Clues about how galaxies like our Milky Way form and evolve and why their stars show surprising chemical patterns have been revealed by a new study.
The research, published today in Monthly Notices of the Royal Astronomical Society , 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".
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 led by researchers at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Centre national de la recherche scientifique (CNRS) uses advanced computer simulations (called the Auriga simulations) to recreate the formation of galaxies like the Milky Way in a virtual universe. By analysing 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," 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 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," said Dr Chervin Laporte, of ICCUB-IEEC, CNRS-Observatoire de Paris and Kavli IPMU.
"Ultimately, these will also help us further refine the physical evolutionary path of our own Milky Way."
