Data from Evolution of the Mass-Metallicity Relation from Redshift 8 to the Local Universe

A tight positive correlation between the stellar mass and the gas-phase metallicity of galaxies has been observed at low redshifts, with only ~ 0.1 dex scatter in metallicity. The shape and normalization of this correlation can set strong constraints on theories of galaxy evolution. In particular, its redshift evolution is thought to be determined by stellar and active galactic nucleus feedback-driven outflows, the redshift evolution of the stellar initial mass function or stellar yields, and broadly the star-formation histories of galaxies. The advent of JWST allows probing the mass-metallicity relation at redshifts far beyond what was previously accessible. Here we report the discovery of two emission-line galaxies at redshifts z = 8.15 and z = 8.16 in JWST NIRCam imaging and NIRSpec spectroscopy of galaxies gravitationally lensed by the cluster RX J2129.4+0005. We measure their metallicities using the strong-line method and their stellar masses through spectral-energy-distribution fitting with a nonparametric star-formation history. We combine these with nine similarly re-analysed galaxies at 7.2 < z < 9.5 to compile a sample of eleven galaxies at z ~ 8 (six with JWST metallicities and five with ALMA metallicities). Based on this sample, we report the first quantitative statistical inference of the mass-metallicity relation at z ~ 8 (median z = 8.15). We measure a ~ 1.0 dex redshift evolution in the normalization of the mass-metallicity relation from z ~ 8 to the local Universe; at fixed stellar mass, galaxies are 10 times less metal enriched at z ~ 8 compared to the present day. Our inferred normalization is in agreement with the predictions of the FIRE simulations. The inferred slope of the mass-metallicity relation is similar to or slightly shallower than the slope predicted by FIRE or observed at lower redshifts. Furthermore, based on emission-line diagnostic diagrams we compare the z ~ 8 galaxies to extremely low metallicity analog candidates in the local Universe, finding that they are generally distinct from extreme emission-line galaxies or “green peas” but are similar in their strong emission-line ratios and metallicities to “blueberry galaxies”. Despite this similarity, at fixed stellar mass, the z ~ 8 galaxies have systematically lower metallicities compared to blueberry galaxies and therefore stand out in the mass-metallicity diagram.