Insights on Metal Enrichment and Environmental Effect at $z\approx5-7$ with JWST ASPIRE/EIGER and Chemical Evolution Model

Zihao Li, Koki Kakiichi, Lise Christensen, Zheng Cai, Avishai Dekel, Xiaohui Fan, Emanuele Paolo Farina, Hyunsung D. Jun, Zhaozhou Li, Mingyu Li, Maria Pudoka, Fengwu Sun, Maxime Trebitsch, Fabian Walter, Feige Wang, Jinyi Yang, Huanian Zhang, Siwei Zou

First listed 2025-04-25 | Last updated 2025-09-19

Abstract

We present the mass-metallicity relation (MZR) for a parent sample of 604 galaxies at $z=5.34-6.94$ with \OIII\ doublets detected, using the deep JWST/NIRCam wide field slitless spectroscopic (WFSS) observations in 26 quasar fields. The sample incorporates the full observations of 25 quasar fields from the JWST Cycle 1 GO program ASPIRE and the quasar SDSS J0100+2802 from the JWST EIGER program. We identify 204 galaxies residing in overdense structures using the friends-of-friends (FoF) algorithm. We estimate the electron temperature of $2.0^{+0.3}_{-0.4}\times10^4$ K from the Hg and OIII4363 lines in the stacked spectrum, indicating a metal-poor sample with median gas phase metallicity 12+$\log(\mathrm{O/H})=7.65^{+0.26}_{-0.15}$. With the most up-to-date strong line calibration based on NIRSpec observations, we find that the MZR shows a metal enhancement of $\sim0.2$ dex at the high mass end in overdense environments. However, compared to the local Fundamental Metallicity Relation (FMR), our galaxy sample at $z>5$ shows a metal deficiency of $\sim0.2$ dex relative to FMR predictions. We explain the observed trend of FMR with a simple analytical model, favoring dilution from intense gas accretion over outflow to explain the metallicity properties at $z > 5$. Those high-redshift galaxies are likely in a rapid gas accretion phase, during which their metal and gas contents are in a non-equilibrium state. According to model predictions, the protocluster members are closer to the gas equilibrium state than field galaxies and thus have higher metallicity and are closer to the local FMR. Our results suggest that the accelerated star formation during protocluster assembly likely plays a key role in shaping the observed MZR and FMR, indicating a potentially earlier onset of metal enrichment in overdense environments at $z\approx5-7$.

Short digest

Using JWST/NIRCam WFSS across 26 quasar fields (ASPIRE+EIGER), the authors build a [O III]-selected sample of 604 galaxies at z=5.34–6.94 and flag 204 protocluster members via a friends-of-friends search. Stacked spectra detect Hγ and [O III] λ4363, giving Te=2.0(+0.3,-0.4)×10^4 K and a metal-poor median 12+log(O/H)=7.65(+0.26,-0.15). With NIRSpec-anchored strong-line calibrations, the MZR shows ≈0.2 dex higher metallicity at the high-mass end in overdense regions, while the full z>5 sample sits ≈0.2 dex below the local FMR. A simple chemical-evolution model points to dilution from intense gas accretion, with protocluster galaxies closer to gas equilibrium—evidence for accelerated early enrichment during protocluster assembly at z≈5–7.

Key figures to inspect

• Figure 1: Check the z=5.34–6.94 distribution and ASPIRE vs EIGER contributions to gauge survey volume and where overdensities pile up; this sets the environment comparison baseline.
• Figure 2: Inspect the stacked spectrum for Hγ and [O III] λ4363 visibility and S/N; these lines underpin the Te≈2×10^4 K and the low median metallicity—also verify the 4959/5007 ratio consistency.
• Figure 3: Use the mass–excitation diagram to see where field and protocluster stacks fall relative to the Coil et al. (2015) AGN demarcation, assessing AGN contamination in the [O III]-selected sample.
• Figure 4: Compare protocluster vs field MZR using the R3 (C24) calibration and note the ≈0.2 dex high-mass enhancement; contrast with the local FMR offset and with literature curves at similar redshift.