2025ApJ...979..247N

We present the results of a Keck and Northern Extended Millimeter Array spectroscopic survey of 507 galaxies, where we confirm the presence of two massive overdensities at z = 3.090-3.110 and z = 3.133-3.155 in the neighborhood of the GOODS-N, each with over a dozen spectroscopically confirmed members. We find that both of these have galaxy overdensities of near-infrared-detected galaxies of δ_gal,obs = 6-9 within corrected volumes of (6-7) × 10³ cMpc³. We estimate the properties of the z = 0 descendants of these overdensities using a spherical collapse model and find that both should virialize by z ≃ 0.5-0.8, with total masses of M_tot ≃ (6-7) × 10¹⁴M_⊙. The same spherical collapse calculations, as well as a clustering-of-clusters statistical analysis, suggest a >80% likelihood that the two overdensities will collapse into a single cluster with M_tot = (1.0-1.5) × 10¹⁵M_⊙ by z ∼ 0.1-0.4. The z = 3.14 substructure contains a core of four bright dusty star-forming galaxies with ΣSFR = 2700 ± 700M_⊙ yr^–1 in a volume of only 280 cMpc³.