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

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 evolution. Identifying close quasar pairs at z ≳ 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 observations. The quasar host galaxies exhibit tidal disturbed features in both the far-infrared continuum emission and the [C ii ] line emission, ruling out the doubly imaged lensed quasar scenario. The two quasar hosts are massive ( M _dyn ≳ 10 ^10 M _⊙ ) and star-forming (star formation rate ≳ 500 M _⊙ yr ^−1 ). The confirmation of J2037–4537 puts a lower limit on the quasar pair fraction at 5.5 < z < 6, F _pair > 1.2%, which is much higher than the quasar pair fraction at z ≲ 4. J2037–4537 is expected to form a gravitationally bound SMBH binary within ≲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.