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

Abstract We present Atacama Large Millimeter/submillimeter Array C ii 158 μ m 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 × 10 9 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 ∼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 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.