The Origin of Strong α -Element Bimodalities in FIRE Simulations of Milky Way–Mass Galaxies
Year: 2026
Type: article
Abstract: One of the Milky Way’s (MW) characteristic features is a strongly bimodal distribution of α -process elements, such as Mg, at fixed [Fe/H] in stellar abundances. We examine patterns in [Mg/Fe] versus [Fe/H] in FIRE-2 simulations of MW-mass galaxies. Out of 16 galaxies, four are capable of producing a strongly bimodal distribution. In all four galaxies, the high- α population corresponds to an older, radially compact, thick disk, and the low- α population corresponds to a younger, radially extended, thin disk, similar to the MW. The transition from high to low α took 0.3–1.2 Gyr and began 5.5–6.5 Gyr ago. [Mg/Fe] decreased at relatively fixed [Fe/H], both in the galaxy overall and at fixed radii: following a rapid star formation rate (SFR) decline, delayed Fe enrichment from white dwarf supernovae continued, while prompt Mg enrichment from core-collapse supernovae weakened. This transition occurred at a relatively low gas fraction (5%–15%), immediately after the SFR halved within a few hundred Myr. Only one case coincided with a major merger coalescence. We find similar trends in measuring stars by their current radius and by their birth radius; therefore, radial redistribution did not play a dominant role in the formation of a bimodality or its spatial dependence today. Overall, in FIRE-2, strong α -element bimodalities are relatively uncommon (∼25%), are often not associated with a major merger, and arise primarily from a rapid decline in star formation during relatively low gas fraction.
Source: The Astrophysical Journal
Institutions: University of California System, University of Alabama
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
Awards: G2782160116, XSEDE 2.0: Integrating, Enabling and Enhancing National Cyberinfrastructure with Expanding Community Involvement, Collaborative Research: Magellanic dwarfs as a key laboratory for dwarf galaxy formation, CAREER: Galactic Archeology: Understanding the Building Blocks of the Milky Way across Cosmic Time, Collaborative Research: Unraveling the Evolving Dynamics of Star Clusters Using Milky Way-like Galaxy Simulations, G7400298905, G7798327059, Simulating the life cycle of star clusters: from birth to death in a cosmological galactic context
OpenAlex ID: W7215106310