Discovery of two quasars at $z=5$ from the OGLE Survey

TL;DR

This paper reports the discovery of two quasars at redshifts around 5 using deep optical and mid-infrared data, highlighting their significant variability compared to lower-redshift quasars, and discusses the potential for future surveys to expand such findings.

Contribution

First identification of high-redshift quasars using combined optical and mid-infrared data with spectroscopic confirmation, demonstrating their high variability and suggesting future large-scale surveys will enhance such discoveries.

Findings

Discovered two quasars at z=5.09 and z=4.98.

High variability amplitude (~0.4 mag) observed in these quasars.

Variability amplitude is larger than in lower-redshift quasars.

Abstract

We have used deep Optical Gravitational Lensing Experiment (OGLE-IV) images ($V \lesssim 23$ mag, $I \lesssim 23$ mag at $3\sigma$) of the Magellanic System, encompassing an area of $\sim$670 deg$^2$, to perform a search for high-$z$ quasar candidates. We combined the optical OGLE data with the mid-IR Wide-field Infrared Survey Explorer (WISE) 3.4/4.6/12 $\mu$m data, and devised a multi-color selection procedure. We have identified 33 promising sources and then spectroscopically observed the two most variable ones. We report the discovery of two high-$z$ quasars, OGLE J015531-752807 at a redshift $z=5.09$ and OGLE J005907-645016 at a redshift of $z=4.98$. The variability amplitude of both quasars at the rest-frame wavelength $\sim$1300\AA\ is much larger ($\sim$0.4 mag) than other quasars ($<0.15$ mag) at the same rest-frame wavelength but lower redshifts ($2<z<4$). To verify if there exist an increased variability amplitude in high-$z$ population of quasars, simply a larger sample of such sources with a decade long (or longer) light curves is necessary, which will be enabled by the Large Synoptic Survey Telescope (LSST) providing light curves for sources 3-4 mag fainter than OGLE.