Probing cluster environments of blazars through gamma-gamma absorption

TL;DR

This paper investigates how the environments of galaxy clusters around blazars affect very high energy gamma-ray absorption and cascade signatures, finding significant absorption only with multiple luminous galaxies at high redshifts.

Contribution

It demonstrates that intracluster light and individual galaxies do not cause substantial gamma-gamma absorption, but multiple luminous galaxies at high redshift could induce notable effects.

Findings

Single galaxy radiation fields do not cause significant gamma-ray absorption.

Multiple luminous galaxies can cause gamma-gamma absorption at high redshifts.

Expected cascade signatures are below Fermi's detection sensitivity.

Abstract

Most blazars are known to be hosted in giant elliptic galaxies, but their cluster environments have not been thoroughly investigated. Cluster environments may contain radiation fields of low-energy photons created by nearby galaxies and/or stars in the intracluster medium that produce diffuse intracluster light. These radiation fields may absorb very high energy gamma rays ($E\gtrsim100$ GeV; VHE) and trigger pair cascades with further production of subsequent generations of gamma rays with lower energies via inverse Compton scattering on surrounding radiation fields leaving a characteristic imprint in the observed spectral shape. The change of the spectral shape of the blazar reflects the properties of its ambient medium. We show, however, that neither intracluster light nor the radiation field of an individual nearby galaxy can cause substantial gamma-gamma absorption. Substantial gamma-gamma absorption is possible only in the case of multiple, $\gtrsim5$, luminous nearby galaxies. This situation is not found in the local Universe, but may be possible at larger redshifts ($z\gtrsim2$). Since VHE gamma rays from such distances are expected to be strongly absorbed by the extragalactic background light, we consider possible signatures of gamma-ray induced pair cascades by calculating the expected GeV flux which appears to be below the Fermi sensitivity even for $\sim10$ nearby galaxies.