Frontier Fields Clusters: Chandra and JVLA View of the Pre-Merging
  Cluster MACS J0416.1-2403
  Merging galaxy clusters leave long-lasting signatures on the baryonic and
non-baryonic cluster constituents, including shock fronts, cold fronts, X-ray
substructure, radio halos, and offsets between the dark matter and the gas
components. Using observations from Chandra, the Jansky Very Large Array, the
Giant Metrewave Radio Telescope, and the Hubble Space Telescope, we present a
multiwavelength analysis of the merging Frontier Fields cluster MACS
J0416.1-2403 (z=0.396), which consists of a NE and a SW subclusters whose cores
are separated on the sky by ~250 kpc. We find that the NE subcluster has a
compact core and hosts an X-ray cavity, yet it is not a cool core.
Approximately 450 kpc south-south west of the SW subcluster, we detect a
density discontinuity that corresponds to a compression factor of ~1.5. The
discontinuity was most likely caused by the interaction of the SW subcluster
with a less massive structure detected in the lensing maps SW of the
subcluster's center. For both the NE and the SW subclusters, the dark matter
and the gas components are well-aligned, suggesting that MACS J0416.1-2403 is a
pre-merging system. The cluster also hosts a radio halo, which is unusual for a
pre-merging system. The halo has a 1.4 GHz power of (1.06 +/- 0.09) x 10^{24} W
Hz^{-1}, which is somewhat lower than expected based on the X-ray luminosity of
the cluster. We suggest that we are either witnessing the birth of a radio
halo, or have discovered a rare ultra-steep spectrum halo.
