The origin of diverse $\alpha$-element abundances in galaxy discs

TL;DR

This study uses the EAGLE simulation to explore how different gas accretion histories lead to diverse $ ext{[ extalpha/Fe]}$ distributions in galaxy discs, revealing that bimodality is rare and linked to early rapid halo growth.

Contribution

It demonstrates that bimodal $ ext{[ extalpha/Fe]}$ distributions arise from specific gas accretion episodes, providing insights into the Milky Way's unique chemical evolution.

Findings

Bimodality in $ ext{[ extalpha/Fe]}$ is rare, occurring in about 5% of simulated galaxies.

Early rapid gas accretion episodes are key to forming high-$ ext{[ extalpha/Fe]}$ sequences.

The Milky Way's abundance pattern may not be typical among similar galaxies.

Abstract

Spectroscopic surveys of the Galaxy reveal that its disc stars exhibit a spread in $\mathrm{[\alpha/Fe]}$ at fixed $\mathrm{[Fe/H]}$, manifest at some locations as a bimodality. The origin of these diverse, and possibly distinct, stellar populations in the Galactic disc is not well understood. We examine the Fe and $\alpha$-element evolution of 133 Milky Way-like galaxies from the EAGLE simulation, to investigate the origin and diversity of their $\mathrm{[\alpha/Fe]}$-$\mathrm{[Fe/H]}$ distributions. We find that bimodal $\mathrm{[\alpha/Fe]}$ distributions arise in galaxies whose gas accretion histories exhibit episodes of significant infall at both early and late times, with the former fostering more intense star formation than the latter. The shorter characteristic consumption timescale of gas accreted in the earlier episode suppresses its enrichment with iron synthesised by Type Ia SNe, resulting in the formation of a high-$\mathrm{[\alpha/Fe]}$ sequence. We find that bimodality in $\mathrm{[\alpha/Fe]}$ similar to that seen in the Galaxy is rare, appearing in approximately 5 percent of galaxies in our sample. We posit that this is a consequence of an early gas accretion episode requiring the mass accretion history of a galaxy's dark matter halo to exhibit a phase of atypically-rapid growth at early epochs. The scarcity of EAGLE galaxies exhibiting distinct sequences in the $\mathrm{[\alpha/Fe]}$-$\mathrm{[Fe/H]}$ plane may therefore indicate that the Milky Way's elemental abundance patterns, and its accretion history, are not representative of the broader population of $\sim L^\star$ disc galaxies.