Two Billion Years Ago, Andromeda Swallowed Another Galaxy

Someday, the Andromeda Galaxy will merge with our Milky Way – and it won’t be the first time Andromeda has swallowed up another galaxy and kept the stars for itself.
The nearby Andromeda Galaxy, like most galaxies, is made of more than just the familiar disk with its graceful arms. A spherical halo of stars and gas surrounds the disk — and many of the stars in that halo are the remnants of another galaxy, swallowed up in a merger two billion years ago.
A team of astronomers led by NOIRLab astronomer Arjun Dey published their findings in a recent paper in The Astrophysical Journal.
2 Billion Years Ago, Andromeda Swallowed Another Galaxy — These Stars are All That Remain