The NIRVANDELS Survey: a robust detection of α-enhancement in star-forming galaxies at z ≃ 3.4

We present results from the NIRVANDELS survey on the gas-phase metallicity (Z_g, tracing O/H) and stellar metallicity (Z_⋆, tracing Fe/H) of 33 star-forming galaxies at redshifts 2.95 < z < 3.80. Based on a combined analysis of deep optical and near-IR spectra, tracing the rest-frame far-ultraviolet (FUV; 1200-2000 Å) and rest-frame optical (3400-5500 Å), respectively, we present the first simultaneous determination of the stellar and gas-phase mass-metallicity relationships (MZRs) at z ≃ 3.4. In both cases, we find that metallicity increases with increasing stellar mass (M_⋆) and that the power-law slope at M_⋆ ≲ 10¹⁰M_⊙ of both MZRs scales as $Z \propto M_{\star }^{0.3}$. Comparing the stellar and gas-phase MZRs, we present direct evidence for super-solar O/Fe ratios (i.e. α-enhancement) at z > 3, finding (O/Fe) = 2.54 ± 0.38 × (O/Fe)_⊙, with no clear dependence on M_⋆.