The Andromeda galaxy, also known as Messier 31, is a spiral galaxy 2.537 million light-years from Earth. It is the Milky Way’s nearest large galaxy and has been the subject of extensive research and observation. While research and speculation about extraterrestrial life in the universe continue, concrete evidence of galactic immigration remains elusive.
Astronomers have discovered compelling new evidence for a massive star migration into the Andromeda Galaxy. Star motion patterns reveal an immigration history very similar to that of the Milky Way.
Over billions of years, galaxies grow and evolve by producing new stars and merging with other galaxies in what are known as “galactic immigration” events. Astronomers study the motions of individual stars in a galaxy and its extended halo of stars and dark matter to try to uncover the histories of these immigration events. Until now, however, such cosmic archaeology was only possible in our own galaxy, the Milky Way.
An international research team has discovered compelling new evidence of a large galactic immigration event in the Andromeda Galaxy, the Milky Way’s nearest large galactic neighbor. The new results were made with the DOE’s Dark Energy Spectroscopic Instrument (DESI) on the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, a Program of NSF’s NOIRLab.
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.
Arjun Dey
The team discovered telltale patterns in the positions and motions of nearly 7500 stars in the inner halo of the Andromeda Galaxy, also known as Messier 31 (M31), by measuring their motions. These patterns 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 observed in any galaxy with such clarity.
“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,” said Arjun Dey, lead author of the paper presenting this research and an astronomer at NSF’s NOIRLab.
“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. “
“We have never before seen this so clearly in the motions of stars, nor had we seen some of the structures that result from this merger,” said Sergey Koposov, an astrophysicist at the University of Edinburgh and coauthor of the paper. “Our emerging picture is that the history of the Andromeda Galaxy is similar to that of our own Galaxy, the Milky Way. The inner halos of both galaxies are dominated by a single immigration event.”
This study sheds light not only on the history of our galaxy’s neighbors, but also on the history of our own. The majority of the stars in the Milky Way’s halo formed in another galaxy and then migrated into ours during a galactic merger 8-10 billion years ago. The relics of a similar, but more recent, galaxy merger in M31 provide astronomers with a window into one of the Milky Way’s major events.
The team turned to DESI to trace the history of migration in M31. DESI was built to map tens of millions of galaxies and quasars in the nearby Universe in order to measure the effect of dark energy on the Universe’s expansion. It is the most powerful multi-object survey spectrograph in the world, and is capable of measuring the spectra of more than 100,000 galaxies a night. DESI’s world-class capabilities can also be put to use closer to home, however, and the instrument was crucial to the team’s survey of M31.
“This science could not have been done at any other facility in the world. DESI’s amazing efficiency, throughput, and field of view make it the best system in the world to carry out a survey of the stars in the Andromeda Galaxy,” said Dey. “In only a few hours of observing time, DESI was able to surpass more than a decade of spectroscopy with much larger telescopes.”
Even though the Mayall Telescope was completed 50 years ago (it achieved first light in 1973), it remains a world-class astronomical facility thanks to continued upgrades and state-of-the-art instrumentation. “Fifty years sounds like a long time, and naïvely one might think that’s the natural lifetime of a facility,” said co-author Joan R. Najita, also at NOIRLab. “But with renewal and reuse, a venerable telescope like the Mayall can continue to make amazing discoveries despite being relatively small by today’s standards.”
The study was conducted in collaboration with two Harvard University undergraduates, Gabriel Maxemin and Joshua Josephy-Zack, who were introduced to the project through the Radcliffe Institute for Advanced Study. From 2021 to 2022, Najita was a Radcliffe Fellow.
The team now intends to explore more of M31’s outlying stars using the unparalleled capabilities of DESI and the Mayall Telescope, with the goal of revealing its structure and immigration history in unprecedented detail.
