A mysterious galaxy called ‘Kraken’ crashed into the Milky Way, scientists uncover while mapping stars’ family tree

The crash happened 11 billion years ago, when the Milky Way was four times smaller than it is now

Adam Smith
Monday 30 November 2020 12:02 GMT
Video of one of the E-MOSAICS simulations, showing the formation of a Milky Way-like galaxy
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The family tree of the Milky Way galaxy has been revealed by scientists, showing how the mass of stars came together and discovering a mysterious galaxy that could have crashed into the Milky Way that scientists did not know about.

The Milky Way is comprised of hundreds of millions of stars, collected into more than 150 ‘global clusters’ which come together to make up the galaxy.

Scientists have suspected that older clusters could be used to reconstruct how they merged with other ones, and therefore shed light on the history of the cosmos.

Using computer simulations called E-MOSAICS, which include a complete model for how clusters form, change, and are eventually destroyed, an international team of researchers were able to infer how the clusters would have come together to form the Milky Way.

Researchers were able to relate the ages, chemical compositions, and original motions of existing clusters to the galaxies they would have formed from over 10 billion years ago.

They could use these insights to determine how many stars these older galaxies contained, and when they would have merged to form the galaxy we know now.

“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,” Dr Diederik Kruijssen of the Center for Astronomy at the University of Heidelberg, who worked with Dr Joel Pfeffer at Liverpool John Moores University to conduct the research, said.

“To make sense of the complex system that is left today, we therefore decided to use artificial intelligence. We trained an artificial neural network on the E-MOSAICS simulations to relate the globular cluster properties to the host galaxy merger history.

“We tested the algorithm tens of thousands of times on the simulations and were amazed at how accurately it was able to reconstruct the merger histories of the simulated galaxies, using only their globular cluster populations.”

The scientists applied this neural network to groups of clusters, finding that they could predict the merger times of these galaxies with a high level of accuracy as well as uncovering a previously unkown collision between the Milky Way and a new galaxy called “Kraken”.

“The collision with Kraken must have been the most significant merger the Milky Way ever experienced,” Kruijssen adds. Prior to that, the largest collision was thought to be one with the Gaia-Enceladus-Sausage galaxy nine billion years ago.

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

Over the course of the Milky Way’s lifespan, the galaxy absorbed five galaxies with over 100 million stars, and fifteen with over 10 million stars. Scientists now hope to use the findings to search for the ‘fossils’ of these older galaxies.

“The debris of more than five progenitor galaxies has now been identified. With current and upcoming telescopes, it should be possible to find them all,” Kruijssen said. Their current findings, entitled “Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations”, are published in Monthly Notices of the Royal Astronomical Society.

In November, another ‘fossil’ galaxy was also found inside the Milky Way; known as Heracles, is thought to have collided with the Milky Way 10 billion years ago, when our galaxy was still at a very young age.

Its remnants make up about a third of Milky Way's spherical halo, researchers report, but despite its scale it was only discovered recently.

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