Quenching Massive Galaxies in a z=3.37 Protocluster

Whereas clusters are dominated by quenched galaxies in the low redshift Universe, at z>2, galaxies in protoclusters are thought to be predominantly star-forming. We present a striking counterexample to the assumed ubiquity of star-forming protoclusters with the discovery of MAGAZ3NE J095924+022537, a spectroscopically-confirmed protocluster at z=3.37 in the COSMOS field. We show that the quenched fraction of galaxies in the protocluster is in excess of the field value and appears to be mass-dependent. The most massive galaxy (M* = 2.34*10^11 Msun) in protocluster MAGAZ3NE J0959 is also quiescent and hosts an X-ray AGN, which may provide insight into how these massive galaxies quench. We discuss the possibility that we might be observing either "early mass quenching" or non-classical "environmental quenching" and suggest that protoclusters exist in a diversity of evolutionary states in the early Universe