ALMA survey of a massive node of the Cosmic Web at z ∼ 3

II. A dynamically cold and massive disk galaxy in the proximity of a hyper-luminous quasar

¹ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy
² DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 155, 2200 Copenhagen, Denmark
³ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, NL-9747 AD Groningen, The Netherlands
⁴ Dipartimento di Fisica, Università di Trieste, Sezione di Astronomia, Via G. B. Tiepolo 11, I-34131 Trieste, Italy
⁵ INAF–Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, I-34131 Trieste, Italy
⁶ Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, 0315 Oslo, Norway
⁷ INAF–Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, I-40129 Bologna, Italy

^⋆ Corresponding author: antonio.pensabene@unimib.it

Received: 26 March 2025
Accepted: 19 July 2025

Abstract

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.