Melkweg swallowed a different galaxy in his youth

No less than 33,000 stars of our Milky Way come from a different galaxy

Thanks to observations made with the Gaia satellite, the researchers discovered that our galaxy was not formed of any kind. ;herself. In its youth, our Milky Way is fused with another big galaxy. The indications on this subject were in fact very current, even if the expertise of the satellite Gaia was necessary for this purpose. During the construction of the telescope, 25 years ago, the investigation into the origins of the Milky Way had already been mentioned as one of the main objectives. And this vision is bearing fruit.

Gaia Satellite
The Space Telescope has been scanning the sky since 2013 with the goal of capturing the position, distance and motions of over a billion stars with greater accuracy than ever before before. In addition to 2D movements of 1.3 billion stars, Gaia also measured the 3D movements of the 7 million brightest stars, or the speed with which they move from us today. or to us. The researchers studied 22 months of observations. And among the 7 million stars, 33,000 seem to belong to an amazing collection.

Images of a simulation showing how Gaia-Enceladus (in red) fused with the Milky Way (in white). Image: HH Koppelman, A. Villalobos and A. Helmi

Stars
Although this group of stars has "mixed" with the other stars of our Milky Way, they are a bit out of the boat . Thus, these stars have elongated paths in the opposite direction to that of the hundreds of billions of stars of the Milky Way. This abnormal behavior has led researchers to believe that this may be related to the genesis of our galaxy. They therefore decided to compare the fusion of two large galaxies with the Gaia observations. And what appears? "The collection of stars we found with Gaia has all the features associated with the remains of a galactic fusion," says lead researcher Amina Helmi.

The above video shows a simulation of the fusion of a galaxy-like galaxy (stars in blue) and a smaller system that resembles the little Magellan cloud (stars in red). Initially, the systems are clearly separated from each other, but the gravity brings them closer together, causing the smaller system to swallow. our milky way. The enslaved galaxy was named in Gaia-Enceladus, named after one of the giants of Greek mythology, sons of Gaia (the primordial mother, the earth) and Uranus (personification of the sky). As evidence for support, Helmi and his team also discovered hundreds of variable stars and thirteen clusters of spherical stars that follow the same orbits, indicating that they were also part of of Gaia-Enceladus.

Dimensions
brought with the galaxy indicates that it was once a large galaxy. Astronomers, for example, think that the dimensions must have been similar to those of Magellan's clouds. It is about ten times smaller than the current Melkweg. However, the Milky Way itself being much smaller ten billion years ago, the ratio at the time of fusion is estimated at 4: 1.

At present, the stars Gaia detected form the largest part of the inner halo – a spherical shape. cloud with stars that can be found around the central disk. The disc itself consists of two parts: a thin disc and a thick disc. The thick disc contains about 20% of the stars of the Milky Way. "It was a fierce meeting in which Enceladus had shaken the Milky Way," Helmi said. "This led to the formation of a thicker disc, whose origin was unknown until now."