A Census of Galaxy Constituents in a Coma Progenitor Observed at z > 3

We present a detailed census of galaxies in and around PC 217.96+32.3, a spectroscopically confirmed Coma analog at z = 3.78. Diverse galaxy types identified in the field include Lyα emitters (LAEs), massive star-forming galaxies, and ultramassive galaxies (≳ 10 ¹¹ M _o ) which may have already halted their star formation. The sky distribution of the star-forming galaxies suggests the presence of a significant overdensity (δ _SFG ≈ 8 ±2), which is spatially offset from the previously confirmed members by 3-4 Mpc to the west. Candidate quiescent and post-starburst galaxies are also found in large excess (a factor of ∼8-15 higher surface density than the field's), although their redshifts are less certain. We estimate that the total enclosed mass traced by the candidate star-forming galaxies is roughly comparable to that of PC 217.96+32.3 traced by the LAEs. We speculate that the true extent of PC 217.96+32.3 may be larger than previously known, half of which is missed by our LAE selection. Alternatively, the newly discovered overdensity may belong to another Coma progenitor not associated with PC 217.96+32.3. Expectations from theory suggest that both scenarios are equally unlikely (<1%) in the cosmic volume probed in our survey. If confirmed as a single structure, its total mass will be well in excess of Coma's, making it an exceptionally large cosmic structure rarely seen even in large cosmological simulations. Finally, we find that the protocluster galaxies follow the same star formation rate-M _∗ scaling relation as the field galaxies, suggesting that the environmental effect at z ∼ 4 is a subtle one at best for normal star-forming galaxies.