Andromeda Ate Milky Way’s Sibling Almost 2 Billion Years Ago


In a new study, scientists just discovered a baffling fact about our galaxy: Milky Way had a big sibling until roughly two billions of years ago when it was torn apart by Andromeda.

The Local Group is a collection of over 50 galaxies packed into space measuring about 10 million light-years across. Both Andromeda and the Milky Way are part of the Local Group, being the largest galaxies of the group. But the third-biggest sibling of the family of galaxies was devoured by Andromeda, says the new study published online on July 23 in the journal Nature Astronomy.

Eric Bell is the co-author of the study and a professor of astronomy at the University of Michigan (UM). He explains how shocked they were to find out about the long-lost sibling, considering astronomers have studied the Local Group for a long time.

Andromeda is known to have shredded many small galaxies over time, but Bell and the study lead author Richard D’Souza – postdoctoral researcher (UM), found that a particular massive galaxy also fell prey to Andromeda’s destruction.

Using computer simulations, the scientists found out that the majority of the stars forming the spherical region around the disk of the galaxy came from a single “prey.”

“It was a ‘Eureka’ moment” – Richard D’Souza

Then, D’Souza said that they “realized we could use this information of Andromeda’s outer stellar halo to infer the properties of the largest of these shredded galaxies.”

More research allowed the team to date the time of the merging to almost 2 billion years ago. They also reconstructed some of the details of the dead galaxy, which the researchers named M32p.

But the dead galaxy is not entirely gone. Astronomers believe that Andromeda’s odd satellite galaxy called M32 is the “corpse” of the lost galaxy, leaving behind the bones.

Bell calls the lost sibling as being a “weirdo” because it looks “like a compact example of an old, elliptical galaxy,” but it also has many young stars, making it a unique galaxy.

The exciting part is that a different independent team found out that sometime between 1.8 billion and 3 billion years ago, Andromeda merged with a galaxy, also forming new stars. This study matches Bell’s simulations.

Why Is This Study Important?

D’Souza and Bell explain that this study will help scientists to understand the evolution and effects of galaxy mergers.

“When I was at graduate school, I was told that understanding how the Andromeda galaxy and its satellite galaxy M32 formed would go a long way toward unraveling the mysteries of galaxy formation,” concludes Bell.

Hopefully, if we’re still around, scientists will find out something about galaxy formation before the next 4 billion years, when Andromeda and Milky Way will merge.


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