Jauzac, M.M.JauzacEckert, D.D.EckertSchaller, M.M.SchallerSchwinn, J.J.SchwinnMassey, R.R.MasseyBahé, Y.Y.BahéBaugh, C.C.BaughBarnes, D.D.BarnesDalla Vecchia, C.C.Dalla VecchiaEbeling, H.H.EbelingHarvey, D.D.HarveyJullo, E.E.JulloKay, S. T.S. T.KayKneib, J. -P.J. -P.KneibLimousin, M.M.LimousinMedezinski, E.E.MedezinskiNatarajan, P.P.NatarajanNONINO, MarioMarioNONINORobertson, A.A.RobertsonTam, S. I.S. I.TamUmetsu, K.K.Umetsu2020-10-082020-10-0820180035-8711http://hdl.handle.net/20.500.12386/27680MN acknowledges PRIN INAF 2014 1.05.01.94.02.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)∼ 2 × 10^{15} M_{☉ }, 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 3.8{-}6.5× 10^{13} M_{☉}, at projected radii 1.6-4.9 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{{ per cent}}. Comparing our data to three similar simulated haloes, 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 ∼ 10^{16} M_{☉} 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.STAMPAenArticle10.1093/mnras/sty23662-s2.0-85054277185000450984100005https://academic.oup.com/mnras/article/481/3/2901/50883682018MNRAS.481.2901JFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation