Rapid, out of equilibrium metal enrichment indicated by a flat mass-metallicity relation at z~6 from NIRCam grism spectroscopy

TL;DR

This study probes metal enrichment in the early Universe by measuring direct-$T_e$ metallicities for ~800 [O III]-selected galaxies at 5<z<7 using JWST/NIRCam grism data. The authors construct a flux-limited, stacked analysis to derive a MZR with normalization $Z_{ m norm}=7.96\pm0.10$ and slope $\gamma=0.12\pm0.08$, finding it flatter than local and some high-redshift samples and difficult to reconcile with several hydrodynamic simulations. They examine strong-line calibrations against direct-$T_e$ results and show that selection effects, particularly the 3D $Z$–$M_\star$–SFR relation, can bias the inferred MZR slope; toy models indicate a weak or absent FMR at $z\sim6$, implying non-equilibrium gas flows and rapid enrichment in low-mass systems. The work underscores the importance of large, carefully selected samples with robust metallicity measurements to disentangle gas inflows, star formation, and feedback in the early stages of galaxy evolution, guiding future JWST surveys toward tighter constraints on the high-redshift FMR.

Abstract

We aim to characterise the mass-metallicity relation (MZR) and the 3D correlation between stellar mass, metallicity and star-formation rate (SFR) known as the fundamental metallicity relation (FMR) for galaxies at $5<z<7$. Using $\sim800$ [O III] selected galaxies from deep NIRCam grism surveys, we present our stacked measurements of direct-$T\rm_e$ metallicities, which we use to test recent strong-line metallicity calibrations. Our measured direct-$T\rm_e$ metallicities ($0.1$-$0.2\,\rm Z_\odot$ for M$_\star$ $\approx5\times10^{7-9}$ M$_{\odot}$, respectively) match recent JWST/NIRSpec-based results. However, there are significant inconsistencies between observations and hydrodynamical simulations. We observe a flatter MZR slope than the SPHINX$^{20}$ and FLARES simulations, which cannot be attributed to selection effects. With simple models, we show that the effect of an [O III] flux-limited sample on the observed shape of the MZR is strongly dependent on the FMR. If the FMR is similar to the one in the local Universe, the intrinsic high-redshift MZR should be even flatter than observed. In turn, a 3D relation where SFR correlates positively with metallicity at fixed mass would imply an intrinsically steeper MZR. Our measurements indicate that metallicity variations at fixed mass show little dependence on the SFR, suggesting a flat intrinsic MZR. This could indicate that the low-mass galaxies at these redshifts are out of equilibrium and that metal enrichment occurs rapidly in low-mass galaxies. However, being limited by our stacking analysis, we are yet to probe the scatter in the MZR and its dependence on SFR. Large carefully selected samples of galaxies with robust metallicity measurements can put tight constraints on the high-redshift FMR and, help to understand the interplay between gas flows, star formation and feedback in early galaxies.

Figures (11)

• Figure 1: Relation between UV magnitude and stellar mass for our H$\beta$ sample (792 galaxies). The $\rm M_{UV}$ and stellar masses have been obtained through SED fitting using Prospector. With data from the ABELL2744 lensing field, we probe galaxies with $\rm M_{UV}$ down to $-16.6$ mag. The UV luminosity increases with stellar mass as expected.
• Figure 2: Median stacked 1D rest-frame emission-line spectrum of the full sample of 792 [Oiii] emitters at $z = 5.5 - 7$. Each galaxy spectrum has been normalised by its ([Oiii]$\lambda$5008$+2.98$[Oiii]$\lambda$4960)/2 flux before stacking. The green shaded region shows the uncertainty estimated through bootstrap resampling. We highlight the wavelengths of H$\delta$, H$\gamma$, [Oiii]$\lambda$4364, H$\beta$, [Oiii]$\lambda\lambda$4960,5008 and HeI$\lambda$5877. For visualisation, we normalise the stacked spectrum by the H$\beta$ luminosity such that H$\beta$ peaks at unity.
• Figure 3: Median stacked 1D rest-frame emission-line spectra of the H$\gamma$ sample ($z = 6.3 - 7$), in three stellar mass bins offset for visualisation purposes. The shaded region shows the uncertainty estimated through bootstrap resampling. In the right panel, we zoom into the wavelength range containing H$\gamma$ and [Oiii]$\lambda$4364. We fit a complex of three Gaussians with the same standard deviation to obtain the fluxes of H$\gamma$ and [Oiii]$\lambda$4364. An additional Gaussian component is fitted to account for possible contamination by [Feii]$\lambda4360$.
• Figure 4: The R3 ratio as a function of stellar mass ratios for the entire dataset. Galaxies with SNR(H$\beta$)$\geq$3 are shown in blue and galaxies with SNR(H$\beta$)$<3$ are shown in green with lower limits. Stars show measurements in stacked spectra.
• Figure 5: The relation between the gas-phase metallicity measured using the direct T$_e-$method and the R3 ratio. We compare our estimates with several strong-line metallicity calibrations from literature. We show the calibration by 2018ApJ...859..175B, which uses local analogues that occupy similar regions to $z\sim2$ star-forming galaxies on the BPT diagram, the calibration based on local galaxies with EW(H$\beta$)$\geq 200$ derived by Nakajima20222023MNRAS.526.3504H calibration for $z>4$ galaxies from the TNG100 simulation, and Sanders2024_MZR and Chakraborty2025_MZR calibrations for high-$z$ galaxies between $2<z<10$.