Gamma-ray probe of cosmic-ray pressure in galaxy clusters and cosmological implications

TL;DR

This paper explores how gamma-ray observations can constrain cosmic-ray pressure in galaxy clusters, impacting mass estimates and cosmological studies, by analyzing the capabilities of GLAST and IACTs.

Contribution

It demonstrates the potential of gamma-ray telescopes to set stringent limits on cosmic-ray pressure in galaxy clusters, improving understanding of their role in cluster physics and cosmology.

Findings

GLAST can limit cosmic-ray to thermal pressure ratio to a few percent in nearby clusters.

Stacking signals from multiple clusters can constrain cosmic-ray pressure to below 10% in distant clusters.

Future gamma-ray observations can track cosmic-ray energy density evolution, aiding cosmological tests.

Abstract

Cosmic rays produced in cluster accretion and merger shocks provide pressure to the intracluster medium (ICM) and affect the mass estimates of galaxy clusters. Although direct evidence for cosmic-ray ions in the ICM is still lacking, they produce gamma-ray emission through the decay of neutral pions produced in their collisions with ICM nucleons. We investigate the capability of the Gamma-ray Large Area Space Telescope (GLAST) and imaging atmospheric Cerenkov telescopes (IACTs) for constraining the cosmic-ray pressure contribution to the ICM. We show that GLAST can be used to place stringent upper limits, a few per cent for individual nearby rich clusters, on the ratio of pressures of the cosmic rays and thermal gas. We further show that it is possible to place tight (<~10%) constraints for distant (z <~ 0.25) clusters in the case of hard spectrum, by stacking signals from samples of known clusters. The GLAST limits could be made more precise with the constraint on the cosmic-ray spectrum potentially provided by IACTs. Future gamma-ray observations of clusters can constrain the evolution of cosmic-ray energy density, which would have important implications for cosmological tests with upcoming X-ray and Sunyaev-Zel'dovich effect cluster surveys.