The origin of the $\alpha$-enhancement of massive galaxies

Abstract

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.

arXiv comment5 pages, 3 figures, accepted for publication in MNRAS Letters

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