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.

Keywords

• Galaxy evolution
• Quasars
• Galaxy mergers
• Galaxy interactions