Growing a ‘cosmic beast’ : observations and simulations of MACS J0717.5+3745.

Abstract

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 × 1015 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 × 1013 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 ∼ 1016 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.