Setting the scene for BUFFALO: A study of the matter distribution in the
  HFF galaxy cluster MACS J0416.1-2403 and its parallel field
  In the context of the BUFFALO (Beyond Ultra-deep Frontier Fields And Legacy
Observations) survey, we present a new analysis of the merging galaxy cluster
MACS\,J0416.1-2403 ($z = 0.397$) and its parallel field using the data
collected by the Hubble Frontier Fields (HFF) campaign. In this work, we
measure the surface mass density from a weak-lensing analysis, and characterise
the overall matter distribution in both the cluster and parallel fields. The
surface mass distribution derived for the parallel field shows clumpy
overdensities connected by filament-like structures elongated in the direction
of the cluster core. We also characterise the X-ray emission of the cluster,
and compare it with the lensing mass distribution. We identify five
substructures at the $>5\sigma$ level over the two fields, four of them being
in the cluster one. Furthermore, three of them are located close to the edges
of the field of view, and border issues can significantly hamper the
determination of their physical parameters. Finally, we compare our results
with the predicted subhalo distribution of one of the Hydrangea/C-EAGLE
simulated cluster. Significant differences are obtained suggesting the
simulated cluster is at a more advanced evolutionary state than
MACS\,J0416.1-2403. Our results anticipate the upcoming BUFFALO observations
that will link the two HFF fields, extending further the \emph{HST} coverage,
and thus allowing a better characterisation of the reported substructures.
