New computer simulations suggest that the Milky Way's unusual split in star chemistry could stem from various evolutionary processes, not just a single ancient collision. Researchers analyzed 30 virtual galaxies to explore this long-standing mystery. The findings challenge previous assumptions about galactic formation.
Astronomers have long puzzled over the Milky Way's chemical bimodality, where stars near the Sun fall into two distinct groups based on their iron (Fe) and magnesium (Mg) abundances. These groups form separate sequences on chemical plots, despite overlapping in overall metallicity.
To unravel this, scientists from the Institute of Cosmos Sciences at the University of Barcelona (ICCUB) and the Centre national de la recherche scientifique (CNRS) turned to the Auriga simulations. These models recreate the formation of Milky Way-like galaxies in a virtual universe. Examining 30 such simulated galaxies, the team identified multiple pathways that could produce the observed split.
One route involves bursts of intense star formation followed by quieter phases. Another arises from changes in incoming gas flows. Streams of metal-poor material from the galaxy's outskirts, specifically the circumgalactic medium (CGM), also contribute significantly. The study, published in Monthly Notices of the Royal Astronomical Society, downplays the role of the ancient Gaia-Sausage-Enceladus (GSE) merger, showing it is not essential for the bimodality.
"This study shows that the Milky Way's chemical structure is not a universal blueprint," said lead author Matthew Orkney, a researcher at ICCUB and the Institut d'Estudis Espacials de Catalunya (IEEC). "Galaxies can follow different paths to reach similar outcomes, and that diversity is key to understanding galaxy evolution."
The simulations link the precise shape of these chemical sequences to a galaxy's star formation history. This offers insights into how the Milky Way and neighbors like Andromeda assembled over time, though Andromeda lacks a similar bimodality.
Future observations from telescopes like the James Webb Space Telescope (JWST), PLATO, and 30m-class instruments will test these ideas. "This study predicts that other galaxies should exhibit a diversity of chemical sequences," noted Dr. Chervin Laporte of ICCUB-IEEC, CNRS-Observatoire de Paris, and Kavli IPMU. "This will soon be probed in the era of 30m telescopes where such studies in external galaxies will become routine. Ultimately, these will also help us further refine the physical evolutionary path of our own Milky Way."