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We present near-infrared spectroscopic confirmations of a sample of 16photometrically-selected galaxies with stellar masses log(M_star/M_sun) > 11 atredshift z > 3 from the XMM-VIDEO and COSMOS-UltraVISTA fields usingKeck/MOSFIRE as part of the MAGAZ3NE survey. Eight of the ultra-massivegalaxies (UMGs) have specific star formation rates (sSFR) < 0.03 Gyr-1, withnegligible emission lines. Another seven UMGs show emission lines consistentwith active galactic nuclei and/or star formation, while only one UMG has sSFR> 1 Gyr-1. Model star formation histories of these galaxies describe systemsthat formed the majority of their stars in vigorous bursts of several hundredMyr duration around 4 < z < 6during which hundreds to thousands of solar masseswere formed per year. These formation ages of < 1 Gyr prior to observation areconsistent with ages derived from measurements of Dn(4000) and EW0(H\delta).Rapid quenching followed these bursty star-forming periods, generally occurringless than 350 Myr before observation, resulting in post-starburst SEDs andspectra for half the sample. The rapid formation timescales are consistent withthe extreme star formation rates observed in 4 < z < 7 dusty starburstsobserved with ALMA, suggesting that such dusty galaxies are progenitors ofthese UMGs. While such formation histories have been suggested in previousstudies, the large sample introduced here presents the most compelling evidenceyet that vigorous star formation followed by rapid quenching is almostcertainly the norm for high mass galaxies in the early universe. The UMGspresented here were selected to be brighter than Ks = 21.7 raising theintriguing possibility that even (fainter) older quiescent UMGs could exist atthis epoch.

astrophysical journal/the astrophysical journal

The Massive Ancient Galaxies At $z>3$ NEar-infrared (MAGAZ3NE) Survey:  Confirmation of Extremely Rapid Star-Formation and Quenching Timescales for  Massive Galaxies in the Early Universe