A. J. Hedge, N. Seymour, J. W. Broderick, A. Gupta, J. Afonso, L. Ighina, M. D. Lehnert, G. Noirot, S. S. Shabala, D. Stern, R. J. Turner

Abstract
We report the discovery of a z ∼ 3.9 protocluster identified from Atacama Large Millimeter/submillimeter Array Band 3 spectral scans of a bright radio source selected from the Galactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. Extended CO(4–3) and [C I] (³P₁ → ³P₀) line emission was detected in GLEAM J005332‑325630 confirming it to be a z = 3.879 powerful radio galaxy with luminosity L_{500 MHz} = 1.3 × 10²⁸ W Hz^‑1. This source is part of a sample of candidate high-redshift radio galaxies with bright radio fluxes, S_{150 MHz} > 0.1 Jy, but host galaxies with K_s(AB) ≳ 23 mag. The molecular gas associated with the radio galaxy host has two kinematically separate components, likely infalling and indicative of a recent interaction or merger with another galaxy. One 100 GHz continuum source ∼120 pkpc away is found to have both CO(4–3) and [C I] (³P₁ → ³P₀) emission lines and a further five protocluster members are identified from CO(4–3) emission alone, all at a similar redshift (Δv < 700 km s^‑1) and within a radius of 1.'1 . Using photometry from the High Acuity Widefield K-band Imager K_s band and the Dark Energy Survey g, r, i, z, and Y bands, we find this protocluster harbours a rare, optically dark, very massive M_* ∼ 10¹² M_⊙ galaxy. Comparisons with the TNG300 cosmological simulation puts this galaxy in a dark matter (DM) halo of M_DM ∼ 3 × 10¹³ M_⊙, which will evolve into a Coma-like DM halo (M_DM ∼ 10¹⁵ M_⊙) by the present day.

Keywords
Radio galaxies / Millimeter-wave spectroscopy / Molecular gas / CO line emission / High-redshift galaxy clusters / High-redshift galaxies / 1343 / 2252 / 1073 / 262 / 2007 / 734 / Astrophysics of Galaxies

The Astrophysical Journal
Volume 988, Number 2, Page 172
2025 August

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