Kavli Affiliate: Robert Simcoe
| Summary:
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 $zgtrsim4$ 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 $(M_textdyngtrsim10^10M_odot)$ and star-forming (SFR $gtrsim500 M_odot~ mathrmyr^-1$). The confirmation of J2037–4537 puts a lower limit on the quasar pair fraction at $5.5<z<6$, $F_textpair>1.2%$, which is much higher than the quasar pair fraction at $zlesssim4$. J2037–4537 is expected to form a gravitationally-bound SMBH binary within $lesssim2$ 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.
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