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