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 ALMA [CII] 158 $\mu$m line and underlying far-infrared (FIR) continuum emission observations ($0^{\prime \prime }.57\times 0^{\prime \prime }.46$ resolution) toward a quasar-quasar pair system recently discovered at $z=6.05$ (Matsuoka et al. 2024). The quasar nuclei (C1 and C2) are faint ($M_{1450}\gtrsim -23$ mag), but we detect very bright [CII] emission bridging the 12 kpc between the two objects and extending beyond them (total luminosity $L_{[\mathrm{CII}]}\simeq 6\times 10^9{L}_{\odot }$). The [CII]-based total star formation rate of the system is $\sim 550M_{\odot }$ yr${}^{-1}$ (IR-based dust-obscured SFR is $\sim 100M_{\odot }$ yr${}^{-1}$), with a [CII]-based total gas mass of $\sim 10^{11}{M}_{\odot }$. The dynamical masses of the two galaxies are large ($\sim 9\times 10^{10}{M}_{\odot }$ for C1 and $\sim 5\times 10^{10}{M}_{\odot }$ 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 there reveals the presence of a broad wing component, which we interpret as the indication of fast outflows with a velocity of $\sim 600$ km s${}^{-1}$. The expected mass loading factor of the outflows, after accounting for multiphase gas, is $\gtrsim 2-3$, which is intermediate between AGN-driven and starburst-driven outflows. Hydrodynamic simulations in the literature predicted that this pair will evolve to a luminous ($M_{1450}\lesssim -26$ mag), starbursting ($\gtrsim 1000M_{\odot }$ yr${}^{-1}$) quasar after coalescence, one of the most extreme populations in the early universe.

16 pages incl. Appendix, 4 main figures, 1 table, 2 appendix figures. Accepted for publication in the Astrophysical Journal on 2024 June 11