2026A&A...705A.214L

High-density environments, such as early galaxy overdensities, play a critical role in hierarchical structure formation and galaxy evolution, providing an ideal setting for accelerated galaxy growth. The GOODS-N overdensity at z ≃ 5.2 has long been known, but its full extent and richness have only recently been revealed by JWST observations. It is highly elongated along the line of sight, spanning nearly 30 cMpc. We investigated its dusty galaxy population using the NIKA2 Cosmological Legacy Survey (N2CLS). Within this overdensity, we identify five luminous dusty starbursts that are spectroscopically confirmed, along with three additional candidates supported by robust photometric redshifts. Three of the spectroscopically confirmed galaxies (N2GN₁01, 06, and 23, known as GN10, HDF850.1, and S3, respectively) had already been recognised as members of this exceptional structure. We report the discovery of N2GN₁13 at z_spec = 5.182, a massive dusty star-forming galaxy that we confirmed as part of the overdensity through targeted NOEMA follow-up observations of the N2CLS. In addition, by combining our analysis with JWST/FRESCO data, we identified another dusty galaxy at z_spec = 5.201 (N2GN₁61). The eight dusty galaxies are massive (with a median stellar mass of ∼9 × 10¹⁰ M_⊙) and highly obscured (with a median A_V of ∼3.3) and caught in a short-lived yet extreme starburst phase at z ∼ 5.2. Their high star formation rates (with a median of ∼590 M_⊙ yr^–1), efficient baryon to stellar mass conversion (ϵ_★ > 20% for 75% of the sample), and substantial gas reservoirs and dust content suggest rapid evolution and imminent quenching. Six of these galaxies reside in overdense filaments; the remaining two may trace new distinct structures, which will have to be spectroscopically confirmed. These few dusty galaxies dominate the star formation within the overdensity, contributing more than the numerous Hα emitters, and surpassing the cosmic average star formation rate density for this epoch. The presence of numerous very massive, dusty, and intensely star-forming galaxies at z ∼ 5.2 shows that rapid stellar and dust mass assembly was already underway within the first billion years of cosmic history in overdense environments. Their properties, likely driven by efficient gas inflows along cosmic filaments in protocluster regions, suggest an accelerated evolution that current models and simulations have difficulty reproducing.