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Posted on November 1, 2018
ESA's Gaia mission made a major breakthrough by unveiling the story of the formation of the Milky Way. Instead of forming alone, our galaxy merged with another big galaxy at the beginning of its life, about 10 billion years ago. The evidence is scattered in the sky all around us, but it took Gaia and her extraordinary precision to show us what was hidden from view at all times.
On the basis of the first 22 months of observation, a team of astronomers led by Amina Helmi, from the University of Groningen (the Netherlands), examined seven million stars – those for which complete 3D positions and velocities are available – and found that 30,000 of them were part of a "strange collection" moving through the Milky Way. The stars observed in particular are currently passing through our solar neighborhood.
We are so deeply rooted in this collection that its stars surround us almost completely and can thus be seen through most of the sky.
Although scattered with other stars, the stars in the collection were distinguishable from Gaia's data because they all moved on elongated trajectories in the opposite direction to most of the other hundred billion stars in the galaxy, including the Sun.
They are also distinguished in the so-called Hertzprung-Russell diagram – which is used to compare the color and brightness of stars – indicating that they belong to a distinctly stellar population.
The untold number of strange stars involved intrigued Amina and her colleagues, who suspected they had a connection to the history of the Milky Way formation and went to work to understand their origins.
In the past, Amina and her research group used computer simulations to study what happens to stars when two large galaxies merge. When she compared these to the Gaia data, the simulated results matched the observations.
"The collection of stars we found with Gaia has all the properties you can expect from the debris of a galactic fusion," says Amina, lead author of the paper published today in Nature.
In other words, the collection corresponds to what they expected from stars that were formerly part of another galaxy and that were consumed by the Milky Way. Stars now form most of the inner halo of our galaxy – a diffuse component of ancient early-born stars that now surround most of the Milky Way, called the bulb and central disk.
The components of the Milky Way
The galactic disk itself is composed of two parts. There is the thin disc, which has a depth of a few hundred light years and contains the spiral arm pattern consisting of bright stars. And there is the thick disc, which is a few thousand light-years deep. It contains about 10 to 20% of the stars in the galaxy, but its origins are difficult to determine.
According to the team's simulations, in addition to providing the halo stars, the galaxy would also have been able to disturb the pre-existing stars of the Milky Way to help the formation of the thick disc.
"We became certain of our interpretation after completing the Gaia data with additional information on the chemical composition of the stars provided by the ground-based APOGEE survey," says Carine Babusiaux, from Grenoble Alpes University, France.
The stars that form in different galaxies have unique chemical compositions that correspond to the conditions of the galaxy of origin. If this collection of stars was indeed the remains of a galaxy that merged with ours, the stars should leave an imprint in their composition. And they did it.
Astronomers have called this Gaia-Enceladus galaxy the name of one of the giants of Greek mythology, which was the offspring of Gaia, the Earth, and Uranus, the Sky.
"Legend has it that Enceladus was buried under Mount Etna in Sicily and was responsible for the local earthquakes. Likewise, the Gaia-Enceladus stars were deeply buried in Gaia's data and shook the Milky Way, resulting in the formation of its thick disk, "says Amina.
Although no additional evidence was really needed, the team also found hundreds of variable stars and 13 globular clusters in the Milky Way that follow similar trajectories to those of Gaia-Enceladus stars. , indicating that they were part of this system.
Globular clusters are groups that can go up to millions of stars, held together by their mutual gravity and gravitating around the center of a galaxy. The fact that so many groups can be linked to Gaia-Enceladus is another indication that Gaia-Enceladus once had to be a large galaxy in its own right, with its own circle of globular groups.
Further analysis revealed that this galaxy was about the size of one of the Magellan clouds, two satellite galaxies about ten times smaller than the current size of the Milky Way.
However, ten billion years ago, when the merger with Gaia-Enceladus took place, the Milky Way itself being much smaller, the ratio between the two was more like four to one. So it was clearly a blow to our galaxy.
"Seeing that we are now beginning to understand the history of the formation of the Milky Way is very exciting," says Anthony Brown, Leiden University (The Netherlands), co-author of the paper and chairman of the Gaia Data Processing and Executive Analysis of the consortium.
Since the very first discussions on the construction of Gaia, 25 years ago, one of the key objectives of the mission was to examine the different stellar currents of the Milky Way and reconstruct its beginnings. This vision is bearing fruit.
"Gaia was built to answer such questions," says Amina. "We can now say that it is so that the galaxy was formed at these beginnings. It's fantastic. It's so beautiful and you feel big and small at the same time. "
"By reading the movements of the stars scattered in the sky, we are now able to go back in the history of the Milky Way and discover a major milestone in its formation, which is possible thanks to Gaia", concludes Timo Prusti, Gaia project manager at ESA.
The second recent publication of the Gaia satellite mission last April provided Professor Helmi with data on about 1.7 billion stars. Helmi has been involved in the development of the Gaia mission for twenty years and was part of the data validation team during the second data release. She is now using the data to look for halo smudges: "We were expecting stars from molten satellites in the halo. What we did not expect was that most of the halo stars actually have a common origin in a very big fusion "
That's what she found. The chemical signature of many halo stars was clearly different from that of the "native" Milky Way stars. "And they form a fairly homogeneous group, which indicates that they share a common origin." By tracing both the trajectory and the chemical signature, the 'invaders' are clearly distinguishable.
Helmi: "The youngest stars of Gaia-Enceladus are actually younger than the stars of the Milky Way in the area of the thick disc. This means that the progenitor of this thick disc was already present at the fusion, and Gaia-Enceladus, because of its large size, shook and inflated it. "
In a previous article, Helmi had already described a huge "blob" of stars sharing a common origin. Now she shows that the stars of this drop in the halo are the debris of the Milky Way fusion with a galaxy slightly more massive than the Little Magellanic Cloud, about ten billion years ago. The galaxy is called Gaia-Enceladus, named after the giant Enceladus, born in the Greek mythology of Gaia (the goddess of the Earth) and Uranus (the god of heaven).
Data on kinematics, chemistry, age and spatial distribution from the stars of the Milky Way and from the remains of Gaia-Enceladus reminds Helmi of the simulations carried out by a former doctoral student a dozen years ago. ; years. His simulations of the fusion of a large disk-shaped galaxy with the young Milky Way produced a distribution of stars from both objects, which fully corresponds to Gaia's data. "It was amazing to watch Gaia's new data and realize that I had already seen it before!" Says the astronomer.
The Daily Galaxy via the University of Groningen
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