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 \Delta \mathrm { v } = 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 \sigma - T relation , and derive a total mass of M \geq 5 \times 10 ^ { 14 } M _ { \odot } for the entire structure .
We demonstrate that the groups are gravitationally bound to one another and will merge into a single cluster with \gtrsim \frac { 1 } { 3 } 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 \sim 0 .