Galaxy cluster assembly at z = 0.37

We present X-ray and spectroscopic confirmation of a cluster assembling from multiple, distinct galaxy groups at z = 0.371. Initially detected in the Las Campanas Distant Cluster Survey, the structure contains at least four X-ray-detected groups that lie within a maximum projected separation of 4 Mpc and within Δv = 550 km s^-1 of one another. Using Chandra imaging and wide-field optical spectroscopy, we show that the individual groups lie on the local σ-T relation and derive a total mass of M ≥ 5 × 10¹⁴ M_⊙ for the entire structure. We demonstrate that the groups are gravitationally bound to one another and will merge into a single cluster with about one-third or more the mass of Coma. We also find that although the cluster is in the process of forming, the individual groups already have a higher fraction of passive members than the field. This result indicates that galaxy evolution on group scales is key to developing the early-type galaxies that dominate the cluster population by z ∼ 0.