Cluster Mergers and the Origin of the ARCADE-2 Excess
  Radio observations at multiple frequencies have detected a significant
isotropic emission component between 22~MHz and 10~GHz, commonly termed the
ARCADE-2 Excess. The origin of this radio emission is unknown, as the
intensity, spectrum and isotropy of the signal are difficult to model with
either traditional astrophysical mechanisms or novel physics such as dark
matter annihilation. We posit a new model capable of explaining the key
components of the excess radio emission. Specifically, we show that the
re-acceleration of non-thermal electrons via turbulence in merging galaxy
clusters are capable of explaining the intensity, spectrum, and isotropy of the
ARCADE-2 data. We examine the parameter spaces of cluster re-acceleration,
magnetic field, and merger rate, finding that the radio excess can be
reproduced assuming reasonable assumptions for each. Finally, we point out that
future observations will definitively confirm or rule-out the contribution of
cluster mergers to the isotropic radio background.
