MAMMOTH-Grism: Revisiting the Mass-Metallicity Relation in Protocluster Environments at Cosmic Noon

Abstract

We present one of the first measurements of the mass-metallicity relation (MZR) in multiple massive protoclusters at cosmic noon, using Hubble Space Telescope (HST) G141 slitless spectroscopy from the MAMMOTH-Grism survey. We identify 63 protocluster member galaxies across three overdense structures at $z=2\text{-}3$ with robust detections of [OIII], H${\hat{I}}^2$, and [OII] emission. The sample spans gas-phase metallicities of $12+\log (\text{O/H})=8.2\text{-}8.6$, dust-corrected H${\hat{I}}^2$-based star formation rates (SFRs) of $10$-$250M_{\odot }{\text{yr}}^{-1}$, and stellar masses of $M_{\ast }\sim 10^{9.4}$-$10^{10.5}{M}_{\odot }$, derived via spectral energy distribution fitting using deep HST and ground-based photometry. We stack spectra in five $M_{\ast }$ bins to obtain average metallicities and SFRs. Relative to field galaxies at similar redshifts, protocluster members show elevated SFRs at $M_{\ast }<10^{10.25}{M}_{\odot }$ and a systematically shallower MZR: $12+\log (\text{O/H})=(6.96\pm 0.13)+(0.143\pm 0.017)\times \log (M_{\ast }/M_{\odot })$. We detect a mass-dependent environmental offset: massive protocluster galaxies are metal-poor compared to field counterparts of similar mass, whereas lower-mass systems exhibit comparable or mildly enhanced metallicities. This trend is consistent with a scenario where cold-mode accretion dilutes the interstellar medium (ISM) across the full mass range, while efficient recycling of feedback-driven outflows preferentially enriches the ISM in low-mass galaxies. Finally, we assess the dependence of metallicity offsets on local overdensity and find no significant trend, likely reflecting the survey's bias toward protocluster cores.

18 pages, 8 fqures, resubmitted after addressing the referee report