Presentation #123.03 in the session Evolution of Galaxies II.
A major outstanding question in galaxy evolution lies in the mechanisms behind the rapid formation and subsequent quenching of massive galaxies at early epochs. Recently, the Massive Ancient Galaxies At z > 3 NEar-infrared (MAGAZ3NE) survey spectroscopically confirmed the existence of rare ultra-massive galaxies (UMGs, M* > 1011 M⊙) at 3 < z < 4. These studies suggest that a significant fraction of UMGs experienced a period of bursty star formation followed by rapid quenching between 4 < z < 5, motivating the need for spectroscopic observation and analysis of UMGs at this crucial epoch. Using wide-field, multi-band imaging in the XMM-VIDEO and COSMOS UltraVISTA fields, we identified a sample of candidate UMGs with zphot > 4 for spectroscopic follow-up with Keck/LRIS. We will present the initial results of this survey, focusing on the spectroscopic confirmation and characterization of this rare galaxy population just ~1.4 Gyr after the Big Bang. Our results offer a new insight into the formation histories of the most massive galaxies and will provide valuable constraints for theoretical galaxy evolution models.