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Peer-reviewedCC BY 4.0The Astrophysical Journal

The Origin of Strong α -Element Bimodalities in FIRE Simulations of Milky Way–Mass Galaxies

  • Megan Barry1
  • Andrew Wetzel1
  • Sarah Loebman1
  • Jeremy Bailin2
  • Hanna Parul3
  1. 1University of California
  2. 2University of Alabama
  3. 3LIRA
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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.

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Cite

BibTeXRIS
BibTeX
@article{barry2026origin,
  title = {{The Origin of Strong α -Element Bimodalities in FIRE Simulations of Milky Way–Mass Galaxies}},
  author = {Megan Barry and Andrew Wetzel and Sarah Loebman and Jeremy Bailin and Hanna Parul},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4357/aea412},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea412},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - The Origin of Strong α -Element Bimodalities in FIRE Simulations of Milky Way–Mass Galaxies
AU  - Megan Barry
AU  - Andrew Wetzel
AU  - Sarah Loebman
AU  - Jeremy Bailin
AU  - Hanna Parul
PY  - 2026
DA  - 2026/10/01
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aea412
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aea412
AB  - 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.
SN  - 0004-637X
SN  - 1538-4357
ER  -