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Obecny czas to 04:52 Sob 18 Wrz 2021

Our Milky Way’s Biggest Collision Was

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Our Milky Way’s Biggest Collision Was With The ‘Kraken Galaxy’ Not The ‘Gaia Sausage,’ Say Scientists

Where does our galaxy come from?

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

The first complete family tree of our home galaxy has been reconstructed by an international team of astrophysicists. They used artificial intelligence to decipher the movements of the 150 globular clusters that orbit the Milky Way.

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

It’s thought that globular clusters—dense clumps of stars older than most in the Milky Way and related to each other—are the leftovers of galaxies that merged to form our galaxy. Scientists have known for some time that galaxies can grow by the merging of smaller galaxies, but until now little has been known about how the Milky Way came to be.

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

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Using globular clusters as “fossils” to reconstruct the early 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 of connecting the properties of globular clusters to the merger history of their host galaxy has always been that galaxy assembly is an extremely messy process, during which the orbits of the globular clusters are completely reshuffled,” said Dr Diederik Kruijssen at the Center for Astronomy at the University of Heidelberg (ZAH) in Germany.

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

In the simulations, the researchers were able to wind the cosmic clock back 10 billion years to place individual globular clusters into 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 found the debris of more than five progenitor galaxies. The first four—Gaia-Enceladus, the Helmi streams, Sequoia and Sagittarius—were already known to astronomers. The team also uncovered a previously unknown collision between the Milky Way and an enigmatic galaxy that they dubbed “Kraken.”

“The collision with Kraken must have been the most significant merger the Milky Way ever experienced,” said Kruijssen. “Before, it was thought that a collision with the Gaia-Enceladus-Sausage galaxy, which took place some 9 billion years ago, was the biggest collision event.”

The merger with Kraken took place 11 billion years ago when the Milky Way was four times less massive. “The collision with Kraken must have truly transformed what the Milky Way looked like at the time,” said Kruijssen.

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

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

“The debris of more than five progenitor galaxies has now been identified,” said Kruijssen. “With current and upcoming telescopes, it should be possible to find them all.”

Wishing you clear skies and wide eyes.
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