Testing Secondary Models for the Origin of Radio Mini-Halos in Galaxy Clusters

Abstract

We present an MHD simulation of the emergence of a radio minihalo in a galaxy cluster core in a "secondary" model, where the source of the synchrotron-emitting electrons is hadronic interactions between cosmic-ray protons with the thermal intracluster gas, an alternative to the "reacceleration model" where the cosmic ray electrons are reaccelerated by turbulence induced by core sloshing, which we discussed in an earlier work. We follow the evolution of cosmic-ray electron spectra and their radio emission using passive tracer particles, taking into account the time-dependent injection of electrons from hadronic interactions and their energy losses. We find that secondary electrons in a sloshing cluster core can generate diffuse synchrotron emission with luminosity and extent similar to observed radio minihalos. However, we also find important differences with our previous work. We find that the drop in radio emission at cold fronts is less prominent than that in our reacceleration-based simulations, indicating that in this flavor of the secondary model the emission is more spatially extended than in some observed minihalos. We also explore the effect of rapid changes in the magnetic field on the radio spectrum. While the resulting spectra in some regions are steeper than expected from stationary conditions, the change is marginal, with differences in the synchrotron spectral index of  a D 0.15-0.25, depending on the frequency band. This is a much narrower range than claimed in the best-observed minihalos and produced in the reacceleration model. Our results provide important suggestions to constrain these models with future observations.

How unusual is the cool-core radio halo cluster CL1821+643?

“…According to recent MHD simulations the magnetic field structure in the cold-fronts can contain the mini-halo sizes to within the cold-front boundaries leading to a sharp fall in the surface brightness in the direction transverse to the coldfronts; however allows further expansion in a direction parallel to the cold fronts (ZuHone et al 2013(ZuHone et al , 2015. In case of CL1821+643 radio halo is about 450 kpc in width in the direction transverse to the outer cold 'swirl' (Fig.…”

“…The origin of mini-haloes has been proposed to be a combination of relativistic electrons produced in hadronic in-⋆ E-mail: ruta@ncra.tifr.res.in teractions and reaccelerated via MHD turbulence (e. g. Gitti, Brunetti & Setti 2002;ZuHone et al 2013ZuHone et al , 2015.…”

Diffuse Radio Emission from Galaxy Clusters

“…Similar to giant radio halos, hadronic (e.g., Pfrommer and Enßlin 2004) or turbulent reacceleration models (Gitti et al 2002) been invoked to explain the presence of the CR synchrotron emitting electrons. Unlike giant radio halos, where the turbulence is induced by major cluster mergers, mini-halos would trace turbulence in the cluster cores generated by gas sloshing (ZuHone et al 2013(ZuHone et al , 2015. The central AGN is a likely candidate for the source of the fossil electrons that are re-accelerated (e.g., Fujita et al 2007).…”

First results from the IllustrisTNG simulations: radio haloes and magnetic fields

“…Another limitation is that to reach high resolution, these calculations use the so-called zoom-in technique (Hahn & Abel 2011), simulating one object at a time. For instance, ZuHone et al (2015) simulate the emergence of a radio mini halo by following the evolution and transport of cosmic rays, through passive tracer particles, in a simulation of cluster formation. While they include a detailed model for the cosmic rays evolution, the cosmological context is missing, together with physics of galaxy forma- Figure 1.…”