The Diffuse Gamma-Ray Flux from Clusters of Galaxies

TL;DR

This study uses simulations to show that galaxy clusters could account for all of the diffuse gamma-ray background observed above 100 GeV, highlighting their significance as gamma-ray sources.

Contribution

It combines cosmological magnetohydrodynamical and Monte Carlo simulations to quantify the gamma-ray flux from galaxy clusters and its potential to explain the entire DGRB.

Findings

Galaxy clusters can contribute up to 100% of the DGRB above 100 GeV.

The flux is mainly from clusters with masses between 10^13 and 10^15 solar masses.

Upcoming gamma-ray observatories could detect high-energy gamma rays from clusters.

Abstract

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 $z\leq 5.0$, and show that the integrated gamma-ray flux from clusters can contribute up to $100\%$ 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 $10^{13} \lesssim M/M_{\odot} \lesssim 10^{15}$ 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).