Exploring Unobscured QSOs in the Southern Hemisphere with KS4

TL;DR

This paper introduces a new catalog of nearly 73,000 unobscured QSOs in the southern hemisphere, identified through combined optical and infrared data, demonstrating high recovery rates and comparable counts to northern surveys.

Contribution

The study presents a novel selection method for unobscured QSOs using multi-wavelength data, resulting in a large, effective catalog for the southern sky.

Findings

Catalog contains 72,964 QSO candidates with high purity.

87% recovery rate for spectroscopically confirmed QSOs at z<2.

Number counts are comparable to northern sky surveys.

Abstract

We present a catalog of unobscured QSO candidates in the southern hemisphere from the early interim data of the KMTNet Synoptic Survey of Southern Sky (KS4). The KS4 data covers $\sim2500\,{\rm deg}^{2}$ sky area, reaching 5$\sigma$ detection limits of $\sim$22.1-22.7 AB mag in the $BVRI$ bands. Combining this with available infrared photometric data from the surveys covering the southern sky, we select the unobscured QSO candidates based on their colors and spectral energy distributions (SEDs) fitting results. The final catalog contains 72,964 unobscured QSO candidates, of which only 0.4% are previously identified as QSOs based on spectroscopic observations. Our selection method achieves an 87% recovery rate for spectroscopically confirmed bright QSOs at $z<2$ within the KS4 survey area. In addition, the number count of our candidates is comparable to that of spectroscopically confirmed QSOs from the Sloan Digital Sky Survey in the northern sky. These demonstrate that our approach is effective in searching for unobscured QSOs in the southern sky. Future spectro-photometric surveys covering the southern sky will enable us to discern their true nature and enhance our understanding of QSO populations in the southern hemisphere.