Growing a `Cosmic Beast': Observations and Simulations of MACS
  J0717.5+3745
  We present a gravitational lensing and X-ray analysis of a massive galaxy
cluster and its surroundings. The core of MACS\,J0717.5+3745 ($M(R<1\,{\rm
Mpc})\sim$\,$2$$\times$$10^{15}\,\msun$, $z$=$0.54$) is already known to
contain four merging components. We show that this is surrounded by at least
seven additional substructures with masses ranging from
$3.8-6.5\times10^{13}\,\msun$, 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 $\sim$\,$10^{16}\,\msun$ 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.
