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

Abstract

We present the results of a study utilising ultra-deep, rest-frame UV, spectroscopy to quantify the relationship between stellar mass and stellar metallicity for 681 starforming 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 ) < 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 * = 3.2 × 10 8 M to Z * /Z = 0.27 at M * = 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 =Ṁ outflow /M * ) scales as η ∝ M β * with β −0.4.

Both starvation and outflows drive galaxy quenching

“…While our model estimates provide a reasonable agreement with the two high-mass measurements, there is considerable disagreement with the three low-mass measurements, even when accounting for the massweight-light-weight offset. We also find some disagreement between our model estimates and the z ∼ 3.5 results from Cullen et al (2019), with our models consistently underestimating the stellar metallicities of these galaxies. Indeed, Cullen et al (2019) find that there is little evolution in the stellar MZR for 2.5 < z < 5.0.…”

“…We also find some disagreement between our model estimates and the z ∼ 3.5 results from Cullen et al (2019), with our models consistently underestimating the stellar metallicities of these galaxies. Indeed, Cullen et al (2019) find that there is little evolution in the stellar MZR for 2.5 < z < 5.0. These results suggest that our assumed evolution of the stellar MZR may become inaccurate at z = 3.…”

The properties of He IIλ1640 emitters at z ∼ 2.5–5 from the VANDELS survey

“…To ensure high enough SNR, we stack both the 'Bright' and the 'Faint' He ii emitters and obtain a stacked spectrum of all He ii emitters presented in this study. The metallicities are then measured following the method of Cullen et al (2019), which relies on a combination of stellar population synthesis modelling and metallicity indicators in the UV. With the low SNR of individual galaxy spectra, accurate metallicities using this method can only be obtained for stacks of objects for now.…”

“…A slight caveat of the stellar metallicity measurement method from Cullen et al (2019) used in this paper is that the SED fitting assumes a constant star-formation history. This assumption is valid for averaging across the general star-forming galaxy population, but if galaxies with He ii emission are very young, then this assumption may bias their derived metallicities.…”

“…We also mask residual sky lines in the spectra. The spectra are then stacked using an error-weighted averaging procedure, and the errors on the spectra are calculated using a bootstrapping method similar to Cullen et al (2019). Additionally, we also stack (using the same procedure as mentioned above) a total of 899 galaxies from the Parent sample that were not classified as He ii emitters.…”