Exploring Unobscured Quasi-stellar Objects in the Southern Hemisphere with KS4

We present a catalog of unobscured quasi-stellar object (QSO) candidates in the Southern Hemisphere from the early interim data of the KMTNet Synoptic Survey of Southern Sky (KS4). The KS4 data covers ∼2500 deg² sky area, reaching 5σ detection limits of ∼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 distribution 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 spectrophotometric surveys covering the southern sky will enable us to discern their true nature and enhance our understanding of QSO populations in the Southern Hemisphere.