2023ApJ...943..166A

We present a new rest-frame color-color selection method using synthetic u_s - g_s and g_s - i_s, (ugi)_s colors to identify star-forming and quiescent galaxies. Our method is similar to the widely used U - V versus V - J (UVJ) diagram. However, UVJ suffers known systematics. Spectroscopic campaigns have shown that UVJ-selected quiescent samples at z ≳ 3 include ∼10%-30% contamination from galaxies with dust-obscured star formation and strong emission lines. Moreover, at z > 3, UVJ colors are extrapolated because the rest-frame band shifts beyond the coverage of the deepest bandpasses at <5 μm (typically Spitzer/IRAC 4.5 μm or future JWST/NIRCam observations). We demonstrate that (ugi)_s offers improvements to UVJ at z > 3, and can be applied to galaxies in the JWST era. We apply (ugi)_s selection to galaxies at 0.5 < z < 6 from the (observed) 3D-HST and UltraVISTA catalogs, and to the (simulated) JAGUAR catalogs. We show that extrapolation can affect (V - J)₀ color by up to 1 mag, but changes ${({g}_{s}-{i}_{s})}_{0}$ color by ≤0.2 mag, even at z ≃ 6. While (ugi)_s-selected quiescent samples are comparable to UVJ in completeness (both achieve ∼85%-90% at z = 3-3.5), (ugi)_s reduces contamination in quiescent samples by nearly a factor of 2, from ≃35% to ≃17% at z = 3, and from ≃60% to ≃33% at z = 6. This leads to improvements in the true-to-false-positive ratio (TP/FP), where we find TP/FP ≳2.2 for (ugi)_s at z ≃ 3.5 - 6, compared to TP/FP < 1 for UVJ-selected samples. This indicates that contaminants will outnumber true quiescent galaxies in UVJ at these redshifts, while (ugi)_s will provide higher-fidelity samples.