M31 shows clear signs of galactic migration

An international research team has discovered clear signs of galactic migration in our neighbor, the Andromeda galaxy M31.
Each point in the image represents a single star in the Andromeda Galaxy. The star’s motion is color-coded from blue (moving towards us) to red (moving away from us). Credit: KPNO/NOIRLab/AURA/NSF/E. Slawik/D. de Martin/M. Zamani

The closest galaxy to us is M31, also known as the Andromeda galaxy, which has unequivocally shown signs of the phenomenon of galactic migration. Throughout their evolutionary history, all galaxies grow by forging new stars and merging with each other.

Astronomers seek to uncover the stories of these events by observing the motions of individual stars and the halo of dark matter. Until now, this approach has only been possible in our own galaxy, the Milky Way.

Tracking 7500 stars

An international team has discovered new and surprising evidence of a major galactic migration event in the Andromeda galaxy. The new results were obtained using the Dark Energy Spectroscopic Instrument (DESI) on the 4-meter Nicholas U. Mayall telescope at the Kitt Peak National Observatory. By measuring the movements of nearly 7500 stars, the team has discovered revealing patterns in the positions and motions of the stars. These stars began their lives as part of another galaxy that merged with M31 about 2 billion years ago. Although such patterns have long been predicted by theory, they have never been seen so clearly before.

“Our observations of the M31 galaxy show signs of galactic migration in exquisite detail” explained Arjun Dey, lead author of the paper (ref.) published in The Astrophysical Journal. “Although the night sky may seem unchanging, the universe is a dynamic place. Galaxies like M31 and our Milky Way are built from the building blocks of many smaller galaxies over cosmic history”.

“We have never seen it so clearly in the movements of the stars, nor have we seen some of the structures resulting from this merger” said Sergey Koposov, astrophysicist at the University of Edinburgh and co-author 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 migration event”.

The Dark Energy Spectroscopic Instrument

To trace the history of migration, the team used a particular instrument. 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 cosmos. It is the world’s most powerful multi-object spectrograph and can measure the spectra of over 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 M31.

“This research could not have been conducted at any other facility in the world. DESI’s incredible efficiency, productivity, and field of view make it the best system in the world for surveying stars in the Andromeda Galaxy” said Dey. “In just a few hours of observation, DESI was able to surpass over a decade of spectroscopy with much larger telescopes”.

“It’s incredible to be able to look up at the sky and read billions of years of another galaxy’s history as written in the movements of its stars – every star tells a part of the story” concluded co-author Joan R. Najita. “Our initial observations exceeded our wildest expectations, and now we hope to conduct a survey of the entire halo of M31 with DESI. Who knows what new discoveries await us”.

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