Gamma Rays from Cosmic-Ray Proton Scattering in AGN Jets: the Intra-Cluster Gas vastly outshines Dark Matter

TL;DR

This paper argues that gamma-ray signals from proton scattering in AGN jets are overshadowed by emissions from intra-cluster gas unless jets are highly collimated, challenging previous models predicting detectable dark matter signals.

Contribution

It highlights the importance of jet collimation in gamma-ray emission models, showing that uncollimated jets produce signals dominated by intra-cluster gas rather than dark matter.

Findings

Proton scattering in AGN jets is overshadowed by intra-cluster gas emissions.

Highly collimated jets are necessary to detect dark matter signals.

Inelastic collisions produce gamma-ray flux much larger than dark matter contributions.

Abstract

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.