Astronomie - Long-lost sibling of the Milky Way was eaten by Andromeda billions of years ago



Scientists have discovered the remnants of a galaxy hiding in plain sight.

The galaxy, named M32p, was once the third largest in the group of galaxies nearest to our Milky Way — known as the Local Group. The Andromeda galaxy and M32p existed in harmony until around 2 billion years ago, but then something shifted. 

According to a new study published in the journal Nature Astronomy today, Andromeda actually devoured the smaller galaxy, and its remains can be see in the clouds, gas, and dust of the huge galaxy today.

Massive galaxies like Andromeda automatically attract other galaxies to it when they're in close enough range due to its size and its gravitational pull. Because of that, it’s not unusual for a galaxy of that mass to effectively cannibalize other nearby galaxies nearby. 

“To give an analogy from the business world, one can grow through mergers and acquisitions. One could possibly grow by metering with other smaller businesses,” author of the study Richard D’Souza said via email. 

“But if one really wants to grow in size to certain level, one needs to acquire and merger with another large business.”

Previously, scientists believed that the population of stars known as a stellar halo surrounding Andromeda were all formerly parts of smaller galaxies Andromeda had merged with. 

But something in the researchers clicked when they realized that there was no way a bunch of individual merging events with smaller galaxies would leave a trail this big.

D’Souza called this their “eureka” moment.

“We realized that Andromeda's stellar halo could only be formed by the merger of a single large galaxy," D'Souza said.

In fact, there aren't enough small galaxies in the universe to merge and create Andromeda's stellar halo, he added.

The stellar halo is larger than Andromeda itself, which led scientists to believe that the size of the intact galaxy is roughly 20 times larger than anything the Milky Way has merged with.

The realization also helped solve the mystery surrounding the formation of M32, an extremely compact satellite galaxy, full of young and old stars, that hovers near the center of Andromeda.

Previously not much was known about M32, but now it seems clear that the smaller galaxy is the surviving center of M32p.

The computational method used in this study will help further investigations other large galaxies as well as advancing the literature on how galaxies evolve in general. 

And it's a good thing too.

One day — about 4 billion years from now — Andromeda and the Milky Way will collide with one another. And while Earth and the sun may not be effected by the cataclysmic event, it'll put on quite the cosmic show. 

Quelle: Mashable

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