The NIRVANDELS Survey: the stellar and gas-phase mass-metallicity relations of star-forming galaxies at z = 3.5

Abstract

We present determinations of the gas-phase and stellar metallicities of a sample of 65 star-forming galaxies at $z\simeq 3.5$ using rest-frame far-ultraviolet (FUV) spectroscopy from the VANDELS survey in combination with follow-up rest-frame optical spectroscopy from VLT/KMOS and Keck/MOSFIRE. We infer gas-phase oxygen abundances ($Z_{\mathrm{g}}$; tracing O/H) via strong optical nebular lines and stellar iron abundances ($Z_{\star }$; tracing Fe/H) from full spectral fitting to the FUV continuum. Our sample spans the stellar mass range $8.5<\log (M_{\star }/{\mathrm{M}}_{\odot })<10.5$ and shows clear evidence for both a stellar and gas-phase mass-metallicity relation (MZR). We find that our O and Fe abundance estimates both exhibit a similar mass-dependence, such that $Fe/H\propto M_{\star }^{0.30\pm 0.11}$ and $O/H\propto M_{\star }^{0.32\pm 0.09}$. At fixed $M_{\star }$ we find that, relative to their solar values, O abundances are systematically larger than Fe abundances (i.e., $\hat{I}\pm$-enhancement).We estimate an average enhancement of $(O/Fe)=2.65\pm 0.16\times (O/Fe{)}_{\odot }$ which appears to be independent of $M_{\star }$. We employ analytic chemical evolution models to place a constraint on the strength of galactic-level outflows via the mass-outflow factor ($\hat{I}\cdot$). We show that outflow efficiencies that scale as $\hat{I}\cdot \propto M_{\star }^{-0.32}$ can simultaneously explain the functional form of of the stellar and gas-phase MZR, as well as the degree of $\hat{I}\pm$-enhancement at fixed Fe/H. Our results add further evidence to support a picture in which $\hat{I}\pm$-enhanced abundance ratios are ubiquitous in high-redshift star-forming galaxies, as expected for young systems whose interstellar medium is primarily enriched by core-collapse supernovae.

18 pages, 10 figures, 4 tables, accepted in MNRAS