Radio Halos From Simulations And Hadronic Models I: The Coma cluster

TL;DR

This study uses cosmological MHD simulations to predict radio and gamma-ray emissions from the Coma cluster, finding hadronic models insufficient to explain observed radio profiles but consistent gamma-ray flux predictions.

Contribution

It introduces a simulation-based approach to model radio halos and gamma-ray flux in galaxy clusters, testing hadronic models against observations.

Findings

Radio halo radial profile requires increasing cosmic-ray proton energy fraction with radius.

Hadronic models cannot reproduce the spectral steepening of the Coma radio halo.

Gamma-ray flux predictions are below current detection limits but could be observed soon.

Abstract

We use the results from a constrained, cosmological MHD simulation of the Local Universe to predict the radio halo and the gamma-ray flux from the Coma cluster and compare it to current observations. The simulated magnetic field within the Coma cluster is the result of turbulent amplification of the magnetic field during build-up of the cluster. The magnetic seed field originates from star-burst driven, galactic outflows. The synchrotron emission is calculated assuming a hadronic model. We follow four approaches with different distributions for the cosmic-ray proton (CRp) population within galaxy clusters. The radial profile the radio halo can only be reproduced with a radially increasing energy fraction within the cosmic ray proton population, reaching $>$100% of the thermal energy content at $\approx$ 1Mpc, e.g. the edge of the radio emitting region. Additionally the spectral steepening of the observed radio halo in Coma cannot be reproduced, even when accounting for the negative flux from the thermal SZ effect at high frequencies. Therefore the hadronic models are disfavored from present analysis. The emission of $\gamma$-rays expected from our simulated coma is still below the current observational limits (by a factor of $\sim$6) but would be detectable in the near future.