The VANDELS survey: the stellar metallicities of star-forming galaxies at 2.5 < z < 5.0

We present the results of a study utilizing ultradeep, rest-frame UV, spectroscopy to quantify the relationship between stellar mass and stellar metallicity for 681 star-forming galaxies at 2.5 < z < 5.0 (<z> = 3.5 ± 0.6) drawn from the VANDELS survey. Via a comparison with high-resolution stellar population synthesis models, we determine stellar metallicities (Z_∗, here a proxy for the iron abundance) for a set of high signal-to-noise ratio composite spectra formed from subsamples selected by mass and redshift. Across the stellar mass range 8.5 < log(< M_{\ast } > /{M}_{⊙ }) < 10.2, we find a strong correlation between stellar metallicity (Z_∗/Z_⊙) and stellar mass, with stellar metallicity monotonically increasing from Z_∗/Z_⊙ < 0.09 at < M_{\ast } > = 3.2 × 10⁸ {M}_{⊙ } to Z_∗/Z_⊙ = 0.27 at < M_{\ast } > = 1.7 × 10^{10} {M}_{⊙ }. In contrast, at a given stellar mass, we find no evidence for significant metallicity evolution across the redshift range of our sample. However, comparing our results to the z = 0 stellar mass-metallicity relation for star-forming galaxies, we find that the <z> = 3.5 relation is consistent with being shifted to lower metallicities by ≃0.6 dex at all stellar masses. Contrasting our derived stellar metallicities with estimates of the gas-phase metallicities of galaxies at similar redshifts and stellar masses, we find evidence for enhanced {O}/{Fe} ratios in z ≳ 2.5 star-forming galaxies of the order (O/Fe) ≳ 1.8 × (O/Fe)_⊙. Finally, by comparing our results to the predictions of three cosmological simulations, we find that the <z> = 3.5 stellar mass-metallicity relation is consistent with current predictions for how outflow strength scales with galaxy stellar mass. This conclusion is supported by an analysis of one-zone analytic chemical evolution models, and suggests that the mass-loading parameter (η =\dot{M}_{outflow}/M_{\ast }) scales as η ∝ M_{\ast }^{β } with β ≃ -0.4.