Merging gas-rich galaxies that harbor low-luminosity twin quasars at z = 6.05: a promising progenitor of the most luminous quasars

We present ALMA [CII] 158 μm line and underlying far-infrared continuum emission observations (0''.57 × 0''.46 resolution) toward a quasar-quasar pair system recently discovered at $z = 6.05$ (Matsuoka et al. 2024). The quasar nuclei (C1 and C2) are very faint (M₁₄₅₀ -23 mag), but we detect very bright [CII] emission bridging the 12 kpc between the two objects and extending beyond them (total luminosity L[CII] 6 × 10⁹~L_). The total star formation rate of the system is ~ 100 to 550 M_ yr⁻¹, with a [CII]-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 [CII], indicating that these quasars are a tidally interacting system. We identified a dynamically distinct, fast [CII] component around C1. Detailed inspection of the line spectrum 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, would be 3-8, as expected for an AGN-driven outflow. Hydrodynamic simulations in the literature predicted that this pair will evolve to a luminous (M₁₄₅₀ -26 mag), starbursting (1000~M_ yr⁻¹) quasar after coalescence, one of the most extreme populations in the early universe.