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 m 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 (M ₁₄₅₀ -23 mag), but we detect very bright CII emission bridging the 12 kpc between the two objects and extending beyond them (total luminosity L ₂₈₈ 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 10^11~M_. The dynamical masses of the two galaxies are large (9 10^10~M_ for C1 and 5 10^10~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 (M ₁₄₅₀ -26 mag), starbursting (1000~M_ yr^-1) quasar after coalescence, one of the most extreme populations in the early universe.