2025A&A...701A.120P

Advancing our understanding of the formation and evolution of early massive galaxies and black holes requires detailed studies of dense structures in the high-redshift Universe. In this work, we present high angular resolution (≃0.​​″3) ALMA observations targeting the CO(4-3) line and the underlying 3 mm dust continuum toward the Cosmic Web node MQN01, a region identified through deep multiwavelength surveys as having one of the densest concentrations of galaxies and active galactic nuclei at cosmic noon. At the center of this structure, we identified a massive, rotationally supported disk galaxy located approximately at a projected-distance of ∼10 kpc and ∼ - 300 km s^–1 from a hyper-luminous quasar at z = 3.2510. By accurately modeling the cold gas kinematics, we determined a galaxy dynamical mass of 2.5 × 10¹¹ M_⊙ within the inner ≃4 kpc and a high degree of rotational support of V_rot/σ ≈ 11. This makes it the first quasar companion galaxy confirmed as a massive, dynamically cold rotating disk at such an early cosmic epoch. Despite the small projected separation from the quasar host, we find no clear evidence of strong tidal interactions affecting the galaxy disk. This might suggest that the quasar is a satellite galaxy in the early stages of a merger. Furthermore, our spectroscopic analysis revealed a broad, blueshifted component in the CO(4-3) line profile of the quasar host, which may trace a powerful molecular outflow or kinematic disturbances induced by its interaction with the massive companion galaxy. Our findings show that rotationally supported cold disks are able to survive even in high-density environments of the early Universe.