Unobscured QSO cand. in the southern sky with KS4

The KS4 data covers ~2500deg2 sky area, reaching 5{sigma} 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 72964 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.