Star-forming galaxies are predicted to lie on a fundamental plane of mass, star formation rate, and α-enhancement

Abstract

Observations show that star-forming galaxies reside on a tight 3D plane between mass, gasphase metallicity, and star formation rate (SFR), which can be explained by the interplay between metal-poor gas inflows, SFR and outflows. However, different metals are released on different time-scales, which may affect the slope of this relation. Here, we use central, starforming galaxies with Mstar = 109.0-10.5 M⊙ from the EAGLE hydrodynamical simulation to examine 3Drelations between mass, SFR, and chemical enrichment using absolute and relative C, N, O, and Fe abundances.We showthat the scatter is smallerwhen gas-phase α-enhancement is used rather than metallicity. A similar plane also exists for stellar α-enhancement, implying that present-day specific SFRs are correlated with long time-scale star formation histories. Between z = 0 and 1, the α-enhancement plane is even more insensitive to redshift than the plane using metallicity. However, it evolves at z > 1 due to lagging iron yields. At fixed mass, galaxies with higher SFRs have star formation histories shifted towards late times, are more α-enhanced, and this α-enhancement increases with redshift as observed. These findings suggest that relations between physical properties inferred from observations may be affected by systematic variations in α-enhancements.