Striking new evidence for mass immigration of stars into the Andromeda galaxy

Galaxies grow and evolve by forging new stars and merging with other galaxies through aptly named "galactic immigration" events. The archaeology of such events, uncovered by studying the motions of individual stars throughout a galaxy and its extended halo of stars and dark matter, had been possible only in the Milky Way. 

Now, an international team of researchers has uncovered striking new evidence of a large galactic immigration event in the Andromeda galaxy, the Milky Way's nearest large galactic neighbor. The results are published in The Astrophysical Journal.

The observations were made with the Dark Energy Spectroscopic Instrument, DESI, on the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, a program of the U.S. National Science Foundation's NOIRLab. Construction and operations of DESI are funded by the U.S. Department of Energy. NSF also supported the work through two research grants to astronomers at the University of Michigan.

By measuring the motions of nearly 7,500 stars in the inner halo of the Andromeda galaxy, also known as 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, then merging 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. 

Arjun Dey, an astronomer at NSF's NOIRLab and the lead author of the paper, explained that "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."

Studying the relics of a similar but more recent galaxy merger in M31 gives astronomers a window into one of the major events in the Milky Way's past. Most of the stars in the Milky Way's halo were formed in another galaxy and later migrated in a galactic merger 8 to 10 billion years ago.

To trace the history of migration, the team turned to DESI, crucial to the team's survey of M31.

The team plans to use the unparalleled capabilities of DESI and the Mayall Telescope to explore more of M31's outlying stars, with the aim of revealing its structure and immigration history in unprecedented detail. 

"It's amazing that we can look out at the sky and read billions of years of another galaxy's history as written in the motion of its stars — each star tells part of the story," said Najita. "Our initial observations exceeded our wildest expectations, and we are now hoping to conduct a survey of the entire M31 halo with DESI. Who knows what new discoveries await."

"This is a great example of how the collaboration between NSF and DOE that supports DESI operations on the Mayall Telescope has resulted in remarkable science, demonstrating that stars formed far from a giant spiral galaxy continue to be pulled in," said Chris Davis, the program director for NOIRLab at NSF. "This shows that galaxies are still growing and changing."

NOIRLab is managed by the Association of Universities for Research in Astronomy under a cooperative agreement with NSF. During the project, the researchers also used the Aspen Center for Physics, a meeting and planning facility supported by NSF.