The Diffuse Gamma-Ray Flux from Clusters of Galaxies

The origin of the diffuse gamma-ray background (DGRB), the one that remains after subtracting all individual sources from observed gamma-ray sky, is unknown. The DGRB possibly encompasses contributions from different source populations such as star-forming galaxies, starburst galaxies, active galactic nuclei, gamma-ray bursts, or galaxy clusters. Here, we combine cosmological magnetohydrodynamical simulations of clusters of galaxies with the propagation of cosmic rays (CRs) using Monte Carlo simulations, in the redshift range $z\le 5.0$, and show that the integrated gamma-ray flux from clusters can contribute up to $100\mathrm{\%}$ of the DGRB flux observed by Fermi-LAT above $100$ GeV, for CRs spectral indices $\alpha =1.5-2.5$ and energy cutoffs $E_{\text{max}}={10}^{16}-{10}^{17}$ eV. The flux is dominated by clusters with masses ${10}^{13}\lesssim M\mathrm{/}{M}_{\odot }\lesssim {10}^{15}$ and redshift $ z \lesssim 0.3$. Our results also predict the potential observation of high-energy gamma rays from clusters by experiments like the High Altitude Water Cherenkov (HAWC), the Large High Altitude Air Shower Observatory (LHAASO), and potentially the upcoming Cherenkov Telescope Array (CTA).