“The collision with Kraken must have been the most significant merger the Milky Way ever experienced,” said astrophysicist Diederik Kruijssen at the Center for Astronomy at the University of Heidelberg (ZAH), about artificial intelligence analysis of ancient globular clusters almost as old as the universe itself orbiting the Milky Way that revealed a stunning picture of successive mergers with neighboring galaxies creating what is, in effect, a galactic “family tree”.
“Before,” Kruijssen adds, “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. However, 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.”
The Ancients –Globular Clusters Reveal Fossil Galaxies
The Milky Way harbors over 150 such clusters–densely packed objects that are the oldest visible objects in the universe – each harboring hundreds of thousands to over ten million stars all born at essentially the same time, and densely packed into a spherical volume with diameter over a thousand times smaller than the diameter of our Galaxy, many of which formed in the smaller galaxies that merged to form the galaxy that we live in today.
Astronomers, led by Kruijssen and Dr. Joel Pfeffer at Liverpool John Moores University, developed a suite of advanced computer simulations called E-MOSAICS that allowed researchers to relate the ages, chemical compositions, and orbital motions of globular clusters to the properties of the “fossil” galaxies in which they formed, more than 10 billion years ago to reconstruct the early assembly histories of galaxies. By applying these insights, they could not only determine how many stars these primordial galaxies contained, but also when they merged into the Milky Way.
“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,” explained Kruijssen. “To make sense of the complex system that is left today, we decided to use artificial intelligence,” Kruijssen added, “training 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.”
ESO image above shows the second largest and second brightest globular cluster, NGC 104 or 47 Tucanae, the light for this image was gathered in 1986 by ESO’s at the La Silla observatory in Chile.
Enter Kraken –“The Unknown”
Enter “Kraken”. By applying the neural network to these groups of globular clusters, the researchers could not only predict the stellar masses and merger times of the progenitor galaxies to high precision, but it also revealed a previously unknown collision between the Milky Way and an ancient, enigmatic galaxy, which the researchers named “Kraken.”
Taken together, these findings allowed the team of researchers to reconstruct the first complete merger tree of our Galaxy. Over the course of its history, the Milky Way cannibalized about five galaxies with more than 100 million stars, and about fifteen with at least 10 million stars. The most massive progenitor galaxies collided with the Milky Way between 6 and 11 billion years ago.
The researchers expect their predictions to stimulate future studies to search for the remains of these progenitor 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 concludes.
Source: J M Diederik Kruijssen et al. Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations, Monthly Notices of the Royal Astronomical Society (2020). DOI: 10.1093/mnras/staa2452
The Daily Galaxy, Max Goldberg, via the Center for Astronomy at the University of Heidelberg
Image at top of page: is the first accurate 3D map of our galaxy revealing its true shape: warped and twisted. Astronomers from Macquarie University and the Chinese Academy of Sciences have used 1339 ‘standard’ stars to map the real shape of our home galaxy in a paper published in Nature Astronomy today. Artist’s impression above of the warped and twisted Milky Way disk. (Chen Xiaodian)