The NIRVANDELS Survey: a robust detection of $\alpha$-enhancement in star-forming galaxies at $z\simeq3.4$

TL;DR

This study uses the NIRVANDELS survey to simultaneously analyze stellar and gas-phase metallicities in star-forming galaxies at z~3.4, providing evidence for alpha-enhancement and characterizing the mass-metallicity relationships at this epoch.

Contribution

First simultaneous measurement of stellar and gas-phase mass-metallicity relationships at z~3.4, revealing alpha-enhancement in early galaxies.

Findings

Metallicity increases with stellar mass in both gas and stars.

Power-law slope of MZRs is approximately 0.3 for M* < 10^10 M_sun.

Direct evidence of super-solar O/Fe ratios (~2.5 times solar) at z>3.

Abstract

We present results from the NIRVANDELS survey investigating the gas-phase metallicity ($\mathrm{Z}_{\mathrm{gas}}$, tracing O/H) and stellar metallicity ($Z_{\star}$, 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 and rest-frame optical respectively, we present the first simultaneous determination of the stellar and gas-phase mass-metallicity relationships (MZRs) at $z\simeq3.4$. In both cases, we find that metallicity increases with increasing stellar mass ($M_{\star}$), and that the power-law slope at $M_{\star} \lesssim 10^{10} \mathrm{M}_{\odot}$ 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., $\alpha$-enhancement) at $z>3$, finding $\mathrm{(O/Fe)}\simeq (2.54 \pm 0.38) \times \mathrm{(O/Fe)}_{\odot}$, with no clear dependence on $M_{\star}$.