Astronomers Uncover Mass Migration of Stars into Andromeda

Astronomers know that galaxies grow over time through mergers with other galaxies. We can see it happening in our galaxy. The Milky Way is slowly absorbing the Large and Small Magellanic Clouds and the Sagittarius Dwarf Spheroidal Galaxy.

For the first time, astronomers have found evidence of an ancient mass migration of stars into another galaxy. They spotted over 7,000 stars in Andromeda (M31), our nearest neighbour, that merged into the galaxy about two billion years ago.

The growth and evolution of galaxies is a hot topic in astronomy and one of the reasons the James Webb Space Telescope is grabbing a lot of headlines lately. One of the JWST’s main scientific objectives is to look back in time to the Universe’s earliest galaxies to understand how they’ve grown and evolved into what they are today. But it’s not the only telescope that can shed light on the issue.

“Galaxies like M31 and our Milky Way are constructed from the building blocks of many smaller galaxies over cosmic history.”

Arjun Dey, NOIRLab

These new observations of Andromeda and the inward migration of stars comes from the Dark Energy Spectroscopic Instrument (DESI.) It was built to measure the effect dark energy has on the expansion of the Universe. It does that by gathering optical spectra on tens of millions of objects, mostly galaxies and quasars, and then constructing a 3D map of the results.

DESI is similar to the more well-known Gaia spacecraft. Gaia has an ambitious goal to precisely map the positions and motions of billions of stars in the Milky Way. Gaia data led to a wealth of discoveries about our own galaxy. But it’s confined to mapping stars in the Milky Way.

Now, thanks to DESI, astronomers have at least a partial map of the stars in Andromeda for the first time. And that map, including the motions of nearly 7500 stars in the inner halo of the Andromeda Galaxy, is revealing their history.

These results are in a new paper titled “DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor.” It will appear in The Astrophysical Journal, and the lead author is Arjun Dey, an astronomer at the National Science Foundation’s NOIRLab, the facility responsible for DESI.

DESI shows that about two billion years ago, another galaxy merged with Andromeda. The positions and motions of about 7500 stars DESI measured reveal that they came from another galaxy. Theory told us this was how Andromeda and other galaxies grew so massive, but now there’s a growing body of clear evidence.

“Our new observations of the Milky Way’s nearest large galactic neighbour, the Andromeda Galaxy, reveal evidence of a galactic immigration event in exquisite detail,” explained lead author Dey. “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.”

The Milky Way experienced a similar merger between 8 to 10 billion years ago. Most of the stars in our galaxy’s halo originated in a different galaxy and joined the Milky Way as a result of the ancient merger. Astronomers can learn more about the Milky Way’s ancient history by closely observing this similar, more recent merger event in Andromeda.

“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.”

The massive halo of gas is outlined in this image captured of the Andromeda galaxy. NASA, ESA, and E. Wheatley (STScI)

For the first time, we get a glimpse of the structures that formed as a result of the merger. “The expected observational signatures of galactic migration include debris streams, shells, rings, and plumes, the expected outcomes of merger interactions between large galaxies and their companions,” the authors write in their paper.

This figure from the research shows some of the stars in Andromeda’s inner halo and their velocities. The merger created a feature called the Giant Stellar Stream that appears as a stream of blueshifted stars approaching the disk from the south. Image Credit: Dey et al. 2023.

“We find clear kinematic evidence for shell structures in the Giant Stellar Stream, the Northeast Shelf and Western Shelf regions,” the paper states. “The kinematics are remarkably similar to the predictions of dynamical models constructed to explain the spatial morphology of the inner halo. The results are consistent with the interpretation that much of the substructure in the inner halo of M31 is produced by a single galactic immigration event 1–2 Gyr ago.”

“While hints of coherent structures have been previously detected in M31, this is the first time they have been seen with such detail and clarity in a galaxy beyond the Milky Way,” the authors write in their paper. “The observations reveal intricate coherent kinematic structure in the positions and velocities of individual stars: streams, wedges, and chevrons.”

This figure from the study shows the velocities vs. distance of some of Andromeda’s stars by colour. Note the stream of red stars and the chevrons of blue and turquoise, the results of the merger. Image Credit: Dey et al. 2023.

Though the positions and velocities of the 7500 stars play a major role in these findings, so did stellar metallicity. The team found high-metallicity stars in all of the sub-structures stemming from the merger. “We find significant numbers of metal-rich stars across all of the detected substructures, suggesting that the progenitor galaxy (or galaxies) had an extended star formation history, one perhaps more representative of more massive galaxies,” the authors explain in their conclusion.

The study highlights similarities between Andromeda and Milky Way, strengthening the theoretical idea that mergers play a key role in galactic evolution and growth. “M31 is remarkably similar to the Milky Way in that the inner halos of both galaxies are dominated by stars from a single accretion event,” the paper states. “Indeed, a recent study of the kinematics of Milky Way stars near the Sun reports chevron-shaped kinematic substructures that are reminiscent of those reported here.”

DESI’s power is on full display in this research. The results stem from DESI’s ability to gather spectra from 5,000 objects simultaneously. This complex instrument is the most powerful multi-object survey spectrograph in the world and can reconfigure its 5,000 separate focal planes in only two minutes as it slews between targets.

“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 motions of its stars.”

Joan R. Najita, NOIRLab.

It was designed to measure the spectra of over 40 billion distant galaxies and quasars to map the large-scale structure of the Universe and how dark energy fuels its expansion. Along the way, it’s showing us how galaxies merge over time.

The Dark Energy Spectroscopic Instrument (DESI) sits inside the Mayall Telescope at the Kitt Peak National Observatory near Tuscon, Arizona. Image Credit: By KPNO/NOIRLab/NSF/AURA/T. Slovinský –, CC BY 4.0,

“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.”

“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 motions of its stars — each star tells part of the story,” said co-author Joan R. Najita, also at NOIRLab. “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!”


Evan Gough

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