2026ApJ...998..191S

Observations of extreme starbursts, often located in the cores of protoclusters, challenge the classical bottom-up galaxy formation paradigm. Giant elliptical galaxies at z = 0 must have assembled rapidly, possibly within few 100 Myr through an extreme growth phase at high-redshift, characterized by elevated star formation rates of several thousand solar masses per year distributed over concurrent, gas-rich mergers. We present a novel view of the z = 4.3 protocluster core SPT2349-56 from sensitive multicycle Atacama Large Millimeter/submillimeter Array dust continuum and [C II]158μm line observations. Distributed across 60 kpc, a highly structured gas reservoir with a line luminosity of L_[C II]_ = 3.0 ± 0.2 × 10⁹L_⊙ and an inferred cold gas mass of M_gas = 8.9 ± 0.7 × 10⁹M_⊙ is found surrounding the central massive galaxy triplet. Like “beads on a string,” the newly discovered [C II] streamers fragment into a few kiloparsec-spaced and turbulent clumps that have a similar column density as local Universe spiral galaxy arms at Σ_gas = 20-60 M_⊙ pc^–2. For a dust temperature of 30 K, the [C II] emission from the ejected clumps carries ≳3% of the far-IR luminosity, translating into an exceptionally low mass-to-light ratio of α_[CII] = 2.95 ± 0.3 M_⊙${L}_{\odot }^{-1}$, indicative of shock-heated molecular gas. In phase space, about half of the galaxies in the protocluster core populate the same caustic as the [C II] streamers (r/r_vir × ∣Δv∣/σ_vir ≈ 0.1), suggesting angular momentum dissipation via tidal ejection while the brightest cluster galaxy (BCG) is assembling. Our findings provide new evidence for the importance of tidal ejections of [C II]-bright, shocked material following multiple major mergers that might represent a landmark phase in the z ≳ 4 coevolution of BCGs with their hot, metal enriched atmospheres.