The diffuse gamma-ray flux from clusters of galaxies

Area of Science:

• Astrophysics and Cosmology
• High-Energy Astrophysics
• Particle Astrophysics

Background:

• The origin of the diffuse gamma-ray background (DGRB) remains unknown, with potential contributions from various celestial sources.
• Previous studies have considered galaxies, active galactic nuclei, and other phenomena as sources of the DGRB.

Purpose of the Study:

• To investigate the contribution of galaxy clusters to the diffuse gamma-ray background (DGRB).
• To determine if galaxy clusters can account for the observed DGRB flux above 100 GeV.

Main Methods:

• Combined cosmological magnetohydrodynamical simulations of galaxy clusters with cosmic ray (CR) propagation using Monte Carlo simulations.
• Simulations covered a redshift range of z ≤ 5.0.
• Analyzed CR spectral indices and energy cutoffs to model gamma-ray emission.

Main Results:

• Integrated gamma-ray flux from galaxy clusters can contribute up to 100% of the DGRB observed by Fermi-LAT above 100 GeV.
• The dominant contribution comes from clusters with masses 10^13 - 10^15 Msun and redshift z ≤ 0.3.
• Specific CR spectral indices (1.5-2.5) and energy cutoffs were considered.

Conclusions:

• Galaxy clusters are a significant, potentially dominant, source of the diffuse gamma-ray background.
• Results predict the detectability of high-energy gamma rays from clusters by current and future experiments like HAWC, LHAASO, and CTA.
• This study provides a crucial piece in understanding the composition of the high-energy gamma-ray sky.