Astronomers Detect a Mass Migration of Stars Converging on 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.


Individual stars from blue (moving toward us) to red (moving away from us) surrounding Andromeda. (KPNO/NOIRLab/AURA/NSF/E. Slawik/D. de Martin/M. Zamani)

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 neighbor that merged into the galaxy about two billion years ago.


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

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 7,500 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 7,500 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 neighbor, 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."


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.

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