Hubble Frontier Fields: The Geometry and Dynamics of the Massive Galaxy
  Cluster Merger MACSJ0416.1-2403
  We use a joint optical/X-ray analysis to constrain the geometry and history
of the ongoing merging event in the massive galaxy cluster MACSJ0416.1-2403
(z=0.397). Our investigation of cluster substructure rests primarily on a
combined strong- and weak-lensing mass reconstruction based on the deep,
high-resolution images obtained for the Hubble Frontier Fields initiative. To
reveal the system's dynamics, we complement this lensing analysis with a study
of the intra-cluster gas using shallow Chandra data, and a three-dimensional
model of the distribution and motions of cluster galaxies derived from over 100
spectroscopic redshifts. The multi-scale grid model obtained from our combined
lensing analysis extends the high-precision strong-lensing mass reconstruction
recently performed to cluster-centric distances of almost 1 Mpc. Our analysis
detects the two well known mass concentrations in the cluster core. A
pronounced offset between collisional and collisionless matter is only observed
for the SW cluster component, while excellent alignment is found for the NE
cluster. Both the lensing analysis and the distribution of cluster light
strongly suggest the presence of a third massive structure, almost 2 arcmin SW
of the cluster centre. Since no X-ray emission is detected in this region, we
conclude that this structure is non-virialised and speculate that it might be
part of a large-scale filament almost aligned with our line of sight. Combining
all evidence from the distribution of dark and luminous matter, we propose two
alternative scenarios for the trajectories of the components of
MACSJ0416.1-2403. Upcoming deep X-ray observations that allow the detection of
shock fronts, cold cores, and sloshing gas (all key diagnostics for studies of
cluster collisions) will allow us to test, and distinguish between these two
scenarios.
