About two billion years ago, the Milky Way’s neighboring spiral galaxy, Andromeda, devoured a third large spiral galaxy that was once a sibling of the two.
Now the two largest galaxies within the Local Group, a region of over 50 galaxies spanning a diameter of more than 10 million light years, Andromeda, also known as M31, and the Milky Way, have both absorbed numerous smaller galaxies during their long lifetimes.
Because Andromeda devoured hundreds of smaller galaxies, it is difficult for scientists to trace evidence of any single one.
However, two astronomers at the University of Michigan (UM), Eric Bell and Richard D’Souza, successfully used computer simulations to study the stars in Andromeda’s outer halo and found that all originated in a single, large collision.
“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,” D’Souza said in a public statement.
Through the computer simulations, the researchers determined the large collision occurred about two billion years ago and even teased out properties of the cannibalized galaxy, which they determined to be 20 times larger than any galaxy consumed by the Milky Way.
These findings match those of another recent study that indicated Andromeda underwent a large merger that triggered major star formation sometime between 1.8 and three billion years ago.
A remnant of the large galaxy Andromeda consumed, designated M32p, still exists within an Andromeda satellite galaxy known as M32. Although M32 has the shape of an elliptical galaxy, it is filled with young stars. Elliptical galaxies are typically composed of older stars.
“It’s one of the most compact galaxies in the universe,” Bell said of M32. “There isn’t another galaxy like it.”
While scientists have long theorized that merging spiral galaxies both lose their spiral shapes during mergers, becoming a single larger elliptical galaxy, this did not happen to Andromeda, which kept its spiral shape after merging with and devouring M32p.
Noting that astronomers have long studied the Local Group, Bell commented, “It was shocking to realize that the Milky Way had a large sibling, and we never knew about it.”
An even larger galactic collision will occur in approximately four billion years, when the Milky Way and Andromeda Galaxy merge.
Findings of the study have been published in the journal Nature Astronomy.