The FENIKS Survey: Spectroscopic Confirmation of Massive Quiescent Galaxies at z ∼ 3–5

The measured ages of massive, quiescent galaxies at z ∼ 3–4 imply that massive galaxies quench as early as z ∼ 6. While the number of spectroscopic confirmations of quiescent galaxies at z < 3 has increased over the years, there are only a handful at z > 3.5. We report spectroscopic redshifts of one secure ( z = 3.757) and two tentative ( z = 3.336 and z = 4.673) massive ( log ( M * / M ⊙ ) > 10.3 ) quiescent galaxies with 11 hr of Keck/MOSFIRE K -band observations. Our candidates were selected from the FLAMINGOS-2 Extragalactic Near-Infrared K -band Split (FENIKS) survey, which uses deep Gemini/Flamingos-2 K b K r imaging optimized for increased sensitivity to the characteristic red colors of galaxies at z > 3 with a strong Balmer/4000 Å break. The rest-frame UVJ and ( ugi ) s colors 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 at z > 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 H β + [O III ] 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 by z ∼ 7–11, reaching ∼10 11 M ⊙ by z ≃ 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 at z > 4 is feasible only with JWST.