A first estimate of radio halo statistics from large-scale cosmological simulation

TL;DR

This study uses cosmological simulations to estimate galaxy cluster radio halo counts for upcoming low-frequency surveys, highlighting uncertainties and potential for model discrimination through detailed imaging.

Contribution

It introduces a simulation-based approach to predict radio halo counts and explores how these counts depend on magnetic fields and cosmic ray models, providing insights for future observations.

Findings

Radio halo counts vary by a factor of two with magnetic field strength.

Fewer low-frequency radio halos are predicted than previous estimates.

Upcoming surveys may struggle to distinguish models due to uncertainties.

Abstract

We present a first estimate based on a cosmological gasdynamics simulation of galaxy cluster radio halo counts to be expected in forthcoming low-frequency radio surveys. Our estimate is based on a FLASH simulation of the LCDM model for which we have assigned radio power to clusters via a model that relates radio emissivity to cluster magnetic field strength, intracluster turbulence, and density. We vary several free parameters of this model and find that radio halo number counts vary by up to a factor of two for average magnetic fields ranging from 0.2 to 3.1 uG. However, we predict significantly fewer low-frequency radio halos than expected from previous semi-analytic estimates, although this discrepancy could be explained by frequency-dependent radio halo probabilities as predicted in reacceleration models. We find that upcoming surveys will have difficulty in distinguishing models because of large uncertainties and low number counts. Additionally, according to our modeling we find that expected number counts can be degenerate with both reacceleration and hadronic secondary models of cosmic ray generation. We find that relations between radio power and mass and X-ray luminosity may be used to distinguish models, and by building mock radio sky maps we demonstrate that surveys such as LOFAR may have sufficient resolution and sensitivity to break this model degeneracy by imaging many individual clusters.