Emergence of an Ultrared, Ultramassive Galaxy Cluster Core atz=4

Recent simulations and observations of massive galaxy cluster evolution predict that the majority of stellar mass buildup happens within cluster members byz = 2, before cluster virialization. Protoclusters rich with dusty, star-forming galaxies (DSFGs) atz > 3 are the favored candidate progenitors for these massive galaxy clusters atz similar to 0. We present here the first study analyzing stellar emission along with cold dust and gas continuum emission in a spectroscopically confirmedz = 4.002 protocluster core rich with DSFGs, the Distant Red Core (DRC). We combine new Hubble Space Telescope and Spitzer data with existing Gemini, Herschel, and Atacama Large Millimeter/submillimeter Array observations to derive individual galaxy-level properties and compare them to coeval field and other protocluster galaxies. All of the protocluster members are massive (>10(10)M), but not significantly more so than their coeval field counterparts. Within uncertainty, all are nearly indistinguishable from galaxies on the star-forming versus stellar mass main-sequence relationship and the star formation efficiency plane. Assuming no future major influx of fresh gas, we estimate that these gaseous DSFGs will deplete their gas reservoirs in similar to 300 Myr, becoming the massive quiescent ellipticals dominating cluster cores byz similar to 3. Using various methodologies, we derive a totalz = 4 halo mass of similar to 10(14)Mand estimate that the DRC will evolve to become an ultramassive cluster core of mass greater than or similar to 10(15)Mbyz = 0.