Our Milky Way’s biggest collision was with the “Kraken Galaxy”, not the “Gaia Sausage”, scientists say

Where does our galaxy come from?

It’s one of the biggest questions in cosmology and yet astronomers are only now beginning to unravel the mysterious mergers that led to the Milky Way.

The first complete family tree of our natal galaxy was reconstructed by an international team of astrophysicists. They used artificial intelligence to decipher the movements of the 150 globular clusters orbiting the Milky Way.

In doing so, they discovered a massive collision billions of years ago between our galaxy and what they dubbed the “Kraken” galaxy, an event that added millions of stars to the Milky Way.

Globular clusters – dense clusters of stars older than most of the Milky Way and linked together – are thought to be the remnants of galaxies that have merged to form our galaxy. Scientists have long known that galaxies can grow from the merger of smaller galaxies, but until now little is known about how the Milky Way was born.

Posted in Royal Astronomical Society Monthly Notices, the paper studied the ages, chemical compositions and orbital motions of globular clusters and analyzed the properties of the satellite galaxies from which the Milky Way was assembled.

Using globular clusters as “fossils” to reconstruct the earliest assembly histories of galaxies, the researchers developed an AI suite of advanced computer simulations called E-MOSAICS that show how globular clusters form, evolve, and are destroyed.

“The main challenge in linking the properties of globular clusters to the merging history of their host galaxy has always been that the assembly of the galaxy is an extremely messy process, during which the orbits of globular clusters are completely reshuffled,” said the dr. Diederik Kruijssen at the Center for Astronomy at the University of Heidelberg (ZAH) in Germany.

Indicates a new artificial neural network. “We tested the algorithm tens of thousands of times on simulations and were amazed at how accurately it was able to reconstruct the merger histories of simulated galaxies using only their populations of globular clusters,” Kruijssen said.

In the simulations, the researchers were able to rewind the 10 billion-year cosmic clock to position individual globular clusters in one of the progenitor galaxies that eventually merged to become the Milky Way.

The researchers were also able to accurately predict when each globular cluster merged with the Milky Way and how many stars it brought with it.

In doing so, the researchers essentially discovered the debris of more than five progenitor galaxies. The first four – Gaia-Enceladus, the Helmi, Sequoia and Sagittarius streams – were already known to astronomers. The team also discovered a previously unknown collision between the Milky Way and an enigmatic galaxy they dubbed the “Kraken”.

“The collision with Kraken must have been the most significant merger ever experienced by the Milky Way,” Kruijssen said. “Earlier it was thought that a collision with the Gaia-Enceladus-Sausage galaxy, which occurred about 9 billion years ago, was the largest collision event.”

The merger with Kraken occurred 11 billion years ago, when the Milky Way was four times less massive. “The collision with Kraken must have really transformed the appearance of the Milky Way at that time,” Kruijssen said.

To date, the Milky Way has merged with about five galaxies of over 100 million stars, but also about 15 others of at least 10 million stars.

So where are the remains of these five galaxies? When the dwarf galaxy Gaia-Enceladus-Sausage collided with the Milky Way, eight globular clusters are thought to have been added to our galaxy. The remnants of that galaxy are called “Gaia Sausage”, which is located near the Large Magellanic Cloud.

“Debris from more than five progenitor galaxies has now been identified,” Kruijssen said. “With current and future telescopes, it should be possible to find them all.”

I wish you clear skies and eyes wide open.