Astronomers have long noted that solar-neighbourhood stars fall into two main sequences when plotted by their iron and magnesium content, a feature known as chemical bimodality. The new study, published in Monthly Notices of the Royal Astronomical Society, tests how that structure might arise by modelling the assembly of galaxies similar to the Milky Way.
Researchers at the Institute of Cosmos Sciences of the University of Barcelona and the French Centre national de la recherche scientifique analysed 30 galaxies from the Auriga simulation suite, which tracks how Milky Way-mass systems grow in a cosmological context. By following how gas cools, forms discs and fuels star formation, they identified conditions that generate two separate chemical tracks in iron-magnesium space.
The team reports that bimodality can emerge through several routes rather than a single universal mechanism. In some simulated galaxies, rapid bursts of star formation, separated by quieter phases, produce chemically distinct stellar populations, while in others the main driver is a change in the rate or composition of gas accreted from the surrounding environment.
The results challenge the idea that the Milky Way's merger with the Gaia-Sausage-Enceladus dwarf galaxy is required to explain the split chemical sequences. Instead, the simulations point to metal-poor gas from the circumgalactic medium as a key ingredient for building a second, chemically different group of stars on top of an existing disc.
Lead author Matthew Orkney of ICCUB and the Institut dEstudis Espacials de Catalunya said the work shows that the Milky Way should not be treated as a template for all disc galaxies. According to the study, systems with similar masses and morphologies can experience different gas accretion and star formation histories yet arrive at broadly comparable present-day structures.
The shape of the chemical sequences in each simulated galaxy tracks its specific star formation history, linking abundance patterns directly to how and when stars formed. That connection offers a way to read past inflows, outflows and merger events from present-day stellar chemistry, and to compare the Milky Way with neighbours such as Andromeda where no clear bimodality has yet been observed.
The authors argue that forthcoming facilities will make it possible to test these predictions in detail. Observations from the James Webb Space Telescope and future missions including PLATO and Chronos, combined with 30-metre-class ground-based telescopes, are expected to map chemical patterns in external galaxies with the precision already achieved in the Milky Way.
Co-author Chervin Laporte of ICCUB-IEEC, CNRS-Observatoire de Paris and Kavli IPMU said the next generation of surveys should reveal whether diverse chemical sequences are common in disc galaxies. Those measurements will in turn refine models of how the Milky Way assembled its disc and populated it with multiple chemically distinct stellar populations.
Research Report:The Milky Way in context: The formation of galactic discs and chemical sequences from a cosmological perspective
Related Links
Royal Astronomical Society
Stellar Chemistry, The Universe And All Within It
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