The core of the massive cluster merger MACS J0417.5-1154 as seen by
  VLT/MUSE
  We present a multi-wavelength analysis of the core of the massive galaxy
cluster MACS\,J0417.5-1154 ($z = 0.441$; MACS\;J0417). Our analysis takes
advantage of VLT/MUSE observations which allow the spectroscopic confirmation
of three strongly-lensed systems. One of these, nick-named \emph{The Doughnut},
consists of three complete images of a complex ring galaxy at $z = 0.8718$ and
a fourth, partial and radial image close to the Brightest Cluster Galaxy (BCG)
only discernible thanks to its strong [OII] line emission. The best-fit mass
model (rms of 0.38\arcsec) yields a two-dimensional enclosed mass of $M({\rm R
< 200\,kpc}) = (1.77\pm0.03)\times10^{14}\,\msun$ and almost perfect alignment
between the peaks of the BCG light and the dark matter of ($0.5\pm0.5$)\arcsec
. Our finding that a significant misalignment results when the radial image of
\emph{The Doughnut} is omitted serves as an important caveat for studies of
BCG-dark matter offsets in galaxy clusters. Using \emph{Chandra} data to map
the intra-cluster gas, we observe an offset between the gas and dark-matter
peaks of ($1.7\pm0.5$)\arcsec, and excellent alignment of the X-ray peak with
the location of optical emission line associated with the BCG. We interpret all
observational evidence in the framework of on-going merger activity, noting
specifically that the coincidence between the gas peak and the peak of blue
light from the BCG may be evidence of dense, cold gas leading to direct star
formation. We use the surface area $\sigma_{\mu}$ above a given magnification
factor $\mu$ as a metric to estimate the lensing power of MACS\,J0417. We
obtain $\sigma(\mu > 3) = 0.22$\,arcmin$^2$ confirming MACS\,J0417 as an
efficient gravitational lens. Finally, we discuss the differences between our
mass model and Mahler et al. (2018).
