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-*>/M-circle dot) < 10.2, we find a strong correlation between stellar metallicity (Z(*)/Z(circle dot)) and stellar mass, with stellar metallicity monotonically increasing from Z(*)/Z(circle dot) < 0.09 at < M-*> = 3.2 x 10(8) M-circle dot to Z(*)/Z(circle dot) = 0.27 at < M-*> = 1.7 x 10(10) M-circle dot. 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 similar or equal to 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 greater than or similar to 2.5 star-forming galaxies of the order (O/Fe) greater than or similar to 1.8x(O/Fe)(circle dot). 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 = (M) over dot(outflow)/M-*) scales as eta proportional to M-*(beta) with beta similar or equal to -0.4.