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Abstract The measured ages of massive, quiescent galaxies atz∼ 3–4 imply that massive galaxies quench as early asz∼ 6. While the number of spectroscopic confirmations of quiescent galaxies atz< 3 has increased over the years, there are only a handful atz> 3.5. We report spectroscopic redshifts of one secure (z= 3.757) and two tentative (z= 3.336 andz= 4.673) massive ( $\log \left(M_{*}/M_{\odot }\right)>10.3$) quiescent galaxies with 11 hr of Keck/MOSFIREK-band observations. Our candidates were selected from the FLAMINGOS-2 Extragalactic Near-InfraredK-band Split (FENIKS) survey, which uses deep Gemini/Flamingos-2K_bK_rimaging optimized for increased sensitivity to the characteristic red colors of galaxies atz> 3 with a strong Balmer/4000 Å break. The rest-frameUVJand (ugi)_scolors of three out of four quiescent candidates are consistent with 1–2 Gyr old stellar populations. This places these galaxies as the oldest objects at these redshifts, and challenges the notion that quiescent galaxies atz> 3 are all recently quenched, post-starburst galaxies. Our spectroscopy shows that the other quiescent-galaxy candidate is a broad-line active galactic nucleus (z= 3.594) with strong, redshifted Hβ+ [OIII] emission with a velocity offset > 1000 km s⁻¹, indicative of a powerful outflow. The star formation history of our highest redshift candidate suggests that its progenitor was already in place byz∼ 7–11, reaching ∼10¹¹M_⊙byz≃ 8. These observations reveal the limit of what is possible with deep near-infrared photometry and targeted spectroscopy from the ground and demonstrate that secure spectroscopic confirmation of quiescent galaxies atz> 4 is feasible only with JWST. 

Abstract We present the construction of a deep multiwavelength point-spread-function-matched photometric catalog in the Ultra-Deep Survey (UDS) field following the final UKIDSS UDS release. The catalog includes photometry in 24 filters, from the MegaCam-uS0.38μm band to the Spitzer-IRAC 8μm band, over ∼0.9 deg²and with a 5σdepth of 25.3 AB in theK-band detection image. The catalog, containing ≈188,564 (136,235) galaxies at 0.2 <z< 8.0 with stellar mass $\log \left(M_{*}/M_{\odot }\right)>8$andK-band total magnitudeK< 25.2 (24.3) AB, enables a range of extragalactic studies. We also provide photometric redshifts, corresponding redshift probability distributions, and rest-frame absolute magnitudes and colors derived using the template-fitting codeeazy-py. Photometric redshift errors are less than 3%−4% atz< 4 across the full brightness range in theKband and stellar mass range $8<\log \left(M_{*}/M_{\odot }\right)<12$. Stellar population properties (e.g., stellar mass, star formation rate, dust extinction) are derived from the modeling of the spectral energy distributions using the codesFASTand Dense Basis. 

We present a new rest-frame color–color selection method using synthetic us − gs and gs − is, (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.