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 (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_ [CII]≃ 6 × 10^9 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.