The G − R vs. I − Z color selection criterion for z ∼ 5 QSO candidates in the HSC-Udeep survey. The black dots are all the objects with magnitude magZ ≤ 20.0, and the red asterisks show the known QSOs with 4.5 ≤ z _spec ≤ 5.2 drawn from SDSS DR17 (Abdurro’uf et al. 2022). The dark-green open pentagons are the known very bright QSOs with 4.5 ≤ z _spec ≤ 5.0 from the QUBRICS survey (Grazian et al. 2022), for which synthetic HSC photometry is derived starting from the observed SkyMapper photometry through SED fitting at their fixed spectroscopic redshift. The big blue circles are the three confirmed QSOs with 4.5 ≤ z _spec ≤ 5.2 in the HSC-Udeep area, while the big dark-green circles indicate known QSOs at 3.5 < z _spec < 4.5. The cyan circle is a confirmed low-redshift object. The green lines indicate the color criteria for selecting bona fide z ∼ 5 QSO candidates, while the green squares are the QSO candidates identified in the HSC-Udeep region. Bona fide stars are identified through accurate measurements of parallaxes and proper motions by Gaia DR3 and are marked by magenta stars. The gray tracks are the simulated colors of synthetic QSOs in the redshift range 4.5 < z < 5.2.