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The Milky Way galaxy stole a few tiny galaxies from a neighbor

Our large galaxy, the Milky Way, is orbited by more than 50 other, smaller galaxies. But astronomers recently discovered that some of these galaxies were kidnapped from neighboring large dwarf galaxy the Large Magellanic Cloud.

The Large Magellanic Cloud, so called because it appears like a faint cloud, is also on a collision course with our galaxy.

The LMC is fairly new to orbiting the Milky Way, entering our corner of the universe 1.5 billion years ago. It’s now the brightest satellite galaxy we have, 163,000 light-years from the Milky Way. Previously, astronomers thought it would hang out in a quiet, long orbit or speed away from the gravity of the Milky Way and move on.

The Large Magellanic Cloud will catastrophically collide with the Milky Way in 2 billion years, according to a study published January in the Monthly Notices of the Royal Astronomical Society. The impact, which they believe is long overdue, has a chance of sending our solar system “hurtling through space.”

Now, researchers using the Gaia space telescope realized that four ultrafaint dwarf galaxies, as well as two bright galaxies called Carina and Fornax, once belonged to the LMC.

But the merger between the LMC and the Milky Way has begun, and our galaxy’s more massive nature is using gravity to rip pieces off of the Large Magellanic Cloud and kidnap some of its orbiting galaxies.

The study results will publish in the November issue of the Monthly Notices of the Royal Astronomical Society.

“These results are an important confirmation of our cosmological models, which predict that small dwarf galaxies in the universe should also be surrounded by a population of smaller fainter galaxy companions,” said Laura Sales, study author and assistant professor of physics and astronomy, at the University of California, Riverside. “This is the first time that we are able to map the hierarchy of structure formation to such faint and ultrafaint dwarfs.”

This discovery also sheds light on the past evolution of our galaxy.

“If so many dwarfs came along with the LMC only recently, that means the properties of the Milky Way satellite population just 1 billion years ago were radically different, impacting our understanding of how the faintest galaxies form and evolve,” Sales said.

Although dwarf galaxies are small, they can still be the home of thousands to billions of stars. But some of the tiny galaxies orbiting the LMC don’t contain any stars. Instead, they are filled with dark matter, a mysterious particle that comprises at least 90% — if not more — of the universe.

The researchers also think that the Large Magellanic Cloud may actually have more satellite galaxies orbiting it than they previously believed. These galaxies may only be made of dark matter and not contain stars.

“The high number of tiny dwarf galaxies seems to suggest the dark matter content of the LMC is quite large, meaning the Milky Way is undergoing the most massive merger in its history, with the LMC, its partner, bringing in as much as one third of the mass in the Milky Way’s dark matter halo — the halo of invisible material that surrounds our galaxy,” said Ethan Jahn, study author and University of California, Riverside graduate student.

In the future, the research team will study how galaxies like the Large Magellanic Cloud form stars. This could also be used to help them determine how much dark matter the galaxies contain.

“It will be interesting to see if they form differently than satellites of Milky Way-like galaxies,” Jahn said.

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