Over the course of billions of years, galaxies grow and evolve by forging new stars and merging with other galaxies through aptly named "galactic immigration" events. Astronomers try to uncover the histories of these immigration events by studying the motions of individual stars throughout a galaxy and its extended halo of stars and dark matter. Such cosmic archaeology, however, has only been possible in our own galaxy, the Milky Way, until now.
By measuring the motions of nearly 7500 stars in the inner halo of the Andromeda Galaxy, also known as Messier 31 (M31), the team discovered telltale patterns in the positions and motions of stars that revealed how these stars began their lives as part of another galaxy that merged with M31 about 2 billion years ago. While such patterns have long been predicted by theory, they have never been seen with such clarity in any galaxy.
"Our new observations of the Milky Way's nearest large galactic neighbor, the Andromeda Galaxy, reveal evidence of a galactic immigration event in exquisite detail," explained Arjun Dey, astronomer at NSF's NOIRLab and the lead author of the paper presenting this research. "Although the night sky may seem unchanging, the Universe is a dynamic place. Galaxies like M31 and our Milky Way are constructed from the building blocks of many smaller galaxies over cosmic history."