New study sheds light on the Milky Way’s mysterious chemical history

A new study led by researchers at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Centre national de la recherche scientifique (CNRS) offers fresh insights into how galaxies like our Milky Way form and evolve and why their stars show surprising chemical patterns.

Published in Monthly Notices of the Royal Astronomical Society, the research explores the origins of a puzzling feature in the Milky Way: the presence of two distinct groups of stars with different chemical compositions, known as the “chemical bimodality.”

What is chemical bimodality?

When scientists study stars near the Sun, they find two main types based on their chemical makeup, specifically, the amounts of iron (Fe) and magnesium (Mg) they contain. These two groups form separate “sequences” in a chemical diagram, even though they overlap in metallicity (how rich they are in heavy elements like iron). This has long puzzled astronomers.

The new study uses advanced computer simulations (called the Auriga simulations) to recreate the formation of galaxies like the Milky Way in a virtual universe. By analyzing 30 simulated galaxies, the team looked for clues about how these chemical sequences form.

Understanding the chemical history of the Milky Way helps scientists piece together how our Galaxy, and others like it, came to be. This includes our sister galaxy, Andromeda, in which no bimodality has yet been detected. It also provides clues about the conditions in the early universe and the role of cosmic gas flows and galaxy mergers.

“This study shows that the Milky Way’s chemical structure is not a universal blueprint,” says lead author Matthew D. A. Orkney, 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.”

Key findings
 

The study reveals that galaxies like the Milky Way can develop two distinct chemical sequences through various mechanisms. In some cases, this bimodality arises from bursts of star formation followed by periods of little activity, while in others it results from changes in the inflow of gas from the galaxy’s surroundings. Contrary to previous assumptions, the collision with a smaller galaxy known as Gaia-Sausage-Enceladus (GSE) is not a necessary condition for this chemical pattern to emerge. Instead, the simulations show that metal-poor gas from the circumgalactic medium (CGM) plays a crucial role in forming the second sequence of stars. Moreover, the shape of these chemical sequences is closely linked to the galaxy’s star formation history.

As new telescopes like the James Webb Space Telescope (JWST) and upcoming missions such as PLATO and Chronos provide more detailed data on stars and galaxies, researchers will be able to test these findings and refine our picture of the cosmos.

“This study predicts that other galaxies should exhibit a diversity of chemical sequences. 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,” adds Chervin Laporte (ICCUB-IEEC, CNRS-Observatoire de Paris and Kavli IPMU). 

Participating Institutions

This research has been led by researchers from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB), the Institute of Space Studies of Catalonia (IEEC) and the CNRS with the collaboration of scientists from Liverpool John Moores University and the Max-Planck-Institut für Astrophysik.

 

Reference:

Orkney, M. D. A., et al. (2025). The Milky Way in context: The formation of galactic discs and chemical sequences from a cosmological perspective. Monthly Notices of the Royal Astronomical Society. https://doi.org/10.1093/mnras/staf1551