Many massive quiescent galaxies at z>2, plausible progenitors of giant ellipticals today, have been discovered in the last decade. How these precursors of giant ellipticals formed in such an early epoch and then how they have evolved into giant ellipticals today is one of the most important questions of observational cosmology. The Lyα filaments extended over one Mpc found in the core of the protocluster at z=3.09 in the SSA22 field is a unique laboratory to test how the characteristics of giant ellipticals today have been developed in the cosmic web in the early Universe. Based on the previous spectroscopic studies and the NIRCam/JWST image analyses, plausible alignments of galaxies along the Lyα filaments, the prevalence of many massive quiescent galaxies, and enhanced formation of low-mass galaxies, which leads to the metallicity gradients, along the cosmic web, are found. To confirm these plausible origins of the properties of giant ellipticals today as early as z∼3 more comprehensively, we propose an extra-deep MOIRCS spectroscopy to confirm more galaxies along the Lyα filaments. To detect the [O III]λ5007 emission as faint as 1×10−18 erg/s/cm2 from ∼50 protocluster members, we request two MOS masks with each two nights (four gray/bright nights in total).
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