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

Observations show that star-forming galaxies reside on a tight 31) plane between mass, gas phase 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, star forming galaxies with M-star = 10(9.0-1.3) M-circle dot from the EAGLE hydrodynamical simulation to examine 3D) relations between mass, SFR, and chemical enrichment using absolute and relative C, N, O, and Fe abundances. We show that the scatter is smaller when gas-phase alpha-enhancement is used rather than metallicity. A similar plane also exists for stellar alpha-enhancement, implying that present-day specific SFRs are correlated with long time-scale star formation histories. Between z = 0 and 1, the alpha-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 alpha-enhanced, and this alpha-enhancement increases with redshift as observed. These findings suggest that relations between physical properties inferred from observations may be affected by systematic variations in alpha-enhancements.