VANDELS survey: the stellar metallicities of star-forming galaxies at |$\mathbf {2.5\,\, \lt\,\, z\,\, \lt\,\, 5.0}$|.

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 |$8.5\,\, \lt\,\, \mathrm{log}(\langle M_{\ast } \rangle /\rm {M}_{\odot })\,\, \lt\,\, 10.2$|⁠ , we find a strong correlation between stellar metallicity (Z _∗/Z_⊙) and stellar mass, with stellar metallicity monotonically increasing from Z _∗/Z_⊙ < 0.09 at |$\langle M_{\ast } \rangle = 3.2 \times 10^{8}\, \rm {M}_{\odot }$| to Z _∗/Z_⊙ = 0.27 at |$\langle M_{\ast } \rangle = 1.7 \times 10^{10}\, \rm {M}_{\odot }$|⁠. 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 |$\rm {O}/\rm {Fe}$| 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 (⁠|$\eta =\dot{M}_{\mathrm{outflow}}/M_{\ast }$|⁠) scales as |$\eta \propto M_{\ast }^{\beta }$| with β ≃ −0.4. [ABSTRACT FROM AUTHOR]