Gamma Rays from Cosmic-Ray Proton Scattering in AGN Jets: the
  Intra-Cluster Gas vastly outshines Dark Matter
  Active Galactic Nuclei (AGN) host powerful jets containing high-energy
electrons and protons. The astrophysical environment where AGNs and their jets
are found is characterized by large concentrations of both dark matter (DM) and
intra-cluster medium (ICM) gas. As the high-energy jet particles transverse the
DM and the ICM, elastic and inelastic scattering processes generically lead to
the production of final-state photons. As first envisioned by Bloom and Wells
(1998), and as more recently pointed out by us and others, the scattering of
electrons off of DM could lead to a potentially detectable gamma-ray signal,
with the parton-level contribution from protons offering dimmer perspectives.
Recently, Chang et al. argued that taking into account photons from
hadronization and showering, the actual photon flux is substantially increased.
Here, we point out that the proton-jets have to be highly collimated, contrary
to what predicted by simple blob-geometry jet-models sometimes employed in
these studies, otherwise they would produce a very large flux of photons from
inelastic collisions with ICM nucleons, which would outshine by many orders of
magnitude the signal from DM, for almost any reasonable ICM and DM density
profiles.
