2025ApJ...993..231Z

The MAMMOTH-MOSFIRE program is a deep Keck/MOSFIRE K-band spectroscopic follow-up of emission-line galaxies identified in the MAMMOTH-Grism Hubble Space Telescope/wide-field camera 3 G141 slitless spectroscopic survey, targeting the core regions of the three most massive galaxy protoclusters at cosmic noon. To introduce this program, we present a comprehensive analysis of the [N II]λ6584, [S II]λλ6717,6731, and [OI]λ6300 Baldwin, Philips, and Terlevich (BPT) diagnostics for a unique sample of 43 protocluster member galaxies at z ∼ 2, investigating how the overdense environment influences their interstellar medium ionization conditions. We find that, similar to their field counterparts at z ∼ 2, protocluster galaxies exhibit a systematic offset in the [N II] BPT diagram relative to the local star-forming sequence, but no such offset in the [S II] BPT diagram. Notably, we detect significantly elevated [O I]/Hα ratios, which can be well reproduced by photoionization models incorporating both H II regions and low-velocity shocks (v ∼ 200 km s^–1). We caution that neglecting shock excitation can bias abundance measurements, leading to an overestimation of nitrogen enrichment. This provides a potential explanation for the long-standing puzzle of enhanced [N II]/Hα ratios observed in z ∼ 2 galaxies. We further suggest that these shocks are likely environmentally driven, e.g., by ram pressure stripping or tidal interactions, which requires future confirmation through direct observations of features such as stripped gas tails.