The Andromeda Galaxy Crash Triggered a Massive Galactic Migration

Astronomers have discovered new evidence that Andromeda, the galaxy next to our own, grew by merging with another galaxy. The event triggered a massive migration of stars into the galaxy.

This event suggests that the migration of stars to Andromeda and the growth history of the galaxy is very similar to the Milky Way. This means that the findings have implications for our understanding of both galaxies.

The evidence came in the form of observations of the individual motions of nearly 7,500 stars in Andromeda’s inner halo. This showed that these stars had started life as part of another galaxy that merged with Andromeda around 2 billion years ago.

Each of the dots in this image represents an individual star in the Andromeda Galaxy, with the star’s motion (relative to the galaxy) color-coded from blue (moving toward us) to red (moving away from us). . (Image credit: KPNO/NOIRLab/AURA/NSF/E. Slawik/D. de Martin/M. Zamani)
Scientists have long predicted that large galaxies like the Milky Way and Andromeda have grown to their current size through collisions and mergers throughout history, but patterns of stellar motion that could confirm this have been elusive.

The survey was conducted by an international team of astronomers using the Dark Energy Spectroscopic Instrument (DESI) on the Nicholas U. Mayall 4-meter telescope at Kitt Peak National Observatory, operated by 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,” said NOIRLab principal investigator and astronomer Arjun Dey in a statement(opens in a new tab). “Although the night sky may seem unchanging, the universe is a dynamic place. Galaxies like M31 and our Milky Way have been built from the building blocks of many smaller galaxies throughout cosmic history.”

It is also believed that most of the stars in the Milky Way’s halo originated from another galaxy that found a new galactic home during a massive merger event believed to have occurred between 8 and 10 billion years ago. Observing relics of a previous merger and stellar migration event in Andromeda could help astronomers search for similar objects in our own galaxy.

“We have never before seen this so clearly in the motions of stars, nor have we seen any of the structures resulting from this merger,” study co-author and University of Edinburgh astrophysicist Sergey Koposov said in the statement. “Our new picture is that the history of the Andromeda galaxy is similar to the history of our own galaxy, the Milky Way. The inner halos of both galaxies are dominated by a single immigration event.”

To track stellar migration in the galaxy, the team turned to DESI because it is the world’s most powerful multi-object survey spectrograph, capable of measuring the spectra of more than 100,000 galaxies in a single night.

“This science could not have been done at any other facility in the world. DESI’s astonishing efficiency, performance and field of view make it the best system in the world for conducting a survey of the stars of the Andromeda Galaxy,” added Dey. “In just a few hours of observing time, DESI was able to overcome more than a decade of spectroscopy with much larger telescopes.”

Although it first opened its eyes to the universe in 1973, the Mayall Telescope can still play a role in groundbreaking investigations like this thanks to five decades of upgrades and improvements.

The team will now continue to use the collaboration between DESI and the Mayall telescope to investigate stars closer to the edge of Andromeda. They hope this will further reveal the structure of the galaxy and the immigration history of the stars.

“It’s amazing that we can look up at the sky and read billions of years of another galaxy’s history as written in the movements of its stars: each star tells a part of the story,” said Joan R. Najita, member from the NOIRLab team and researcher.

The team’s research has been published in The Astrophysical Journal.