2025ApJ...988..172H

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 $1.^{\prime} 1$. 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_* ∼ 10¹²M_⊙ galaxy. Comparisons with the TNG300 cosmological simulation puts this galaxy in a dark matter (DM) halo of M_DM ∼ 3 × 10¹³M_⊙,which will evolve into a Coma-like DM halo (M_DM ∼ 10¹⁵M_⊙) by the present day.