Scientists have concluded that the Andromeda Galaxy, our next major galactic neighbor, tore up and exploded a huge galaxy two billion years ago.
Although this vast galaxy has been largely shredded, it left a rich trail of evidence: an almost invisible halo of stars, larger than the Andromeda Galaxy itself, an elusive star stream, and a separate enigmatic compact galaxy, M32. The discovery and exploration of this decimated galaxy will help astronomers understand how disk galaxies develop like the Milky Way and survive large mergers.
This broken galaxy, named M32p, was the third largest member of the Local Galaxy Group and Andromeda galaxies after the Milky Way. Using computer models, Richard D & # 39; Souza and Eric Bell of the University of Michigan Astronomical Department were able to put together this evidence to expose the Milky Way's long-lost brother. Their results appear in Nature Astronomy .
& # 39; & # 39 Eureka!;
Scientists have long known that this almost invisible, large halo of stars surrounding galaxies contains the remains of smaller cannibalized galaxies. A galaxy like Andromeda was expected to consume hundreds of her smaller companions. The researchers thought that would make it difficult to learn about one of them.
Using new computer simulations, scientists could understand that although Andromeda consumed many companion galaxies, most stars in Andromeda's outer faint halo came from the crushing of a single large galaxy.
"It was a" Eureka "moment, and we realized that we could use this information from Andromeda's outer stellar halo to infer the properties of the largest of these rugged galaxies," says lead author D & # 39; Souza postdoc.
"Astronomers have studied the local group ̵
This galaxy, called M32p, which ripped apart the Andromeda galaxy, was at least 20 times larger than any galaxy that linked to the Milky Way during its lifetime. M32p would have been massive, making it the third largest galaxy in the Local Group after the Andromeda and Milky Way galaxies.
"M32 is a nutcase."
This work could also solve a long-standing mystery: the formation of the Andromeda enigmatic M32 satellite galaxy, say the scientists. They suggest that the compact and dense M32 is the surviving center of the Milky Way's long-lost siblings, like the indestructible pit of a plum.
"M32 is a nutcase," says Bell. "While it looks like a compact example of an old, elliptical galaxy, it actually has many young stars, it is one of the most compact galaxies in the universe, there is no other galaxy like this."
The Milky Way ate 11 other galaxies  Their study could change the traditional understanding of galaxy evolution, researchers say. They realized that the disk of the Andromeda survived an impact with a massive galaxy, which would challenge the general belief that such large interactions would destroy disks and form an elliptical galaxy.
The timing of the fusion could also explain the thickening of the disk The Andromeda galaxy as well as a burst of star formation two billion years ago, a result that French researchers independently achieved earlier this year.
How much does our galaxy weigh?
"The Andromeda galaxy, with a spectacular burst of star formation, would have looked so different two billion years ago," says Bell. "When I was at the Graduate School, I was told that understanding the genesis of the Andromeda galaxy and its satellite galaxy M32 would make a great contribution to the elucidation of the mysteries of galaxy formation."
The researchers say they could use the method to study other galaxies that allow measurements of their most massive galaxy fusion. With this knowledge, scientists can better understand the complex network of cause and effect that drives galaxy growth and what galactic fusions do.
Source: University of Michigan