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Clues of additional fusions have been spotted in star clusters called globular clusters. Diederik Kruijssen, an astronomer at the University of Heidelberg in Germany, used galaxy simulations to train a neural network to examine globular clusters. He had him study their age, makeup and eye sockets. From this data, the neural network could reconstruct the collisions that have assembled the galaxies. Then he dropped it on the data from the real Milky Way. The program pieced together well-known events such as Gaia-Enceladus, as well as an older, more significant merger that the group dubbed Kraken.
In August, Kruijssen’s group released a Milky Way lineage fusion and the dwarf galaxies that formed it. They also predicted the existence of 10 more past collisions which they hope will be confirmed by independent observations. “We haven’t found the other 10 yet,” Kruijssen said, “but we will.”
All these mergers have led some astronomers to to suggest that the halo can consist almost exclusively of immigrant stars. Models of the 1960s and 1970s predicted that most of the Milky Way’s halo stars should have formed in place. But as more and more stars have been identified as galactic intruders, astronomers may not need to assume that many stars, if any, are native, Di Matteo said.
An ever-growing galaxy
The Milky Way has had a relatively quiet history of late, but newcomers keep coming. Astronomers in the southern hemisphere can spot a pair of dwarf galaxies called the Large and Small Magellanic Clouds with the naked eye. Astronomers have long believed that the couple were our faithful companions in orbit, like the moons of the Milky Way.
Then a series of Hubble Space Telescope Observations between 2006 and 2013, they looked more like incoming meteorites. Nitya Kallivayalil, an astronomer at the University of Virginia, timed the clouds to be hot at about 330 kilometers per second – almost twice as fast as expected.
When a team led by Jorge Peñarrubia, an astronomer at the Royal Observatory in Edinburgh, analyzed the numbers a few years later, they concluded that the fast clouds must be extremely heavy – perhaps 10 times larger. than previously thought.
“It was surprise after surprise,” Peñarrubia said.
Various groups have predicted that the surprisingly beefy dwarves could drag parts of the Milky Way, and this year Peñarrubia teamed up with Petersen to find proof of that.
The problem with galaxy-wide motion research is that the Milky Way is a raging blizzard of stars, with astronomers looking out from one of the snowflakes. So Peñarrubia and Petersen spent most of the lockdown figuring out how to neutralize the movements of the Earth and the Sun, and how to average the motion of the halo stars so that the outer fringe of the halo can serve as a stationary backdrop. .
When they calibrated the data in this way, they found that the Earth, the sun, and the rest of the disk they were on were wobbling in one direction – not toward the current position of the Large Magellanic Cloud, but toward its position. around a billion years ago (the galaxy is a heavy beast with slow reflexes, Petersen explained). They recently detailed their conclusions in Nature astronomy.
The sliding of the disc against the halo undermines a fundamental assumption: that the Milky Way is an object in equilibrium. It can spin and glide in space, but most astronomers have assumed that after billions of years, the mature disc and halo have settled into a stable configuration.
Peñarrubia and Petersen’s analysis proves this assumption to be false. Even after 14 billion years, mergers continue to sculpt the general shape of the galaxy. This awareness is just the latest change in how we understand the great flow of milk across the sky.
“Everything we thought we knew about the future and history of the Milky Way,” said Petersen, “we need a new model to describe this.”
Original story reprinted with permission from Quanta Magazine, an independent editorial publication of Simons Foundation whose mission is to improve public understanding of science by covering developments and research trends in mathematics and the physical and life sciences.
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