In 2018, a long-hidden and especially tumultuous chapter in the Milky Way’s history was revealed: a smash-up between the young Galaxy and a colossal companion that once orbited the Milky Way like a planet around a star. “The two collided, massively altering the Galactic disk and scattering stars far and wide,” reported Nature about data captured by the Gaia Spacecraft that radically transformed how we see the evolution of our Galaxy. “It is the last-known major crash the Galaxy experienced before it assumed the familiar spiral shape seen today.”
The Gaia Mission
ESA’s Gaia spacecraft. with its extraordinary precision, made a major breakthrough in unravelling the formation history of the Milky Way. Evidence of alien star streams and ancient globular clusters is littered across the sky that our Galaxy merged with another large galaxy slightly more massive than the Small Magellanic Cloud early in its life, around 10 billion years ago when the Milky Way was much smaller, at a ratio of four to one.
In 2018, using the first 22 months of Gaia observations, a team of astronomers led by Amina Helmi at the University of Groningen in the Netherlands looked at seven million stars for which the full 3D positions and velocities were available. They found that some 30,000 of them were part of an ‘odd collection’ moving through the Milky Way. The observed stars in particular are currently passing by our solar neighborhood.
A clearly distinct stellar population — “Moving in the Opposite Direction”
Even though they are interspersed with other stars, the stars in the collection stood out in the Gaia data because they all move along elongated trajectories in the opposite direction to the majority of the Galaxy’s other hundred billion stars, including the Sun.
They also stood out in the so-called Hertzprung-Russell diagram – which is used to compare the color and brightness of stars – indicating that they belong to a clearly distinct stellar population. In particular, stars with different chemical compositions will have different temperatures and luminosities for a given mass.
The sheer number of odd-moving stars involved intrigued Dr. Helmi and her colleagues, who suspected they might have something to do with the Milky Way’s formation history and set to work to understand their origins.
A Match with the Gaia Data
In the past, Dr. Helmi and her research group had used computer simulations to study what happens to stars when two large galaxies merge. When she compared those to the Gaia data, the simulated results matched the observations.
“The collection of stars we found with Gaia has all the properties of what you would expect from the debris of a galactic merger,” says Amina, lead author of the paper published in Nature.
Form the Milky Way’s Inner Halo of Ancient Stars
In other words, the collection is what they expected from stars that were once part of another galaxy and have been consumed by the Milky Way. The stars now form most of our Galaxy’s inner halo – a diffuse component of old stars that were born at early times and now surround the main bulk of the Milky Way known as the central bulge and disc.
Thin Disk and Thick Disk
The Galactic disk itself is composed of two parts. There is the thin disk, which is a few hundred light years deep and contains the pattern of spiral arms made by bright young stars. And there is the thick disk, which is a few thousand light years deep. It contains about 10–20 percent of the Galaxy’s stars yet its origins have been difficult to determine.
According to the team’s simulations, as well as supplying the halo stars, the accreted galaxy could also have disturbed the Milky Way’s pre-existing stars to help form the thick disk.
“We became only certain about our interpretation after complementing the Gaia data with additional information about the chemical composition of stars, supplied by the ground-based APOGEE survey,” says Carine Babusiaux, Université Grenoble Alpes, France, and second author of the paper.
Stars that form in different galaxies have unique chemical compositions that match the conditions of the home galaxy. If this star collection was indeed the remains of a galaxy that merged with our own, the stars should show an imprint of this in their composition.
Named Galaxy Gaia-Enceladus
The astronomers called this galaxy Gaia-Enceladus after one of the Giants in ancient Greek mythology, who was the offspring of Gaia, the Earth, and Uranus, the Sky.
“According to the legend, Enceladus was buried under Mount Etna, in Sicily, and responsible for local earthquakes. Similarly, the stars of Gaia-Enceladus were deeply buried in the Gaia data, and they have shaken the Milky Way, leading to the formation of its thick disc,” explains Helmi.
Even though no more evidence was really needed, the team also found hundreds of variable stars and 13 globular clusters in the Milky Way that follow similar trajectories as the stars from Gaia-Enceladus, indicating that they were originally part of that system.
Globular Clusters Point to a Big Galaxy
Globular clusters are groups of up to millions of stars, held together by their mutual gravity and orbiting the centre of a galaxy. The fact that so many clusters could be linked to Gaia-Enceladus is another indication that this must have once been a big galaxy in its own right, with its own entourage of globular clusters.
Further analysis revealed that this galaxy was about the size of one of the Magellanic Clouds – two satellite galaxies roughly ten times smaller than the current size of the Milky Way.
“Seeing that we are now starting to unravel the formation history of the Milky Way is very exciting,” says Anthony Brown, Leiden University, The Netherlands, who is a co-author of the paper and also chair of the Gaia Data Processing and Analysis Consortium Executive.
Since the very first discussions about building Gaia 25 years ago, one of the mission’s key objectives was to examine the various stellar streams in the Milky Way, and reconstruct its early history.
“Gaia was built to answer such questions,” says Amina Helmi. “We can now say this is the way the Galaxy formed in those early epochs. It’s fantastic. It’s just so beautiful and makes you feel so big and so small at the same time.”
“By reading the motions of stars scattered across the sky, we are now able to rewind the history of the Milky Way and discover a major milestone in its formation, and this is possible thanks to Gaia,” concludes Timo Prusti, Gaia project scientist at ESA.
The second data release from the Gaia satellite mission in April of 2018 provided Helmi with data on around 1.7 billion stars. Helmi has been involved in the development of the Gaia mission for some twenty years and was part of the data validation team on the second data release. She has now used the data to look for traces of mergers in the halo: “We expected stars from fused satellites in the halo. What we didn’t expect to find was that most halo stars actually have a shared origin in one very large merger.”
This is indeed what she found. The chemical signature of many halo stars was clearly different from the ‘native’ Milky Way stars. “And they are a fairly homogenous group, which indicates they share a common origin”. By plotting both trajectory and chemical signature, the ‘invaders’ stood out clearly.
“The youngest stars from Gaia-Enceladus are actually younger than the native Milky Way stars in what is now the thick disk region. This means that the progenitor of this thick disk was already present when the fusion happened, and Gaia-Enceladus, because of its large size, shook it and puffed it up.”