The FENIKS Survey: Spectroscopic Confirmation of Massive Quiescent Galaxies at z > 3

Abstract

We report spectroscopic confirmation of a z = 3.757 ± 0.0011 massive quiescent galaxy with 11 hours of Keck/MOSFIRE K-band observations. This galaxy was selected from the FENIKS survey, which uses deep Gemini/Flamingos-2 Kb Kr imaging optimized for increased sensitivity to galaxies with strong Balmer/4000 Å breaks. Its rest-frame colors are consistent with a ∼1 Gyr old stellar population. This places it as one of the oldest objects at these redshifts, and challenges the notion that quiescent galaxies at z > 3 are all young post-starbursts. The star formation history suggests that its progenitor was in place by z ∼4 -7, reaching 1010.6 M » by z ∼ 5, which we have confirmed using ultra-deep medium resolution spectroscopy from JWST. These results are in line with the rapid formation and quenching timescales reported for similar massive quiescent galaxies at z > 3, implying extreme baryonic physics within the first billion years of cosmic history.