stable, almost eternal structure. But the serene image is deceptive. Our galaxy was built from collisions, engulfments, and mergers with smaller galaxies. Some of the stars that orbit the center of the Milky Way today were never born there. They are the remnants of galaxies that were torn apart and swallowed up billions of years ago.
One of the most important testimonies to this turbulent past is the structure known as Gaia-Sausage-Enceladus — The remains of a dwarf galaxy that merged with the Milky Way about 8 to 11 billion years ago. This merger was one of the largest events in the history of our galaxy, and perhaps the last of the great mergers it has ever undergone. It left behind stars in unusual orbits, which can still be identified in data from modern astronomical missions.
Archaeology without excavations
The field of trying to reconstruct the history of the Milky Way is sometimes called “galactic archaeology.” Instead of digging in the ground, researchers search through vast amounts of stellar data: positions, velocities, orbits, and chemical composition. Ancient stars retain in their chemical memory the conditions in which they were born, and their orbits reveal what dynamic events shook them.
The great revolution in the field came with open, large-scale astronomical surveys. The Sloan Digital Sky Survey, which began in the early 2000s, showed how public databases could transform astronomical research. It was followed by the Gaia of the European Space Agency, which has been mapping the positions and movements of almost two billion stars since 2014. For Milky Way researchers, Gaia has turned the galaxy into a kind of cosmic excavation site.
When analyzing the motion of stars, a clear distinction emerges between “locals” and “migrants.” Stars born in the disk of the Milky Way tend to move together, in relatively regular orbits around the center of the galaxy. In contrast, stars originating in a swallowed galaxy move in more elongated and extreme orbits: they plunge into the inner regions of the galaxy and then are thrown out again into the halo.
The stars that betrayed the collision
Gaia-Sausage-Enceladus got its name because of the unusual shape of its stars’ motions in velocity space. These stars do not form a single thin stream, but a broad population of stars in very radial orbits, which appeared to researchers as a kind of “sausage” in velocity graphs.
Their chemical composition reinforced the identification. Many of them are poorer in heavy elements than stars born in the Milky Way's disk. This is a signature characteristic of a dwarf galaxy, where the rate of star formation and chemical enrichment was slower. Thus, these stars became two things at once: fossils of a violent past, and natural sensors reaching far into the galactic halo.
The merger didn’t just add stars to the Milky Way. It also changed the galaxy itself. According to studies and simulations, the collision heated and shook the ancient disk, scattered some of its stars into the halo, added globular clusters, and may even have influenced how the Milky Way’s thick disk formed. In other words, the galaxy we live in isn’t just the result of slow internal evolution. It’s also the product of a major collision that changed its structure.
Dark matter enters the picture
The importance of migrating stars does not end with the history of the galaxy. They also allow us to measure the gravitational field of the Milky Way, thereby mapping the dark matter that surrounds it.
Dark matter does not emit light, but its gravity determines how stars move in the outer regions of the galaxy. Because the Milky Way is the only galaxy where the motions of individual stars can be measured with such high precision, it serves as a unique laboratory for studying the distribution of dark matter: how wide the halo is, whether it is round or elongated, and how smooth or lumpy it is.
The ancient Gaia-Sausge-Enceladus merger probably distorted this halo as well. Rather than a quiet, spherical halo of dark matter, the Gaia data hint at a more complex structure, shaped by past galactic encounters.
The Large Magellanic Cloud is already pulling us in.
After that early collision, the Milky Way enjoyed a relatively long period of quiet. It doesn't appear to have undergone another major merger for billions of years. But this quiet isn't eternal. Today, our galaxy is dominated by the Large Magellanic Cloud, one of the most massive dwarf galaxies that surrounds it.
The Large Magellanic Cloud is about 160 light-years away from us. Despite its modest name, it is massive enough to pull on the entire Milky Way and cause reactive motion of the disk and halo. Studies of the motion of stars in the halo show that the Milky Way can no longer be considered an isolated, calm system. It is already responding to the gravity of its large companion.
In the distant future, the Large Magellanic Cloud is expected to merge with the Milky Way. This is not an event that threatens Earth or the solar system in any human time, but galactically it marks the beginning of a new chapter: another wave of engulfment, mixing, and adaptation.
Galaxy as a living memory
This story changes the way we look at the Milky Way. The bright streak in the sky is not just a permanent fixture. It is a living record of a chain of events: galaxies swallowed, stars displaced, a disk reshaped, a halo stretched, and dark matter whose structure is still being revealed through the motions of the stars.
In this sense, the stars are both memory and prediction. Their orbits tell what happened billions of years ago, and the small deviations in their motion reveal what is beginning to happen now. The Milky Way is not a structure frozen in time. It is a dynamic system, built from past events and still changing.
Secondary testing sources:
The identification of Gaia-Sausage-Enceladus as the remnant of a galaxy that merged with the Milky Way about 8–11 billion years ago also appears in astrophysical reviews and articles; the article “Sausage & Mash” discusses the impact of the merger on the Milky Way's thick disk and halo. (arXiv)
Studies of the Large Magellanic Cloud's influence indicate a reactive motion of the Milky Way's disk and that models of the galaxy need to take into account the Large Magellanic Cloud's fall into the halo. (arXiv)
Recent studies also emphasize that the future collision with Andromeda is not as certain as previously thought, partly because of the influence of the Large Magellanic Cloud and the Triangulum Galaxy, which is why I refrained from presenting Andromeda as the certain collision target in the article. (arXiv)
Short FAQ:
What is Gaia-Sausage-Enceladus?
This is the name given to the remnants of a dwarf galaxy that merged with the Milky Way about 8–11 billion years ago, leaving a population of stars in unusual orbits.
How do we know that certain stars came from another galaxy?
They move in elongated and irregular orbits, and their chemical composition differs from that of stars born in the disk of the Milky Way.
What does this have to do with the Gaia mission?
Gaia measures the positions and motions of almost two billion stars with high precision, thus allowing the dynamic history of the galaxy to be reconstructed.
What is the Large Magellanic Cloud?
This is a relatively massive dwarf galaxy that accompanies the Milky Way. Its gravity is already affecting the motion of our galaxy's disk and halo.
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