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Milky Way Collision With Sausage Galaxy 10 Billion Years Ago Was The Defining Moment In Galaxy’s History

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The remnants of a powerful collision between the Milky Way and a smaller galaxy, which happened billions of years ago can still be seen today.

An international team of astronomers has found that certain stars traveling in a peculiar orbit near the center of the Milky Way are the remains of a head-to-head encounter with a dwarf galaxy called Sausage.

The cataclysmic crash is one of the biggest moments in the history of the Milky Way, according to experts. Although the galaxy has been merging with a dozen of its satellites since its inception, this particular merger reshaped the structure of the Milky Way, creating the inner bulge at the center and the spherical halo circling the outer edges.

Milky Way Crash With Sausage Galaxy

Around 8 to 10 billion years ago, a dwarf galaxy smashed head-on into the Milky Way. The smaller galaxy did not survive the crash, as the strong gravitational force coming from the Milky Way tore it apart.

The researchers, however, have spotted evidence of the collision in the form of stars traveling in long, narrow orbits close to the galactic center. These sausage-like orbits, after which the galaxy was named, are called radial orbits.

"This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed," says Vasily Belokuro of the University of Cambridge.

The researchers reached their conclusion after extensively studying the data gathered from the ambitious Gaia mission of the European Space Agency. The objective was to collect as much information about the positions and trajectories of the millions of stars in the Milky Way and create a three-dimensional map of the galaxy based on the data collected.

Details of the study are published in three papers in the Monthly Notices of the Royal Astronomical Society, The Astrophysical Journal Letters, and the pre-print website ArXiv.

Gaia Sausage

Using data from Gaia, the researchers were able to analyze the velocities of the stars and plot their trajectories. They found a distinctively sausage-like orbit, prompting the scientists to christen them the Gaia Sausage stars.

"These Sausage stars are what's left of the last major merger of the Milky Way," says Wyn Evans, also of the University of Cambridge.

The Milky Way continues to swallow up smaller galaxies within its vicinity. There are nine dwarfs orbiting the galaxy, but the Milky Way is currently in gobbling up the dwarf galaxy Sagittarius.

In the next 100 million years or so, the gravitational pull from the Milky Way will likely rip Sagittarius apart in a scenario similar to what happened to the Sausage galaxy.

Sausage, however, was likely more powerful than Sagittarius. Despite its size, it wreaked a lot of havoc on the Milky Way following the collision, causing the galactic center to puff up and scattering the wreckage along the inner parts of the galaxy. The researchers believe it was this collision that shaped the Milky Way into the cosmic structure it is today.

They also found eight globular clusters left over from the crash. Globular clusters are ancient spherical clusters of stars typically found in larger galaxies. Although Sausage was a dwarf galaxy, the scientists believe it must have been large enough to harbor its own star clusters.

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