Mystery solved: discovery of extended radio emission in the merging
  galaxy cluster Abell 2146
  Abell 2146 ($z=0.232$) is a massive galaxy cluster currently undergoing a
spectacular merger in the plane of the sky with a bullet-like morphology. It
was the first system in which both the bow and upstream shock fronts were
detected at X-ray wavelengths (Mach$\sim2$), yet deep GMRT 325 MHz observations
failed to detect extended radio emission associated with the cluster as is
typically seen in such systems. We present new, multi-configuration $1-2$ GHz
Karl G. Jansky Very Large Array (VLA) observations of Abell 2146 totalling 16
hours of observations. These data reveal for the first time the presence of an
extended ($\approx850$ kpc), faint radio structure associated with Abell 2146.
The structure appears to harbour multiple components, one associated with the
upstream shock which we classify as a radio relic and one associated with the
subcluster core which is consisted as being a radio halo bounded by the bow
shock. The newly detected structures have some of the lowest radio powers
detected thus far in any cluster ($P_{\rm 1.4 GHz,
halo}=2.4\pm0.2\times10^{23}$ W Hz$^{-1}$ and $P_{\rm 1.4 GHz,
relic}=2.2\pm0.2\times10^{23}$ W Hz$^{-1}$). The flux measurement of the halo,
as well as its morphology, also suggest that the halo was recently created
($\approx0.3$ Gyr after core passage), consistent with the dynamical state of
the cluster. These observations demonstrate the capacity of the upgraded VLA to
detect extremely faint and extended radio structures. Based on these
observations, we predict that many more radio relics and radio halos in merging
clusters should be detected by future radio facilities such as the Square
Kilometre Array (SKA).
