Beyond UVJ: Color Selection of Galaxies in the JWST Era

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 μ m (typically Spitzer/IRAC 4.5 μ m 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.