The Milky Way was part of the cosmic collision 10 billion years ago

Astronomers using the Gaia space telescope from the European Space Agency have discovered that our galaxy was involved in a cosmic fusion 10 billion years ago.

It is thought that the Milky Way was formed more than 13 billion years ago. But soon after – at least cosmologically, at least – another galaxy crashed on it, scattering its stars while creating new ones. in the milky way.

Spiral galaxies like ours are composed of several parts: the central bulge, the spiral arms, the disk and a surrounding halo.

Researchers We found evidence of the merger by studying the movement of seven million stars in the inner halo of the Milky Way, a region located around the thick disk of stars of the galaxy. They discovered that about 30,000 of these stars were moving in the opposite direction to other stars in the galaxy – a clear sign that they could have come from elsewhere.

This clip shows a simulation of the fusion of a galaxy similar to the Milky Way (with its stars in blue) and a smaller galaxy (with its stars in red). Initially, the two galaxies are clearly separated, but gravity brings them together and merges them.

Prior to this discovery, the team had performed galactic merger simulations. What they observed with Gaia data – and with data from the Galactic Evolution Experiment (APOGEE) of the Apache Point Observatory in Chile – corresponds to these simulations, the leading to the conclusion that another galaxy had merged with the Milky Way.

The researchers say that one of the clues was found in the evidence of 13 globular clusters in the same region of the galaxy – Thousands, even millions of stars, all gravitational, move in the same way as the 30,000 stars observed.

"The discovery that … the inner halo of the Milky Way is proving to be a different galaxy that has fundamentally contributed to the creation of all the stars in our own galaxy, I think it was a big surprise, "said lead author Amina Helmi, astronomer at the Kapteyn Institute of Astronomy at the University of Groningen in the Netherlands.

The Omega Centauri Globular Group – with no less than ten million stars – appears on this captured image from the La Silla Observatory of the European Southern Observatory. (European Southern Observatory)

Another piece of evidence was the composition of the stars themselves. Stars of different galaxies have their own type of fingerprint. And it was the case of the 30,000 moving stars that they discovered in the halo of the Milky Way.

"It's very cool that stars formed in another galaxy are hiding next to us," said Kim Venn, a professor in the Department of Physics and Astronomy at the University of Victoria, who did not participate in the study.

Stellar Explosions

The fusion would have produced brilliant stellar explosions – supernovas – and the rapid birth of stars.

"If you were there … you would see young blue and bright stars, a bit like fireworks," Helmi said.

Three billion years after the Big Bang, galaxy mergers began to slow down, but they were much more common than today. In about four billion years, the Milky Way and the neighboring galaxy of Andromeda will have a similar collision.

The researchers named the galaxy that merged with our Gaia-Enceladus, according to the telescope and the mythical Greek figure who was the son of Gaia, the mother of all life.

It is thought that Gaia-Enceladus was about the size of one of the Magellan clouds, two satellite galaxies and about ten times smaller.

But 10 billion years ago, the Milky Way itself was much smaller, illustrating the explosive power of fusion.

This Hubble image of antenna galaxies is the clearest of their fusion. (ESA / Hubble and B. Whitmore (Space Telescope Science Institute) / Reuters)

The researchers hope that understanding the collision in the Milky Way will allow them to better understand the process in other galaxies.

"The other thing we would like to do is go beyond these 10 billion years, sooner and earlier, and see if we can find evidence of the mergers that took place early and their nature, "said Helmi. "By studying these stars in these galaxies, you get a way to understand their properties."

The research, published today in the journal Nature, is far from over. With the latest version of Gaia, there is still much to do.

"What I really like is that Gaia data has been combined with … APOGEE data," Venn said. "It took the two investigations to work together and these two projects will continue, so we could find more stars from the fused galaxy and probably start rebuilding its whole story."

And this galactic dig is exactly what is so appealing to Helmi.

"The Milky Way is our home, and people like to know their origins, they like to know their own story, and for me, that's what's fascinating."