Spectroscopic Confirmation of Two Massive Protoclusters at 𝑧 ≈ 3.1

Submillimeter galaxy-rich protoclusters are the most highly star-forming extended structures in the Universe, and are believed to be the ancestors of today’s most massive galaxy clusters. However, only a few such protoclusters have enough spectroscopically confirmed members to assess accurately their total SFRs, masses, and the expected properties of their descendants. Here we present the results of a Keck and NOEMA spectroscopic survey of 507 galaxies, where we confirm the presence of two massive protoclusters at z = 3.10 and z = 3.14 in the neighborhood of the GOODS-N, each with over a dozen spectroscopically confirmed members. We find that both protoclusters have galaxy overdensities of δ_gal,obs ~ 9 within corrected volumes of (5 − 7) × 10³ cMpc³. We estimate the properties of the z = 0 descendants of these protoclusters using a spherical collapse model. We find that both should virialize by z ≈ 0.6 − 0.8, with total masses of M_tot = 6 − 8 × 10¹⁴ M_⊙. The z = 3.14 protocluster contains four bright dusty star-forming galaxies, which constitute a core within the larger structure. This core has ΣSFR = 4900 ± 700 M_⊙ yr^-1 in a volume of only 280 cMpc³, making it one of the most densely star-forming structures in the Universe.