Realistic simulated galaxies form [α/Fe]–[Fe/H] knees due to a sustained decline in their star formation rates
A. Mason, Robert A. Crain, Ricardo P. Schiavon, David H. Weinberg, Joel Pfeffer, Joop Schaye, Matthieu Schaller, Tom Theuns
Abstract
ABSTRACT We examine the stellar [$\alpha$/Fe]–[Fe/H] distribution of $\simeq 1000$ present-day galaxies in a high-resolution EAGLE simulation. Roughly half of the galaxies exhibit the canonical distribution, characterized 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. Knees are found only in galaxies with star formation histories (SFHs) featuring a sustained decline from an early peak ($t\simeq 7~{\rm Gyr}$), which enables enrichment by Type Ia supernovae to dominate that due to Type II supernovae (SN II), reducing [$\alpha$/Fe] in the interstellar gas. The simulation thus indicates that, contrary to the common interpretation implied by analytic galactic chemical evolution (GCE) models, knee formation is not a consequence of the onset of enrichment by SN Ia. We use the SFH of a simulated galaxy exhibiting a knee as input to the vice GCE model, finding it yields an $\alpha$-rich plateau enriched only by SN II, but the plateau comprises little stellar mass and the galaxy forms few metal-poor ([Fe/H] $\lesssim$$-$1) stars. This follows from the short constant gas consumption time-scale typically assumed by GCEs, which implies the presence of a readily enriched low-mass gas reservoir. When an initially longer, evolving consumption time-scale is adopted, vice reproduces the simulated galaxy’s track through the [$\alpha$/Fe]–[Fe/H] plane and its metallicity distribution function.