A Candidate Kiloparsec-scale Quasar Pair at $z=5.66$

Abstract

We report the discovery of a close quasar pair candidate at $z=5.66$, J2037--4537. J2037--4537 is resolved into two quasar images at the same redshift in ground-based observations. Followup spectroscopy shows significant differences in both the continuum slopes and emission line properties of the two images. The two quasar images have a projected separation of $1\farcs24$ ($7.3\text{ kpc}$ at $z=5.66$) and a redshift difference of $\Delta z\lesssim 0.01$. High-resolution images taken by {\em Hubble Space Telescope} do not detect the foreground lensing galaxy. The observational features of J2037--4537 strongly disfavor the lensing hypothesis. If J2037--4537 is a physical quasar pair, it indicates a quasar clustering signal of $\sim 10^5$ at a separation of $\sim 10$ proper kpc (pkpc), and gives the first observational constraint on the pair fraction of $z>5$ quasars, $f_{\text{pair}}(r<30\text{ pkpc})>0.3\%$. The properties of J2037--4537 are consistent with those of merger-triggered quasar pairs in hydrodynamical simulations of galaxy mergers.

8 pages, 4 figures. Accepted by ApJL; Corrected minor errors of the previous version

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