On the variation in stellar alpha-enhancements of star-forming galaxies in the EAGLE simulation

2021 
The ratio of $\alpha$-elements to iron in galaxies holds valuable information about the star-formation history 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 cosmological EAGLE simulation to investigate the origin of variations in $\alpha$-enhancement among star-forming galaxies at $z=0$. 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 star formation history. 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 which 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 star formation histories can guide the interpretation of new observations of star-forming galaxies.
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