Metal enrichment of galaxies in a massive node of the cosmic web at <i>z</i> ∼ 3

We present the mass-metallicity relation for star-forming galaxies in the protocluster MUSE Quasar Nebula 01 (MQN01) field, a massive cosmic-web node at z ∼ 3.245, hosting one of the highest overdensities of galaxies and active galactic nuclei found so far at z > 3. Through James Webb Space Telescope (JWST) Near Infrared Spectrograph (NIRSpec) spectra and images from JWST and Hubble Space Telescope (HST), we identified a sample of nine star-forming galaxies in the MQN01 field with a detection of nebular emission lines (H β , [OIII], H α , [NII]), covering the mass range of 10 7.5 M ⊙ − 10 10.5 M ⊙ . We present the relations of the emission line flux ratios versus stellar mass for the sample and derive the gas-phase metallicity (12 + log(O/H)) based on the strong-line diagnostics of [OIII] λ 5008/H β and [NII] λ 6585/H α . Compared to the typical field galaxies at similar redshifts, MQN01 galaxies show relatively higher [NII] λ 6585/H α and lower [OIII] λ 5008/H β at the same stellar mass, which implies a higher metallicity by about 0.26 ± 0.07 dex than on the field mass-metallicity relation. These differences decrease when we consider the fundamental metallicity relation, that is, when we also take the galaxy star formation rates into account. We argue that these results are consistent with a scenario in which galaxies in overdense regions assemble their stellar mass more efficiently (or, equivalently, start forming mass at earlier epochs) than field galaxies at similar redshifts.