On the Variation in Stellar α-enhancements of Star-forming Galaxies in the EAGLE Simulation

The ratio of alpha-elements to iron in galaxies holds valuable information about the star formation history (SFH) since their enrichment occurs on different timescales. The fossil record of stars in galaxies has mostly been excavated for passive galaxies, since the light of star-forming galaxies is dominated by young stars, which have much weaker atmospheric absorption features. Here we use the largest reference cosmological simulation of the EAGLE project to investigate the origin of variations in stellar alpha-enhancement among star-forming galaxies at z = 0, and their impact on integrated spectra. The definition of alpha-enhancement in a composite stellar population is ambiguous. We elucidate two definitions-termed "mean" and "galactic" alpha-enhancement-in more detail. While a star-forming galaxy has a high "mean" alpha-enhancement when its stars formed rapidly, a galaxy with a large "galactic" alpha-enhancement generally had a delayed SFH. We find that absorption-line strengths of Mg and Fe correlate with variations in alpha-enhancement. These correlations are strongest for the "galactic" alpha-enhancement. However, we show that these are mostly caused by other effects that are cross-correlated with alpha-enhancement, such as variations in the light-weighted age. This severely complicates the retrieval of alpha-enhancements in star-forming galaxies. The ambiguity is not severe for passive galaxies, and we confirm that spectral variations in these galaxies are caused by measurable variations in alpha-enhancements. We suggest that this more complex coupling between alpha-enhancement and SFHs can guide the interpretation of new observations of star-forming galaxies.