We didn’t know it until now, but we live in a “split personality” galaxy. This has just been revealed by a new study with significant Spanish participation, and the results provide new and valuable clues about how galaxies like ours form and evolve. … and why its stars exhibit such surprising chemical patterns.
The research, just published in Monthly Notices of the Royal Astronomical Society, explains for the first time the origins of a puzzling feature of the Milky Way: the presence of two distinct groups of stars, each with its own chemical “personality”, known as “chemical bimodality”.
When scientists study stars near the Sun, they encounter two main types that differ in composition. More precisely, in the quantities of iron (Fe) and magnesium (Mg) they contain. The two groups form distinct “sequences” on a chemical diagram, although they overlap in terms of “metallicity” (the relative abundance of all chemical elements heavier than hydrogen and helium). Something that has long confused astronomers.
Looking for a solution
Led by researchers from the University of Barcelona’s Institute of Cosmos Sciences (ICCUB) and the Center National de la Recherche Scientifique (CNRS) in Paris, the new study used advanced computer simulations (called Auriga simulations) to recreate the formation of galaxies such as the Milky Way in a virtual universe. Thus, and by analyzing up to 30 simulated galaxies, the team searched for clues about how these disparate chemical sequences form.
Understanding the chemical history of the Milky Way is not just a scientific curiosity, but the key to reconstructing its origin, as well as that of other similar galaxies. This includes our “galactic sister”, Andromeda, in whom no bimodality has yet been detected. This is certainly strange when you consider that Andromeda, without taking into account the small satellite galaxies surrounding ours, is the closest galaxy to us, “only” 2.5 million light years away.
In addition to this, the study provides clues about the conditions that prevailed in the early Universe and, incidentally, the role of cosmic gas flows and galaxy mergers. “This study – says Matthew Orkney, lead author of the article – shows that the chemical structure of the Milky Way is not a universal ‘recipe’. Galaxies can follow different paths to obtain similar results, and this diversity is essential to understand their evolution.”
Dual personality
The study reveals that galaxies like the Milky Way can develop two different chemical “personalities” through various mechanisms. In some cases, this bimodality results from intense bursts of star formation followed by periods of low activity, while in others it results from changes in the supply of gas surrounding the galaxy.
Until now, this dual chemical pattern was thought to be a necessary condition for the Milky Way to collide with a smaller galaxy known as Gaia-Sausage-Enceladus (GSE). But it’s not like that. Instead, the simulations clearly show that it is the metal-poor gas from the galactic environment that actually plays a crucial role in the formation of the second sequence of stars.
Furthermore, the shape of these chemical sequences is also closely linked to the star formation history of the galaxy.
So as new telescopes like James Webb or upcoming missions like PLATO and Chronos provide more detailed data about stars and galaxies, researchers will be able to test these findings and significantly refine our picture of the cosmos.
“This study – says Chervin Laporte, co-author of the article – predicts that other galaxies should also present a diversity of chemical sequences. This will soon be explored in the era of 30m telescopes, when the study of distant galaxies becomes routine. “Ultimately, these studies will also help us better understand the evolutionary path followed by our own Milky Way.”
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