Free Astronomy Magazine May-June 2023

34 MAY-JUNE 2023 ASTRO PUBLISHING began their lives as part of another galaxy that merged with M31 about 2 billion years ago. While such pat- terns have long been predicted by theory, they have never been seen with such clarity in any galaxy. “Our new observations of the Milky Way’s nearest large galactic neigh- bor, the Andromeda Galaxy, reveal evidence of a galactic immigration event in exquisite detail,” explained Arjun Dey, astronomer at NSF’s NOIRLab and the lead author of the paper presenting this research. “Al- though the night sky may seem un- changing, 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.“ “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 galax- ies are dominated by a single immigration event.” This research sheds light on not only the history of our galactic neighbors but also the history of our own galaxy. Most of the stars in the Milky Way’s halo were formed in another galaxy and later mi- grated into our own in a galactic merger 8–10 billion years ago. Studying the relics of a similar, but more recent, galaxy merger in M31 gives astronomers a window onto one of the major events in the Milky Way’s past. O ver the course of billions of years, galaxies grow and evolve by forging new stars and merging with other galaxies through aptly named “galactic im- migration” events. Astronomers try to uncover the histories of these im- migration events by studying the motions of individual stars through- out a galaxy and its extended halo of stars and dark matter. Such cosmic archaeology, however, has only been possible in our own galaxy, the Milky Way, until now. An international team of re- searchers has uncovered strik- ing new evidence of a large galactic immigration event in the Andromeda Galaxy, the Milky Way’s nearest large galactic neighbor. The new re- sults were made with the DOE’s Dark Energy Spectro- scopic Instrument (DESI) on the Nicholas U. Mayall 4- meter Telescope at Kitt Peak National Observatory, a Pro- gram of NSF’s NOIRLab. By measuring the motions of nearly 7500 stars in the inner halo of the Andromeda Galaxy, also known as Messier 31 (M31), the team discovered telltale patterns in the posi- tions and motions of stars that revealed how these stars Galactic immigration uncovered in Andromeda galaxy by NOIRLab − Charles Blue A team of researchers led by astronomers at NSF’s NOIRLab has uncovered striking new evidence for a mass migration of stars into the Andromeda Galaxy. Intricate patterns in the motions of stars reveal an mmigration history very similar to that of the Milky Way. This video summarizes the discovery. [Images and Videos: CTIO/KPNO/NOIRLab/NSF/AURA/R.T. Sparks/E. Slawik/M. Zaman/N. Bartmann. Image Pro- cessing: KPNO/NOIRLab/AURA/NSF/E. Slawik/D. de Martin/M. Zamani. Music: Stellardrone - Airglow]