In a study published today in Nature Astronomy, researchers showed that about 2 billion years ago, the Andromeda Galaxy cannibalized one of the largest galaxies in the Local Group, turning it into the unusual compact galaxy known as M32 that we see bound to Andromeda today.
To investigate how Andromeda accumulated its mass, the authors of the new study ran cosmological simulations of galaxy formation to show that Andromeda's observed properties - including a massive yet almost invisible halo of stars - can be well explained by a single major merger with what was once the third-largest galaxy in the Local Group, M32p.
"We realized that Andromeda's stellar halo could only be formed by the merger of a single large galaxy", D'Souza said. When they realized that they could use this information of Andromeda's outer stellar halo to infer the properties of the largest of these shredded galaxies.
Andromeda and the Milky Way are the two largest members of the Local Group, a collection of more than 50 galaxies packed into a dumbbell-shaped region of space about 10 million light-years across.
M32's size is between Milky Way and Andromeda's, but it does not outer layers as big as theirs.
"M32 is a weirdo", Bell said in a statement. It looks like a compact example of an ancient elliptical galaxy, but it actually has many young stars. Which is more significant than Andromeda Galaxy itself. "There isn't another galaxy like it".
Andromeda and the Milky Way are the two largest members of the Local Group
The method used to piece together Andromeda's dramatic past can now be applied to other galaxies, allowing scientists to gain a greater understanding of large galactic mergers, and how they influence the evolution of some of the most massive structures to populate the universe. Due to which researchers thought it would be hard to learn about any of those.
"Astronomers have been studying the Local Group - the Milky Way, Andromeda and their companions - for so long", said Bell, who teaches astronomy at the university.
Researchers believe that Andromeda's satellite galaxy M32 is actually the surviving center of the Milky Way's long-lost sibling.
The study of M32 will help astronomers understand how disk galaxies such as ours evolve and survive large mergers.
And, it could support previous findings from last year, which shed light on the thickening of Andromeda's disk and a burst of star formation two billion years ago. But Andromeda has retained its spiral disk, suggesting that the conventional wisdom does not always hold. Andromeda has a large, stretched stream of stars orbiting it, separate from its spiral arms, that is thought to have originated from some collision.