A Close Quasar Pair in a Massive Galaxy Merger at $z=5.7$

Abstract

Close quasar pairs are rare products of galaxy mergers in which both supermassive black holes (SMBHs) are actively accreting, offering strong constraints on merger-driven active galactic nuclei (AGN) evolution. Identifying close quasar pairs at $z\gtrsim 4$ is challenging due to the declining quasar number density in the early Universe. Here we report the confirmation of a close quasar pair at $z=5.7$, J2037--4537, utilizing high-resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations. The quasar host galaxies exhibit tidal disturbed features in both the far-infrared continuum emission and the {\cii} line emission, ruling out the doubly-imaged lensed quasar scenario. The two quasar hosts are massive $(M_{\text{dyn}}\gtrsim 10^{10}{M}_{\odot })$ and star-forming (SFR $\gtrsim 500M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$). The confirmation of J2037--4537 puts a lower limit on the quasar pair fraction at $5.5<z<6$, $F_{\text{pair}}>1.2\%$, which is much higher than the quasar pair fraction at $z\lesssim 4$. J2037--4537 is expected to form a gravitationally-bound SMBH binary within $\lesssim 2$ Gyr. The elevated quasar pair fraction at $z>5.5$, as indicated by J2037--4537, likely contributes to the high gravitational-wave background reported by recent Pulsar Timing Array experiments.

10 pages, accepted by ApJL