Scientists find evidence of galactic immigration outside the Milky Way

Astronomers found that the Andromeda Galaxy's past is remarkably similar to that of the Milky Way.
Chris Young
Stars in the Andromeda Galaxy represented with color.
Stars in the Andromeda Galaxy represented with color.

KPNO/NOIRLab/AURA/NSF/E. Slawik/D. de Martin/M. Zamani 

For the first time, astronomers have uncovered evidence of a large galactic immigration event occurring outside of our galaxy, according to a press release shared by

The cosmic event occurred in our nearest galactic neighbor, the Andromeda Galaxy, and it provides a window into the role of dark matter and the motion of individual stars throughout the cosmos.

Up until the new discovery, astronomers had only detected galactic immigration events in the Milky Way.

New evidence of Andromeda Galaxy immigration

A galactic immigration event, as the name implies, refers to the movement of a vast amount of stars, typically into the halo of a galaxy. These have been known to occur in the Milky Way due to galaxy mergers, which have been recorded due to these star movements.

Now, a team of researchers has uncovered new evidence of a large galactic immigration event in the Andromeda Galaxy. They made the new discovery using observations from the DOE's Dark Energy Spectroscopic Instrument (DESI) on the Nicholas U. Mayall four-meter Telescope at Kitt Peak National Observatory, a Program of NSF's NOIRLab.

The astronomers, who published their findings in The Astrophysical Journal, measured the motions of approximately 7,500 stars in the inner halo of the Andromeda Galaxy, which is also known as Messier 31 (M31). During this investigation, they found patterns in the positions and motions of stars that were indicative of immigration. Specifically, they observed stars that merged with M31 during a galaxy merger some two billion years ago.

Shedding new light on galaxy formation

The researchers believe their new observations shed new light on galaxy formation as it shows that the Andromeda Galaxy had a surprisingly similar evolution to our own Milky Way galaxy. Most of the stars in the Milky Way's inner halo also migrated to our galaxy during a merger event that occurred eight to ten billion years ago.

"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," Sergey Koposov, an astrophysicist at the University of Edinburgh and co-author of the paper, explained in a press statement. "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 new observations reveal a great deal about the cannibalistic past of both the Milky Way and our nearest neighbor, the Andromeda Galaxy. Next, the team behind the discovery plans to carry out more observations of M31's outlying stars in order to reveal its structure and immigration history in greater detail.

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