3 years ago

# Snapshots of structure formation of a Cosmic Beast: Full-scale observations and simulations of MACS J0717.5+3745.

A. Robertson, E. Medezinski, K. Umetsu, S. T. Kay, R. Massey, M. Nonino, Y. Bahé, D. Eckert, C. Dalla Vecchia, E. Jullo, D. Barnes, D. Harvey, J. Schwinn, M. Schaller, S. I. Tam, P. Natarajan, J.-P. Kneib, H. Ebeling, M. Jauzac, C. Baugh, M. Limousin

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 $1$--$4\%$. The typical growth rate and substructure infall velocity of simulated clusters suggests that MACS\,J0717 will 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.

Publisher URL: http://arxiv.org/abs/1711.01324

DOI: arXiv:1711.01324v1

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