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 = 2–3 with robust detections of [O iii ], H β , and [O ii ] emission. The sample spans gas-phase metallicities of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>12</mml:mn> <mml:mo>+</mml:mo> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>8.2</mml:mn> <mml:mo>−</mml:mo> <mml:mn>8.6</mml:mn> </mml:math> , dust-corrected H β -based star formation rates (SFRs) of 10–250 M ⊙ yr −1 , and stellar masses of M * ∼ 10 9.4 –10 10.5 M ⊙ , derived via spectral energy distribution fitting using deep HST and ground-based photometry. We stack spectra in five M * bins to obtain average metallicities and SFRs. Relative to field galaxies at similar redshifts, protocluster members show elevated SFRs at M * &lt; 10 10.25 M ⊙ and a systematically shallower MZR: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>12</mml:mn> <mml:mo>+</mml:mo> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> =(6.96 ± 0.13) + (0.143 ± 0.017) <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>×</mml:mo> <mml:mspace width="0.25em"/> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:math> . 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.