Over billions of years, galaxies expand and evolve through the aptly named “galactic immigration” events, which involve the formation of new stars and mergers with other galaxies. Scientists are investigating the motions of individual stars within a galaxy and the broad halo of stars and dark matter to learn more about the history of these immigration episodes. But until now, such cosmic archeology has only been possible in our galaxy, the Milky Way.

An international team of researchers has found striking new evidence of a major galactic immigration event in the Andromeda galaxy. Star movements show complex patterns that indicate a migration history similar to the Milky Way. The new findings were achieved using Nicholas U.’s Dark Energy Spectroscopic Instrument, which serves the DOE.

A team of astronomers studied the motions of nearly 7,500 stars in the inner halo of the Andromeda galaxy, also known as Messier 31 (M31). They found patterns in their positions and motions that indicated how these stars formed as part of an earlier galaxy that joined M31 about 2 billion years ago. While such patterns have long been anticipated by theory, they have never been observed with such clarity in a galaxy.

The new observations reveal evidence of a galactic immigration event in minute detail. Throughout cosmic history, smaller galaxies like M31 and our Milky Way served as building blocks for larger galaxies like M31 and ours.

Sergey Koposov, an astrophysicist at the University of Edinburgh and co-author of the paper, said: “We’ve never seen this so clearly in the motions of stars, nor had we seen some of the structures resulting from this merger. Our emerging view is that the history of the Andromeda Nebula is similar to that of our Milky Way galaxy, the Milky Way. The inner halos of both galaxies are dominated by a single immigration event.”

Galactic Immigration in the Andromeda Galaxy
Striking new evidence for a massive immigration of stars into the Andromeda Galaxy has been discovered by researchers led by astronomers from NSF’s NOIRLab. The team used the DOE’s Dark Energy Spectroscopic Instrument on Kitt Peak National Observatory’s Nicholas U. Mayall 4-meter telescope, a program of NSF’s NOIRLab, to reveal intricate structures in this galaxy with unprecedented detail and clarity. Each of the dots in this image represents an individual star in the Andromeda Galaxy, with the star’s movement (relative to the galaxy) color-coded from blue (moving towards us) to red (moving away from us). Credit: KPNO/NOIRLab/AURA/NSF/E. Slavik/D. de Martin/M. Zamani

The team used DESI to investigate the history of migration in M31. It is the most powerful multi-object survey spectrograph in the world and can measure the spectra of more than 100,000 galaxies per night.

This research provides information about the past of our galaxy and our galactic neighbours. During a galactic merger 8-10 billion years ago, most of the stars in the Milky Way’s halo originated in another galaxy and eventually ended up in our own. Astronomers can gain insight into one of the pivotal events in the history of the Milky Way by examining the remains of a similar but more recent galaxy merger in M31.

Arjun Dey, an astronomer at NSF’s NOIRLab and the lead author of the paper presenting this research, said: “This science could not have been done in any other facility in the world. DESI’s amazing efficiency, throughput and field of view make it the best system in the world to probe the stars in the Andromeda galaxy. In just a few hours of observation time, DESI was able to more than a decade of spectroscopy with much larger telescopes.”

The research was conducted in collaboration with two Harvard University students, Gabriel Maxemin and Joshua Josephy-Zack, who were involved in the project through the Radcliffe Institute for Advanced Study. Najita was a Radcliffe Fellow from 2021 to 2022.

Magazine reference:

  1. Dey, A. et al., (2023) “DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor” appears in The Astrophysical Journal. DOI: 10.48550/arXiv.2208.11683