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, z=4.673) massive (log(M_∗/M_⊙) > 10.3) quiescent galaxies with 11 hours of Keck/MOSFIRE K-band observations. Our candidates were selected from the 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 strong Balmer/4000 Å breaks. The rest-frame UVJ and (ugi)_s colors of 3/4 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 AGN (z = 3.594) with strong, redshifted Hβ+[O III] emission with a velocity offset >1000 km/s, 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 only feasible with JWST.