We present a gravitational lensing and X-ray analysis of a massive galaxy cluster and its surroundings .
The core of MACS J0717.5+3745 ( M ( R < 1 { Mpc } ) \sim 2 \times 10 ^ { 15 } { M } _ { \sun } , z = 0.54 ) is already known to contain four merging components .
We show that this is surrounded by at least seven additional substructures with masses ranging from 3.8 - 6.5 \times 10 ^ { 13 } { M } _ { \sun } , at projected radii 1.6 to 4.9 Mpc .
We compare MACS J0717 to mock lensing and X-ray observations of similarly rich clusters in cosmological simulations .
The low gas fraction of substructures predicted by simulations turns out to match our observed values of 1 – 4 \% .
Comparing our data to three similar simulated halos , we infer a typical growth rate and substructure infall velocity .
That suggests MACS J0717 could evolve into a system similar to , but more massive than , Abell 2744 by z = 0.31 , and into a \sim 10 ^ { 16 } { M } _ { \sun } supercluster by z = 0 .
The radial distribution of infalling substructure suggests that merger events are strongly episodic ; however we find that the smooth accretion of surrounding material remains the main source of mass growth even for such massive clusters .