On the physics of radio haloes in galaxy clusters: scaling relations and luminosity functions

Abstract

The underlying physics of giant and mini radio halos in galaxy clusters is still an open question. We find that mini halos (such as in Perseus and Ophiuchus) can be explained by radio-emitting electrons that are generated in hadronic cosmic ray (CR) interactions with protons of the intracluster medium. By contrast, the hadronic model either fails to explain the extended emission of giant radio halos (as in Coma at low frequencies) or would require a flat CR profile, which can be realized through outward streaming and diffusion of CRs (in Coma and A2163 at 1.4 GHz). We suggest that a second, leptonic component could be responsible for the missing flux in the outer parts of giant halos within a new hybrid scenario and we describe its possible observational consequences. To study the hadronic emission component of the radio halo population statistically, we use a cosmological mock galaxy cluster catalog built from the Multi-Dark simulation. Because of the properties of CR streaming and the different scalings of the X-ray luminosity (L X ) and the Sunyaev-Zel'dovich flux (Y ) with gas density, our model can simultaneously reproduce the observed bimodality of radio-loud and radio-quiet clusters at the same L X as well as the unimodal distribution of radio-halo luminosity versus Y ; thereby suggesting a physical solution to this apparent contradiction. We predict radio halo emission down to the mass scale of galaxy groups, which highlights the unique prospects for low-frequency radio surveys (such as the LOFAR Tier 1 survey) to increase the number of detected radio halos by at least an order of magnitude.

Cosmic Rays in Galaxy Clusters and Their Nonthermal Emission

“…In addition a number of authors have identified the very broad (flat) spatial profiles of giant radio halos as a potential challenge for a purely hadronic origin of these sources, because in some cases a large, or radially increasing, energy density of CRp and/or magnetic field is required to generate the observed radio emission from radially distant, low target-ICM density, regions. [255][256][257][258][259]274 Potential constraints on the thermal -non-thermal connection and on the physics of radio halos also come from possible trends between the local synchrotron spectral indices and ICM temperatures. 38 Still, these relationships are only hints at the full picture.…”

“…Consequently Mpc-scale radio emission should be generated at some level in "all" clusters with luminosities that should vary depending on the cluster dynamical histories (e.g., accretion shock histories) and/or magnetic properties (e.g., histories of magnetic flux injection and amplification). 23,36,239,273,274 In this respect future radio surveys with sensitivities to diffuse cluster-scale emission much better than current surveys, will provide unique and critical constraints to the role played by secondaries. For reference, Hybrid models, that assume radio halos to be generated by the (re)acceleration of secondary particles by turbulence in cluster mergers, 172,177 predict that secondaries generated in more relaxed systems produce radio emission with luminosity marginally smaller than current upper limits in Figure 12.…”

The Extended GMRT Radio Halo Survey

“…Mazzotta & Giacintucci 2008;Giacintucci et al 2014b) suggests that turbulence may be reaccelerating electrons to produce the mini-haloes (Mazzotta & Giacintucci 2008;ZuHone et al 2013;Giacintucci et al 2014a). In addition, the secondary electron models similar to those for radio haloes have also been suggested as explaining the mini-haloes that indeed trace the region where the number density of thermal targets in proton-proton collisions is higher (Pfrommer & Enßlin 2004;Keshet & Loeb 2010;Zandanel et al 2014). Based on the role of turbulent motions in the ICM and the transport of cosmic rays, a potential evolutionary connection between mini-haloes and radio haloes has also been suggested (e.g.…”

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

“…Still, contrary to the case of nearby giant radio halos, these limits do not put significant tension on a secondary origin of mini-halos (Brunetti & Jones 2014). For example, Zandanel et al (2014) proposed that mini-halos are primarily of hadronic origin, while giant radio halos experience a transition from a central hadronic emission to a leptonic emission component in the external regions due to CRe reacceleration. In the hadronic model, under relatively quiescent conditions, the electron spectrum will reach a steady-state condition where the energy losses and the gains due to injection balance each other out (Sarazin 1999).…”