2022ApJ...926...37M

We report the discovery of MAGAZ3NE J095924+022537, a spectroscopically confirmed protocluster at z={3.3665}_{-0.0012}^{+0.0009} around a spectroscopically confirmed UVJ-quiescent ultramassive galaxy (UMG; {M}_* ={2.34}_{-0.34}^{+0.23}×{10}^{11} {M}_☉) in the COSMOS UltraVISTA field. We present a total of 38 protocluster members (14 spectroscopic and 24 photometric), including the UMG. Notably, and in marked contrast to protoclusters previously reported at this epoch that have been found to contain predominantly star-forming members, we measure an elevated fraction of quiescent galaxies relative to the coeval field ({73.3}_{-16.9}^{+26.7} % versus {11.6}_{-4.9}^{+7.1} % for galaxies with stellar mass M _* >= 10¹¹ M _☉). This high quenched fraction provides a striking and important counterexample to the seeming ubiquitousness of star-forming galaxies in protoclusters at z > 2 and suggests, rather, that protoclusters exist in a diversity of evolutionary states in the early universe. We discuss the possibility that we might be observing either "early mass quenching" or nonclassical "environmental quenching." We also present the discovery of MAGAZ3NE J100028+023349, a second spectroscopically confirmed protocluster, at a very similar redshift of z={3.3801}_{-0.0281}^{+0.0213}. We present a total of 20 protocluster members, 12 of which are photometric and eight spectroscopic including a poststarburst UMG ({M}_*={2.95}_{-0.20}^{+0.21}×{10}^{11} {M}_☉). Protoclusters MAGAZ3NE J0959 and MAGAZ3NE J1000 are separated by 18' on the sky (35 comoving Mpc), in good agreement with predictions from simulations for the size of "Coma"-type cluster progenitors at this epoch. It is highly likely that the two UMGs are the progenitors of Brightest Cluster Galaxies seen in massive virialized clusters at lower redshift.