ALMA survey of a massive node of the Cosmic Web at $z\sim 3$. II. A dynamically cold and massive disk galaxy in the proximity of a hyperluminous quasar

A. Pensabene, S. Cantalupo, W. Wang, C. Bacchini, F. Fraternali, M. Bischetti, C. Cicone, R. Decarli, G. Pezzulli, M. Galbiati, T. Lazeyras, N. Ledos, G. Quadri, A. Travascio

First listed 2025-07-22 | Last updated 2025-07-22

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 ($\simeq0.3''$) 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 one of the densest concentrations of galaxies and AGN at cosmic noon. At the center of this structure, we identify a massive, rotationally supported disk galaxy located approximately at $\sim10\,{\rm kpc}$ projected-distance and $\sim-300\,{\rm km\,s^{-1}}$ from a hyperluminous quasar at $z=3.2510$. By accurately modeling the cold gas kinematics, we determine a galaxy dynamical mass of $2.5\times10^{11}\,{M_{\odot}}$ within the inner $\simeq 4\,{\rm kpc}$, and a high degree of rotational support of $V_{\rm rot}/σ\approx 11$. 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 reveals 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.

Short digest

High-resolution (∼0.3″) ALMA Band-3 CO(4–3) and 3‑mm continuum mapping of the MQN01 node isolates a massive companion, MQN01‑QC, ∼10 kpc and −300 km s⁻¹ from the hyperluminous quasar CTS G18.01 (z=3.251). Kinematic modeling of the cold gas yields a dynamical mass of 2.5×10^11 M⊙ within ≈4 kpc and a dynamically cold disk with Vrot/σ≈11, establishing the first quasar companion confirmed as a massive rotating disk at this epoch. Despite the proximity, the disk shows no clear tidal disturbances, while the quasar’s CO(4–3) profile exhibits a broad blueshifted wing consistent with a powerful molecular outflow or interaction. The system suggests dynamically cold disks can persist even in dense, early cosmic‑web nodes and hints the quasar may be a satellite in an incipient merger.

Key figures to inspect

• Figure 1: Compare the PSF‑subtracted JWST/NIRCam view with the ALMA 3‑mm map to verify the MQN01‑QC centroid, its projected ∼10 kpc separation from CTS G18.01, and the alignment between stellar light and dust continuum within the 0.3″ beam.
• Figure 2: Inspect the CO(4–3) line profiles—multi‑Gaussian fit for the quasar and the 3DBarolo rotating‑disk model for MQN01‑QC—to see the ∼−300 km s⁻¹ offset of the companion and the quasar’s broad blueshifted wing; check residuals to gauge model adequacy.
• Figure 3: Use the moment 0/1/2 maps to confirm an ordered velocity gradient across MQN01‑QC and low dispersions consistent with Vrot/σ≈11, and to contrast this with the higher‑σ emission near the quasar position.
• Figure 4: Examine the modified‑blackbody SED fits to assess dust temperatures and masses for the quasar and MQN01‑QC, and how assumptions on Td shift inferred continuum‑based ISM properties.