Merging Gas-rich Galaxies That Harbor Low-luminosity Twin Quasars at z = 6.05: A Promising Progenitor of the Most Luminous Quasars

We present Atacama Large Millimeter/submillimeter Array [C ii] 158 μm line and underlying far-IR continuum emission observations (0.″57 × 0.″46 resolution) toward a quasar-quasar pair system recently discovered at z = 6.05. The quasar nuclei (C1 and C2) are faint (M ₁₄₅₀ ≳ −23 mag), but we detect very bright [C ii] emission bridging the 12 kpc between the two objects and extending beyond them (total luminosity L _{[C ii]} ≃ 6 × 10⁹ L _⊙). The [C ii]-based total star formation rate of the system is ∼550 M _⊙ yr⁻¹ (the IR-based dust-obscured star formation is ∼100 M _⊙ yr⁻¹), with a [C ii]-based total gas mass of ∼10¹¹ M _⊙. The dynamical masses of the two galaxies are large (∼9 × 10¹⁰ M _⊙ for C1 and ∼5 × 10¹⁰ M _⊙ for C2). There is a smooth velocity gradient in [C ii], indicating that these quasars are a tidally interacting system. We identified a dynamically distinct, fast-[C ii] component around C1: detailed inspection of the line spectrum there reveals the presence of a broad-wing component, which we interpret as the indication of fast outflows with a velocity of ∼600 km s⁻¹. The expected mass-loading factor of the outflows, after accounting for multiphase gas, is ≳2 − 3, which is intermediate between AGN-driven and starburst-driven outflows. Hydrodynamic simulations in the literature predict that this pair will evolve to a luminous (M ₁₄₅₀ ≲ −26 mag), starbursting (≳1000 M _⊙ yr⁻¹) quasar after coalescence, one of the most extreme populations in the early Universe.