Zusammenfassung
We study the origin of the stellar $\alpha$-element-to-iron abundance ratio,
$\alpha/Fe_\ast$, 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$_ødot$, $\alpha/Fe_\ast$
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
(ISM) that was mostly $\alpha$-enriched by massive stars. In the absence of
feedback from active galactic nuclei (AGN), however,
$\alpha/Fe_\ast$ in $M_\ast > 10^10.5$ M$_ødot$ 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.
Nutzer