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

TL;DR

This paper reports the discovery of a close quasar pair at redshift 5.66, providing insights into quasar clustering and galaxy mergers in the early universe.

Contribution

It presents the first observational constraint on the pair fraction of high-redshift quasars and discusses its implications for galaxy merger models.

Findings

Quasar pair separated by 7.3 kpc at z=5.66

Pair fraction of >0.3% for separations <30 pkpc

Supports merger-triggered quasar activity in early universe

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 $\Delta z\lesssim0.01$. 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 $\sim10^5$ at a separation of $\sim10$ 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.