Evidence of galactic cannibalism detected in Andromeda

The Andromeda galaxy shredded and cannibalised a massive galaxy two billion years ago, a new study published in Nature Astronomy has found. 

Even though it was mostly shredded, this massive galaxy left behind a rich trail of evidence: an almost invisible halo of stars larger than the Andromeda galaxy itself, an elusive stream of stars and a separate enigmatic compact galaxy, M32. Discovering and studying this decimated galaxy will help astronomers understand how disk galaxies like the Milky Way evolve and survive large mergers.

This disrupted galaxy, named M32p, was the third-largest member of the Local Group of galaxies, after the Milky Way and Andromeda galaxies. Using computer models, Richard D’Souza and Eric Bell of the University of Michigan were able to piece together this evidence. 

Scientists have long known that the nearly invisible large halo of stars surrounding galaxies contains the remnants of smaller cannibalised galaxies. A galaxy like Andromeda was expected to have consumed hundreds of its smaller companions. Researchers thought this would make it difficult to learn about any single one of them.

However, using new computer simulations, the scientists were able to use the size and qualities of the halo around Andromeda to understand that most of the stars in it must have come from a single large galaxy.

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

This galaxy, called M32p, which was shredded by the Andromeda galaxy, was at least 20 times larger than any galaxy which merged with the Milky Way over the course of its lifetime. M32p would have been the third largest galaxy in the Local Group after Andromeda and the Milky Way.

This work might also solve a long-standing mystery: the formation of Andromeda’s enigmatic M32 satellite galaxy, the scientists say. They suggest that the compact and dense M32 is the surviving centre of the Milky Way’s long-lost sibling, like the indestructible pit of a plum.

“M32 is a weirdo,” Bell said. “While it looks like a compact example of an old, elliptical galaxy, it actually has lots of young stars. It’s one of the most compact galaxies in the universe. There isn’t another galaxy like it.”

The study may alter the traditional understanding of how galaxies evolve, the researchers say. They realised that the Andromeda’s disk survived an impact with a massive galaxy, which would question the common wisdom that such large interactions would destroy disks and form an elliptical galaxy.

The method used in this study can be used for other galaxies, permitting measurement of their most massive galaxy merger, the researchers say. With this knowledge, scientists can better untangle the complicated web of cause and effect that drives galaxy growth and learn about what mergers do to galaxies.