Realistic simulated galaxies form [$\alpha$/Fe]-[Fe/H] knees due to a sustained decline in their star formation rates

Understand

We examine the stellar [$\alpha$/Fe]-[Fe/H] distribution of $\simeq1000$ present-day galaxies in a high-resolution EAGLE simulation.

• A slight majority of galaxies exhibit the canonical distribution, characterised by a sequence of low-metallicity stars with high [$\alpha$/Fe] that transitions at a "knee" to a sequence of declining [$\alpha$/Fe] with increasing metallicity.
• This population yields a knee metallicity - galaxy-mass relation similar to that observed in Local Group galaxies, both in slope and scatter.
• However, many simulated galaxies lack a knee or exhibit more complicated distributions.