Simulating cosmic rays in clusters of galaxies - III. Non-thermal scaling relations and comparison to observations

TL;DR

This paper uses high-resolution simulations to predict non-thermal emission scaling relations in galaxy clusters, comparing them with observations to test models of cosmic ray physics and magnetic fields.

Contribution

It introduces self-consistent cosmic ray physics in cluster simulations and calibrates magnetic fields with Faraday rotation, providing predictions for multi-wavelength observables and their observational prospects.

Findings

Synchrotron emission matches observed radio halos well.

Predicted gamma-ray fluxes suggest about ten clusters detectable by GLAST.

Inverse Compton X-ray emission is much lower than observed in some clusters.

Abstract

Complementary views of galaxy clusters in the radio synchrotron, hard X-ray inverse Compton, and high-energy gamma-ray regimes are critical in calibrating them as high-precision cosmological probes. We present predictions for scaling relations between cluster mass and these non-thermal observables. To this end, we use high-resolution simulations of a sample of galaxy clusters spanning a mass range of almost two orders of magnitudes, and follow self-consistent cosmic ray physics on top of the radiative hydrodynamics. Calibrating the magnetic fields of our model with Faraday rotation measurements (RM), the synchrotron emission of our relativistic electron populations matches the radio synchrotron luminosities and morphologies of observed giant radio halos and mini-halos surprisingly well. Using the complete sample of the brightest X-ray clusters observed by ROSAT in combination with our gamma-ray scaling relation, we predict GLAST will detect about ten clusters allowing for Eddington bias due to the scatter in the scaling relation. The brightest gamma-ray clusters are Ophiuchus, Fornax, Coma, A3627, Perseus, and Centaurus. We provide an absolute lower flux limit for the gamma-ray emission of Coma in the hadronic model which can be made tighter for magnetic field values derived from RM values to match the GLAST sensitivity, providing thus a unique test for the possible hadronic origin of radio halos. Our predicted hard X-ray emission, due to inverse Compton emission of shock accelerated and hadronically produced relativistic electrons, falls short of the detections in Coma and Perseus by a factor of 50. This casts doubts on the inverse Compton interpretation and reinforces the known discrepancy of magnetic field estimates from Faraday RM values and those obtained by combining synchrotron and inverse Compton emission. [abridged]