Gaia has found remnants of galaxies that collided with the Milky Way

The Milky Way was formed about 12 billion years ago. Since then, like the other galaxies that populate theUniverseshe grew up in mass and in size, thanks to numerous collisions. Over time, it has attracted smaller galaxies or clusters ofstars that she literally absorbed. Integrating into its ranks, these foreign stars. And to understand how galaxies form, astronomers need to know more about these collisions.

They tirelessly seek the traces left in the Milky Way by these events merger. In the data returned by the Gaia mission, for example. Because it aims to accurately measure the position, distance and movement of stars in the Milky Way. This is how a international team managed to draw a little more clearly thetree genealogy of our Galaxy.

Everything is played in the milky way halo. A region that extends far beyond the main part of our Galaxy. This is where we find what astronomers call stellar currents. They form from the stars of small galaxies that collide relatively slowly with the Milky Way. When the collision is more brutal, the stars — but also the globular clusters in particular — small galaxies find themselves dispersed in a random way in the halo of ours.

Five collisions… plus at least one

Astronomers have this time studied some 170 globular clusters, 41 stellar currents and 46 satellite galaxies of the Milky Way. And by analyzing their trajectories and all the kinematic data at their disposal using ultra-powerful algorithms, they show that 25% of them fall into six large groups. Six large groups, each corresponding to a past collision.

Most of the galactic mergers identified were already known to researchers. But this work makes it possible to specify the parent galaxies of different globular clusters, stellar currents or satellite galaxies. Thus currents deficient in metals C-19, Sylgr and phoenix would come from the fusion with the LMS-1/Wukong galaxy. What to consider it as a galaxy that would have formed very early after the big bang.

This collision with LMS-1/Wukong, which astronomers already knew about, just like those with the Cetus, Gaia-Sausage/Enceladus or Arjuna/Sequoia/I’itoi galaxies all occurred around eight to ten billion years ago. years. The collision with Sagittarius — also already identified in the past — seems more recent. It could date from 5 to 6 billion years ago ” only “. Because she remains very “active”.

A sixth hitherto unknown collision also appeared at the eyes researchers. They named it Pontus — which means ” Wed “ — as one of the first children of Gaia — the Greek goddess of the Earth. This event must also have occurred about 8 to 10 billion years ago. And there are already rumors of a seventh merger could still be hiding in the data…

Galaxy: the genealogy of the Milky Way reveals a big surprise

Use the globular clusters hosted by the Milky Way as markers of its history, of its evolution. Astronomers dreamed of it. They have now finally managed to put the pieces of the puzzle back together to reveal the family tree of our Galaxy. All thanks to the contribution of theartificial intelligence.

Article of Nathalie Mayer published on 16/11/2020

In the vastness of the Universe, the collisions between galaxies, as violent as they are, are not uncommon. It is even these collisions that are believed to have shaped the face of the galaxies we observe today. For the first time, researchers at the University of Heidelberg (Germany), among others, have succeeded in reconstructing the history of the collisions suffered by the Milky Way.

To establish the family tree of our Galaxy, astronomers have relied on two tools: globular clusters and artificial intelligence. The Milky Way, indeed, hosts more than 150 known globular clusters, dense groupings of hundreds of thousands of stars nearly as old as the Universe itself. Most formed in small galaxies that merged to eventually form ours. But it is only the most recent observations and the most sophisticated models that have made it possible to trace the thread of history.

The simulations played by the researchers — which they call E-Mosaics — incorporate a comprehensive model of the formation, evolution and destruction of globular clusters. Enough to link the ages, chemical compositions and orbital movements of these clusters to the properties of the galaxies in which they were born more than 10 billion years ago. And determine not only the number of stars that made up these galaxies, but also when they collided with the Milky Way.

Amazingly accurate simulations

It is to tame the complexity of the process that astronomers have used artificial intelligence. “We formed a artificial neural network on our simulations. We have tested the algorithm tens of thousands of times and have been amazed at how accurately it is able to reconstruct the histories of simulated galaxy mergers from their globular cluster populations alone”comments Diederick Kruijssen, researcher, in a press release from the University of Heidelberg.

Applied to the Milky Way, the method has shed light on the history of mergers that have shaped our galaxy. Thus the Milky Way would have engulfed approximately five galaxies of more than 100 million stars and about fifteen of at least 10 million stars. The biggest collisions having taken place between 6 and 11 billion years ago. Like the collision with the Gaia-Enceladus galaxy that happened some 9 billion years ago.

The method even revealed a previously unknown collision between the Milky Way and a galaxy named Kraken. “The largest collision our galaxy has ever seen. » It would have happened 11 billion years ago. The Milky Way was then four times less massive.