A Survey for High-redshift Gravitationally Lensed Quasars

Gravitationally lensed quasars are valuable objects that enable various studies in extragalactic astronomy and cosmology. Despite intensive efforts, to date only one lensed quasar at $z>5$ has been discovered, much fewer than the predictions of previous theoretical models. This discrepancy motivates us to conduct a new survey for high-redshift gravitationally lensed quasars at $z\gtrsim5$. We revisit the models of high-redshift lensed quasar population using both mock catalogs and analytical methods, finding that there should be $\sim10$ discoverable lensed quasars at $z>5$ in present-day sky surveys. Previous theoretical models overestimated the number of lensed quasars at $z\sim6$, and traditional survey strategies for high-redshift quasars are incomplete for lensed ones. We design a new candidate selection method for lensed quasars, utilizing the DESI Legacy Imaging Survey and other optical and infrared imaging surveys. The depth and deblending ability of the Legacy Survey allow discoveries of faint and small-separation lensed quasars. We select objects with both morphology and colors consistent with lensed quasars as candidates, and carry out follow-up observations to confirm the nature of these candidates. This process naturally yields close quasar pairs, a population of useful objects that exhibit observed features similar to doubly-imaged lensed quasars. In the initial stage of the survey, we have observed $\sim200$ lens candidates, among which we identify one lensed quasar and two close quasar pairs at $z\gtrsim5$. In particular, we report the discovery of J2037--4537 $(z=5.66)$, which is the most distant kpc-scale quasar pair known so far. J2037--4537 gives the first observational constraint on the pair fraction of $z>5$ quasars, $f_\text{pair}(r0.3\%$. Finally, we present the ALMA observation of J0439+1634, a lensed quasar at $z=6.52$. With the help of lensing magnification, the observation reaches an effective spatial resolution of $\sim0.8$ kpc. The host galaxy of J0439+1634 is a rotation-supported system with a regular S\'ersic profile, yet detailed analysis indicates complex structures in the quasar host galaxy. J0439+1634 is a gas-rich system that is undergoing extreme star formation processes