Found the long-lost sibling of the Milky Way

Messier 31, Andromeda Spiral Galaxy, located some 2.5 million light-years from Earth, the largest galactic neighbor to our own Milky Way, contains hundreds of billions of stars. Credit: Boris Štromar, Wikipedia.

M32 is a well-known small compact satellite galaxy, a companion to our closest galaxy, Andromeda or M31. Now a recent work by two scientists at the University of Michigan, Richard D’Souza and Eric Bell, show that M32 may be the core of a previously more massive sibling to our own Milky Way, they have deduced that the Andromeda galaxy, shredded and cannibalized a massive galaxy two billion years ago, and M32 is what remains of it, in fact, it may be the core of it. 

This disrupted galaxy was named M32p, been the third largest member of the Local Group after the Andromeda and the Milky Way galaxies. By studying the stars in the distant outskirts of M31 which were left behind as debris from this merger, the team was able to reconstruct the collision. “It’s kind of like a child eating dinner, and then looking on the floor afterwards and finding breadcrumbs all around,” D’Souza told The Guardian. “You know what’s been eaten.” 

In this image, the Andromeda galaxy shreds the large galaxy M32p, which eventually resulted in M32 and a giant halo of stars. Credit: Richard D’Souza. Image of M31 courtesy of Wei-Hao Wang. Image of stellar halo of M31 courtesy of AAS/IOP.

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. Their findings were published in Nature Astronomy. 

Scientists have long known that this nearly invisible large halo of stars surrounding galaxies contains the remnants of smaller cannibalized 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. 

To reveal this the scientists employed new computer simulations, “It was a ‘eureka’ moment. We realized we could use this information of Andromeda’s outer stellar halo to infer the properties of the largest of these shredded galaxies,” said lead author D’Souza, a postdoctoral researcher at U-M. “Astronomers have been studying the Local Group—the Milky Way, Andromeda and their companions—for so long. It was shocking to realize that the Milky Way had a large sibling, and we never knew about it,” said co-author Bell, U-M professor of astronomy. 

This is a family portrait of our local neighborhood of galaxies, called the Local Group as it would have looked more than 2 billion years ago. The missing galaxy, M32p would have been the third largest galaxy in the Local Group after the Andromeda and Milky Way galaxies. Credit: Richard D’Souza; LMC, M33 and M31, courtesy of Wei-Hao Wang; Milky Way, NASA/JPL; M64, NOAO/AURA/NSF.

The works points to solving the long-standing mystery of how the enigmatic M32 galaxy companion of Andromeda, formed in the first place. Their study may alter the traditional understanding of how galaxies evolve, the researchers say. They realized 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. 

Also, the timing of the merger may also explain the thickening of the disk of the Andromeda galaxy as well as a burst of star formation two billion years ago, a finding which was independently reached by French researchers earlier this year. 

The larger disc in this image, galaxy M64, represents what astronomers suspect the M32p galaxy looked like before the Andromeda galaxy cannibalized it nearly 2 billion years ago. The smaller image at the top shows is the M32p’s present-day remnant, the compact elliptical galaxy, M32. Credit: Richard D’Souza. 

“The Andromeda Galaxy, with a spectacular burst of star formation, would have looked so different 2 billion years ago,” Bell said. “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 towards unraveling the mysteries of galaxy formation.” 

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. 

Sources: University of Michigan, Wikipedia,