A team of researchers led by astronomers from NSF’s NOIRLab has found striking new evidence of a massive migration of stars into the Andromeda galaxy. Intricate patterns in the movement of stars reveal an immigration history very similar to that of the Milky Way. The new results were obtained with the DOE’s Dark Energy Spectroscopic Instrument on the Nicholas U. Mayall 4-meter telescope at Kitt Peak National Observatory, a program of NSF’s NOIRLab.
Over billions of years, galaxies grow and evolve by forging new stars and merging with other galaxies through aptly named “galactic immigration” events. Astronomers are trying to uncover the history of these immigration events by studying the motions of individual stars in a galaxy and the vast halo of stars and dark matter. However, such cosmic archeology has until now only been possible in our own galaxy, the Milky Way.
An international team of researchers has found striking new evidence of a major galactic immigration event in the Andromeda galaxy, the Milky Way’s closest large galactic neighbor. The new results were created 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.
By measuring the motions of nearly 7,500 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 from another galaxy. galaxy that merged with M31 about 2 billion years ago. While such patterns have long been predicted by theory, they have not been observed so clearly 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 fine detail,” explained Arjun Dey, an astronomer at NSF’s NOIRLab and the lead author of the paper presenting this research. “While the night sky appears unchanging, the universe is a dynamic place. Galaxies like M31 and our Milky Way are built from the building blocks of many smaller galaxies throughout cosmic history. ”
“We’ve never seen this so clearly in the motions of stars, nor had we seen some of the structures resulting from this merger,” said Sergey Koposov, an astrophysicist at the University of Edinburgh and co-author of the paper. “Our view 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 research sheds light not only on the history of our galactic neighbours, but also on the history of our own galaxy. Most of the stars in the Milky Way’s halo formed in another galaxy and later migrated to ours in a galactic merger 8-10 billion years ago. Studying the remains of a similar, but more recent, galaxy merger in M31 gives astronomers a glimpse into one of the most important events in the Milky Way’s past.
To trace migration history in M31, the team turned to DESI. DESI was built to map tens of millions of galaxies and quasars in the nearby Universe to measure the effect of dark energy on the expansion of the Universe. It is the most powerful multi-object survey spectrograph in the world and can measure the spectra of more than 100,000 galaxies per night. However, DESI’s world-class capabilities can also be used closer to home, and the instrument was crucial to the team’s investigation of the M31.
“This science could not have been done in any other facility in the world. DESI’s amazing efficiency, throughput and field of view make it the best system in the world to conduct a survey of the stars in the Andromeda Galaxy. Dei said. “In just a few hours of observation time, DESI was able to surpass more than a decade of spectroscopy with much larger telescopes.”
Although the Mayall Telescope was completed 50 years ago (it first reached light in 1973), it remains a world-class astronomical facility thanks to constant upgrades and state-of-the-art instruments. “Fifty years sounds like a long time, and you might naively think that’s the natural lifespan 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 research was conducted in collaboration with two Harvard University students, Gabriel Maxemin and Joshua Josephy-Zack, who were involved in the project through the Radcliffe Institute for Advanced Study. Najita was a Radcliffe Fellow from 2021 to 2022.
The team now plans to use the unparalleled capabilities of DESI and the Mayall telescope to explore more of M31’s distant stars, aiming to reveal its structure and immigration history in unprecedented detail.
“It’s amazing that we can look at the sky and read billions of years of another galaxy’s history as written in the motions of its stars — each star tells part of the story,” concluded Najita. “Our initial observations exceeded our wildest expectations and we now hope to examine the entire M31 halo with DESI. Who knows what new discoveries await!”
Magazine reference
- Dey, A. et al., (2023) “DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor” appears in The Astrophysical Journal. DOI: 10.48550/arXiv.2208.11683
