Diffuse Pionic Gamma-Ray Emission from Large Scale Structures in the FERMI Era

TL;DR

This paper models gamma-ray emission from large-scale structure shocks to evaluate their contribution to the diffuse gamma-ray background, suggesting they could be significant at high energies and guiding future observations.

Contribution

It introduces a new model of gamma-ray emission from structure formation shocks calibrated with Fermi-LAT data, highlighting their potential role in the EGRB.

Findings

Large-scale shocks could significantly contribute to the EGRB at high energies.

Current limits from Fermi-LAT are weak but allow for models matching the observed background.

Future gamma-ray detections of clusters could refine these models and improve understanding of structure formation shocks.

Abstract

For more than a decade now the complete origin of the diffuse gamma-ray emission background (EGRB) has been unknown. Major components like unresolved star-forming galaxies (making <50% of the EGRB) and blazars (<23%), have failed to explain the entire background observed by \emph{Fermi}. Another, though subdominant, contribution is expected to come from the process of large-scale structure formation. The growth of structures is accompanied by accretion and merger shocks, which would, with at least some magnetic field present, give rise to a population of structure-formation cosmic rays (SFCRs). Though expected, this cosmic-ray population is still hypothetical and only very weak limits have been placed to their contribution to the EGRB. The most promising insight into SFCRs was expected to come from \emph{Fermi}-LAT observations of clusters of galaxies, however, only upper limits and no detection have been placed. Here, we build a model of gamma-ray emission from large-scale accretion shocks implementing a source evolution calibrated with the \emph{Fermi}-LAT cluster observation limits. Though our limits to the SFCR gamma-ray emission are weak (above the observed EGRB) in some case, in others, some of our models can provide a good fit to the observed EGRB. More importantly, we show that these large-scale shocks could still give an important contribution to the EGRB, especially at high energies. Future detections of cluster gamma-ray emission would help place tighter constraints on our models and give us a better insight into large-scale shocks forming around them.