How do you make space sausage?

About 10 million years ago, a small galaxy collided with our Milky Way, creating a cosmic sausage. This so-called Gaia-Enceladus-Sausage (GES) merger event stirred up the stars in our galaxy, throwing some of them into sausage-like elongated orbits around the galaxy’s central black hole and blowing the Milky Way’s disk to its current thick state , a pancake-like shape.

Astronomers now believe that the GES merger may also be responsible for the formation of the Milky Way’s signature band, a straight line of stars at the center of the galaxy’s spiral. Their findings were recently presented to the journal Monthly Notices of the Royal Astronomical Society and are currently available on arXiv as preprint.

This artist’s concept illustrates a view of the Milky Way. The two large arms of the galaxy can be seen attached to the ends of a thick central bar. Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

“Our paper shows for the first time that the Milky Way’s bar may have been created as a direct result of the Galaxy’s largest merger [the Gaia-Enceladus-Sausage merger]whose remnants we can see in the motions of nearby stars,” say authors Alex Merrow and Robert Grand, astronomers at Liverpool John Moores University PopSci.

Almost two-thirds of all spiral galaxies have bars, and they are a crucial piece of the puzzle of how stars, gas, and energy move within a galaxy due to their gravitational influence. However, astronomers do not fully understand how they appeared. Although we cannot travel back in time to see the origins of the Milky Way, astronomers can study nearby stars of different ages in great detail, providing clues about the past. “Observational clues lie in starlight, much like fossils that inform us about Earth’s history,” Merrow and Grand explain. “In particular, the positions, motions and chemical compositions of stars throughout the galaxy tell stories about our cosmic past.”

Recent observations have suggested that our galaxy’s bar may be quite old—perhaps 10 million years old, around the same time as the GES merger. To see if the merger could push the stars in the right way to form a bar, the researchers generated a computer simulation of a GES-like galaxy crashing into a Milky Way-like galaxy and then watched the stars move with “gravity” ” over time. With this setup, the stars in the simulation formed into a bar rather quickly, indicating that it is possible for this type of merger event to make a galactic bar.

“This is a great result, especially since there is now a lot of evidence showing how the GES merger has had a significant impact on a number of modern properties of the Milky Way,” Pratik Gandhi, an astronomer at UC Davis who was not involved in the new work, said. PopSci.

Our galaxy is also an important player in how we got here – living things on the tiny rock that is Earth. “In the Milky Way, we live well outside the bar region, but we are not outside its influence,” Merrow and Grand explain. As the bar moved around the stars, it may have displaced our sun as well – and where you live in a galaxy has huge consequences for how comfortable the planet you live on is. The Sun may “have been born in a completely different part of the galaxy than where it is now, due to the gravitational influence of the belt that moves it around,” they add.

Learning more about our home galaxy is important to our own origin story, explaining why we ended up in this particular part of the galactic neighborhood. This information could be key to unlocking other galactic stories as well. As Merrow and Grand say, this work will “provide a new perspective on the history of other barred galaxies in the Universe.”

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