Growing a `Cosmic Beast': Observations and Simulations of MACS J0717.5+3745

TL;DR

This paper combines gravitational lensing and X-ray observations with simulations to analyze the complex structure and growth of the massive galaxy cluster MACS J0717.5+3745, revealing detailed substructure and evolution insights.

Contribution

It provides the first detailed comparison of observed substructures in MACS J0717 with cosmological simulations, highlighting the cluster's growth and infall dynamics.

Findings

Observed substructures match simulation predictions of low gas fractions.

Cluster is likely to evolve into a supercluster by z=0.

Merger events are episodic, with smooth accretion dominating mass growth.

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 ($M(R<1\,{\rm Mpc})\sim$\,$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.