Insights on metal enrichment and environmental effects at z ≈ 5–7 with JWST ASPIRE/EIGER and the chemical evolution model

¹ Cosmic Dawn Center (DAWN), Denmark
² Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK2200 Copenhagen N, Denmark
³ Department of Astronomy, Tsinghua University, Beijing 100084, China
⁴ Center for Astrophysics and Planetary Science, Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel
⁵ Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, CA 95064, USA
⁶ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
⁷ International Gemini Observatory/NSF NOIRLab, 670 N A’ohoku Place, Hilo, Hawai’i 96720, USA
⁸ Department of Physics, Northwestern College, 101 7th Street SW, Orange City, Iowa 51041, USA
⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁰ LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 77 Av. Denfert-Rochereau, F-75014 Paris, France
¹¹ Max Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
¹² Department of Astronomy, University of Michigan, 1085 S. University Avenue, Ann Arbor, MI 48109, USA
¹³ Department of Astronomy, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
¹⁴ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, China
¹⁵ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile

^⋆ Corresponding author: zihao.li@nbi.ku.dk

Received: 2 May 2025
Accepted: 16 September 2025

Abstract

We present the mass–metallicity relation (MZR) for a parent sample of 604 galaxies at z = 5.34 − 6.94 with [O III] doublets detected that was obtained from 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 identified 204 galaxies residing in overdense structures using the friends-of-friends (FoF) algorithm. We estimated the electron temperature of 2.0^+0.3_−0.4 × 10⁴ K from the Hγ and [O III]₄₃₆₃ lines in the stacked spectrum, indicating a metal-poor sample with a median gas phase metallicity of 12 + log(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 ∼0.2 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 ∼0.2 dex relative to FMR predictions. We explain the observed trend of FMR with a simple analytical model, and we favor dilution from intense gas accretion over outflow to explain the metallicity properties at z > 5. The high-redshift galaxies are likely in a rapid gas accretion phase when 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 a 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 ≈ 5 − 7.