New insights into Andromeda’s turbulent past emerge from stellar footprints
American researchers analyzed the composition and motion of 556 stars to uncover convincing evidence that the Andromeda Nebula, known as M 31, experienced a dramatic collision with a neighboring galaxy long ago. The findings were shared at a major gathering of the American Astronomical Society held in Pasadena, highlighting a moment when a neighboring galaxy shaped Andromeda’s current form and structure.
Galaxies grow by gathering material from nearby sources, including other galaxies and dense star clusters called globular clusters. Sometimes these gatherings culminate in mergers with larger neighbors, events that leave recognizable traces in the form of tidal remnants. These remnants may appear as long stellar streams or curved features near the growing galaxy. In Andromeda’s case, the researchers point to the northeast, west, and southeast shelves alongside the Giant Andromeda star stream as visible signatures of past mergers.
Ivanna Escala from the Carnegie Observatory, together with a team, examined these features to reconstruct the galaxy’s history and the role of its former neighbor in shaping Andromeda’s appearance and chemical makeup today. The team explains that remnants of such mergers can be identified by studying how stars move and what they are made of chemically. In essence, this information acts as a fingerprint that reveals which stars joined Andromeda as a result of the collision. For the first time, a detailed study traced the chemical composition and motion of stars in these regions of the nearest galactic neighbor, convincingly showing that the northeast shelf is a tidal remnant composed mainly of fragments from the collision. The data further indicate that the Giant Stellar Stream and the northeast shelf represent the initial and subsequent layers of material accretion, aligning with simulations of galactic growth through mergers.
The latest analysis reinforces prior ideas about Andromeda’s chaotic past and helps astronomers understand how material from such encounters influences a galaxy’s shape and long-term evolution. This refined view of Andromeda’s history strengthens the connection between stellar motions, chemical fingerprints, and the large-scale structure observed in one of the Milky Way’s closest galactic neighbors.