The origin of the $\alpha$-enhancement of massive galaxies
  We study the origin of the stellar $\alpha$-element-to-iron abundance ratio,
$[\alpha/\mathrm{Fe}]_{\ast}$, of present-day central galaxies, using
cosmological, hydrodynamical simulations from the Evolution and Assembly of
GaLaxies and their Environments (EAGLE) project. For galaxies with stellar
masses of $M_{\ast} > 10^{10.5}$ M$_{\odot}$, $[\alpha/\mathrm{Fe}]_{\ast}$
increases with increasing galaxy stellar mass and age. These trends are in good
agreement with observations of early-type galaxies, and are consistent with a
`downsizing' galaxy formation scenario: more massive galaxies have formed the
bulk of their stars earlier and more rapidly, hence from an interstellar medium
that was mostly $\alpha$-enriched by massive stars. In the absence of feedback
from active galactic nuclei (AGN), however, $[\alpha/\mathrm{Fe}]_{\ast}$ in
$M_{\ast} > 10^{10.5}$ M$_{\odot}$ galaxies is roughly constant with stellar
mass and decreases with mean stellar age, extending the trends found for
lower-mass galaxies in both simulations with and without AGN. We conclude that
AGN feedback can account for the $\alpha$-enhancement of massive galaxies, as
it suppresses their star formation, quenching more massive galaxies at earlier
times, thereby preventing the iron from longer-lived intermediate-mass stars
(supernova Type Ia) from being incorporated into younger stars.
