Andromeda May Have Eaten the Milky Way's Long-lost Sibling

The Andromeda Galaxy, located some 2.5 million light-years from Earth, burns brightly in ultraviolet light in this image taken by NASA’s Galaxy Evolution Explorer. (Credit: NASA/JPL-Caltech) The Andromeda Galaxy (M31) is the largest member of the Milky Way’s gang of galactic neighbors, known as the Local Group. With around a trillion suns worth of mass, Andromeda’s gravitational influence is a force to be reckoned with. And according to new research, no galaxy in the Local Group knows this better than M32, an oddball satellite galaxy now orbiting Andromeda. 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 strange compact galaxy known as M32 that we see bound to Andromeda today. This massive collision stripped M32’s progenitor galaxy (dubbed M32p) of most of its mass – taking it from a hefty 25 billion solar masses to just a few billion solar masses. “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 Eric Bell, an astronomer from the University of Michigan, in a press release.