The Andromeda galaxy is the closest galaxy to ours. Apparently, it has been keeping some secrets. Scientists from the University of Michigan discovered that Andromeda ate its sister galaxy two billion years ago. At one point, the Milky Way had a sister galaxy and Andromeda literally ate it. How’s that for sibling rivalry? I know that my brother and I don’t always get along, but I feel like these are extreme measures. All jokes aside, scientists have suspected this for a really long time. In fact, this whole concept is what produces the halo of stars that surround Andromeda. But there was really no way to determine how many galaxies it had consumed over the course of its lifetime.
That said, they were able to determine this halo fact using computer models. Eric Bell, co-author of the paper that was published in Nature Astronomy about the discovery had this to say:
“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.”
The sister galaxy, known as M32p, was huge. It was a third in size to Andromeda and the Milky Way. What’s more, this revelation may also solve a secondary mystery: Andromeda has a small satellite galaxy, M32, and scientists don’t know where it originated. Using this theory, they can extrapolate that the dense, compact galaxy is actually the remnant of M32p. Andromeda and the Milky Way are the two largest members of the Local Group, which is a collection of more than 50 galaxies that are packed into a region of space that spans about 10 million light-years across.
It’s believed that Andromeda might have done this to other galaxies over the years. That said, the number and complexity of these galaxies has been difficult to know – until this particular team of researchers came along. Using computer simulations, the researchers were able to determine that most of the stars in the faint outer reaches of Andromeda’s halo came from a single smashup. Further modeling work allowed them to date the merger to about 2 billion years ago, and then to be able to reconstruct some basic details of that long-dead galaxy.
M32p was likely 20 times bigger than any galaxy that the Milky Way has ever merged with. That said, M32p isn’t gone completely. The researchers think that an odd satellite galaxy of Andromeda called M32 is the lost galaxy’s corpse (for lack of a better term). The timing of the merger matches up as well. Another research team independently determined earlier this year that Andromeda likely underwent a big merger, and a concomitant surge of star formation, between 1.8 billion and 3 billion years ago.
As dramatic as the Andromeda-M32p collision likely was, something much bigger is on the horizon. About 4 billion years from now, the Milky Way and Andromeda will come together in an epic crash that will shake up the Local Group. The merger will spark some pretty impressive star-formation fireworks in Earth’s night sky.